Spiroketal natural products isolated from traditional Chinese medicine: isolation, biological activity, biosynthesis, and synthesis

Contents

• 1. Introduction

• 2. Terpenoid spiroketals
  • 2.1 Monoterpenoids
  • 2.2 Sesquiterpenoids
  • 2.3 Diterpenoids
  • 2.4 Triterpenoids
  • 2.5 Meroterpenoids

• 3. Polyketide spiroketals
  • 3.1 Cephalosporolide B derived
  • 3.2 Polycyclic polyprenylated acylphloroglucinols
  • 3.3 Other phenolic polyketides

• 4. Polyacetylene spiroketals

• 5. Glycoside spiroketals
  • 5.1 Acylphloroglucinol glycosides
  • 5.2 Flavonoid glycosides
  • 5.3 Pyrrolomorpholine spiroketal alkaloids
  • 5.4 Lycibarbarines A–C

• 6. Summary and perspectives

• 7. Conflicts of interest

• Acknowledgements

• References

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Abstract

Covering: 2010 to December 2024

Traditional Chinese medicine is an ancient knowledge base of therapeutic plants and preparations. Today, the isolation of bioactive natural products from traditional Chinese medicine is a valuable tool to identify new scaffolds for drug discovery. One such scaffold, the spiroketal moiety, is widespread in bioactive natural products, often crucial to the bioactivity of the compound. The convergent evolution of the spiroketal moiety in natural products arising from diverse phylogenetic and biosynthetic origins is a hallmark of the biological importance of this moiety. This review aims to highlight the diverse biosynthetic origins and ensuant structural diversity of spiroketal natural products isolated from traditional Chinese medicine, along with their potent and wide array of biological activities, and synthetic approaches to access these natural products to date.

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Eilidh G. Young

Eilidh Young obtained her BSc (Honours) in Medicinal Chemistry from The University of Auckland in 2021, where her research focused on iminium-catalysed [4 + 2] cycloadditions supervised by Assoc. Prof. Daniel Furkert and Dist. Prof. Dame Margaret Brimble. In 2021, she commenced her PhD studies under the supervision of Dist. Prof. Dame Margaret Brimble at the University of Auckland. Her doctoral research focused on the synthesis of N-heterocyclic spiroketal alkaloids isolated from traditional Chinese medicines. As part of this she also conducted research as a visiting graduate scholar in the lab of Prof. Simon Blakey at Emory University in Atlanta, USA.

Freda F. Li

Freda F. Li obtained her PhD in 2017 in organic chemistry by completing the synthesis of a chromanone natural product and relevant methodology studies. She then started her research assistant position in Brimble research group at the University of Auckland in New Zealand and completed the syntheses and activity studies of culicinin D anticancer peptide analogues. Her subsequent research has expanded to medicinal chemistry, including the development of anti-methanogens, nitrification inhibitors, antimicrobial agents, and live-cell imaging probes. Currently, as a Research Fellow in the group of Dame Margaret Brimble, her focus lies on the synthesis of spirocyclic imine marine toxins and pharmaceutically relevant natural spiroketals.

Margaret A. Brimble

Professor Dame Margaret Brimble FRS is a Distinguished Professor at the University of Auckland where her research focuses on the synthesis of bioactive natural products and peptides and has been reported in 700+ papers. She is a Dame companion of the New Zealand Order of Merit and a Fellow of the Royal Society. Her awards include the 2023 Davy Medal (Royal Society London), the 2023 RSC Pedler Award, the 2023 Ernest Guenther Award in Chemistry of Natural Products, the 2010 RSC Natural Product Chemistry Award, and the 2012 RSNZ Rutherford medal. She led the medicinal chemistry team that resulted in the discovery of the FDA approved drug DAYBUE™ (trofinetide) to treat Rett Syndrome drug with Neuren Pharmaceuticals in partnership with Acadia Pharmaceuticals.

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1. Introduction

In China, the implementation of flora and fauna for use in traditional medicine dates back over 4500 years.¹ Since then, thousands of traditional medicines have been developed.² Traditional Chinese medicine is based on the concept of ‘qi’, or the circulation of vital energy, which is viewed as an equilibrium between ‘Yin’ and ‘Yang’, the two complementary forces of nature.³ A disruption in the Yin-Yang balance can disturb a person's vital energy and lead to disease.³ Today, traditional Chinese medicine is standardised by the Chinese Pharmacopoeia, which is updated every 5 years.^4,5 The isolation of individual bioactive natural products from traditional medicine allows for scientific evaluation of safety and efficacy, improved control of dosage, as well as the ability to design and screen synthetic analogues to improve upon the pharmacological profile of the parent natural product.

Arguably the greatest success story of modern drug discovery from traditional Chinese medicine is the development of the antimalarial drug artemisinin (1) (Fig. 1). The plant Artemesia annua has been established in traditional Chinese medicine for antimalarial properties as early as the 4th century AD, first recorded by Ge Hong in “Zhouhou Beiji Fang” (“Handbook of Prescriptions for Emergency”).³ In 1972, artemisinin (1) was isolated from the plant A. annua by Youyou Tu, who in 2015 was awarded the 2015 Nobel Prize in Physiology or Medicine for this discovery.⁶ The more potent semisynthetic derivative dihydroartemisinin (2) (Fig. 1) is now listed on the World Health Organization Model List of Essential Medicines.⁷

Fig. 1 Artemisinin (1) and dihydroartemisinin (2).

The isolation of new bioactive natural products from traditional Chinese medicine remains an active field of study. As new natural product isolation reports continue to emerge, trends in key bioactive scaffolds and structural motifs emerge over time. One such scaffold present in a wide array of bioactive natural products is the spiroketal moiety.

The spiroketal moiety is well established as a ‘privileged scaffold’ in drug discovery.^8–11 Most commonly, spiroketals occur as [5,5]-, [6,5]-, or [6,6]-spiroketal scaffolds (Fig. 2). The dynamic, three-dimensional spiroketal core confers unique architectural complexity, and the spiroketal core is often directly implicated in key binding interactions with the biological target.^9,10,12 The stereochemistry of the spiroketal motif is governed by a range of factors including the anomeric effect, minimisation of steric interactions, and intramolecular hydrogen bonding effects.^13,14

Fig. 2 Common naturally occurring spiroketal scaffolds.

The spiroketal motif is ubiquitous in biologically-active natural products isolated from traditional Chinese medicine, with the scaffold arising from diverse biosynthetic and phylogenetic origins, implicating an evolutionary driving force towards the construction of this privileged scaffold.⁸ Since 2010, a plethora of research has been directed towards such natural products; including the isolation of over 87 novel structures. This review summarises the isolation, biosynthesis, biological activity, and synthesis of such natural products, with an emphasis on the traditional Chinese medicine of origin. Due to their distinct biogenesis and physicochemical properties, oxaspirolactone natural products are omitted from this review.¹⁵ Despite being a distinct biosynthetic source of the spiroketal motif, steroids with an embedded spiroketal moiety are also excluded from this review.¹⁶

2. Terpenoid spiroketals

Terpenoids are natural products comprised of repeating isoprene units and are classified in accordance to the number of isoprene units incorporated in the structure.¹⁷ Late-stage oxidation, cyclisation, and rearrangement transformations give rise to impressive structural diversity in this class.

2.1 Monoterpenoids

Japonones A (3) and B (4) are [5,5]-spiroketals isolated from Hypericum japonicum by Hu and co-workers in 2016 (Scheme 1).¹⁸ The herb H. japonicum was first registered in traditional Chinese medicine during the Qi Dynasty, used to treat fever, haemostasis, hepatitis, gastrointestinal disorders, and wound-healing.^19,20 (+)-Japonone A ((+)-3) showed modest inhibition of KSHV (EC₅₀ 166 μM, selectivity index >3.01).¹⁸

Scheme 1 Biosynthesis of japonones A (3) and B (4) proposed by Hu and co-workers.¹⁸

Biosynthetically, japonones A (3) and B (4) arise from the universal monoterpenoid precursor geranyl pyrophosphate (GPP) (5) (Scheme 1).¹⁸ Alkylation of acetate–acetate–propionate derived intermediate 6 with GPP (5) gives intermediate 7, which may oxidise to key linear intermediate 8. Cyclisation of 8 then affords japonones A (3) and B (4). No total synthesis of japonones A (3) and B (4) have been reported to date.

2.2 Sesquiterpenoids

Pleurospiroketals A–F (9–14) are [5,5]-spiroketal sesquiterpenes (Scheme 2), isolated from mushrooms of the genus Pleurotus. Pleurospiroketals A–E (9–13) were isolated in 2013 by Liu and co-workers, from P. cornucopiae.²¹P. cornucopiae is an edible mushroom with medicinal properties frequently consumed in China, known for its anti-hypertensive and anti-oxidant effects.^22,23 Pleurospiroketals A–C (9–11) demonstrated anti-oxidant activity in lipopolysaccharide-activated macrophages (IC₅₀ 6.8 μM, 12.6 μM, 20.8 μM, respectively). Pleurospiroketal F (14) was isolated in 2016 by Tao and co-workers, from P. citrinopileatus.²⁴P. citrinopileatus is also an edible mushroom common in China, known for its anti-oxidant and anti-hyperglycaemic effects.^22,25 The bioactivity of pleurospiroketal F (14) remains undetermined.

Scheme 2 Biosynthesis of pleurospiroketals A–F (9–14), proposed by Liu and co-workers.²¹

As sesquiterpenes, the biosynthesis of pleurospiroketals A–F (9–14) arises from farnesyl pyrophosphate (FPP) (15), which is proposed to undergo cyclisation to form intermediate 16 (Scheme 2).²¹ Subsequent oxidative transformations gives key furan intermediate 17, which can be further elaborated to the natural products 9–14 in a divergent manner.

In 2018, the asymmetric total synthesis of pleurospiroketals A (9) and B (10) was achieved by Ito and co-workers (Scheme 3).²⁶ Installation of a chiral auxiliary into known carboxylic acid 18 afforded 19 in two steps. A syn-selective Evans aldol reaction then afforded 20 as a single isomer. A Grubbs II catalysed ring-closing metathesis gave cyclohexene 21, which was elaborated to 22 in three steps, via a Grignard addition to the Weinreb amide derivative and subsequent O-TBS protection. From here, dihydroxylation with potassium osmate gave the desired diol 23 in 97% yield (d.r. >95 : 1). Acetonide protection, and treatment with phenyl triflimide, gave the vinyl triflate 24. Then, 2,4,6-trichlorophenyl formate was implemented as a CO surrogate to give 25, the absolute stereochemistry of which was confirmed by X-ray diffraction (XRD) analysis. Subsequent reduction gave 26, and oxidation of the primary alcohol gave aldehyde 27. Derivatisation with dithiane 28, followed by removal of the dithiane group and subsequent oxidation, gave 1,2-diketone 29 in 50% yield over three steps. Final spiroketalisation, and tandem silyl deprotection, gave pleurospiroketals A (9) and B (10), with no diastereoselective control with regards to the spiroketal core. The absolute stereochemistry of pleurospiroketal A (9) was unambiguously confirmed with XRD analysis.

Scheme 3 Synthesis of pleurospiroketals A (9) and B (10) by Ito and co-workers.²⁶

In 2021, Kontham and co-workers reported a diastereoselective synthesis of pleurospiroketal A (9) (Scheme 4).²⁷ Cyclohexenone 30 was transformed into Weinreb amide derivative 31 in three steps, as a mixture of inseparable diastereomers. Diastereoselective dihydroxylation gave 32 as the major isomer, which was carried forward for the rest of the synthesis. Subsequent acetonide protection, reduction, and α-methylenation using Eschenmoser's salt gave aldehyde 33. Dithiane 34 was then coupled with 33 to give alcohol 35. From here, the stereoselective synthesis of pleurospiroketal A (9) was achieved by oxidation of 35 to give 36, and subsequent acid-catalysed spiroketalisation provided 37 as a single diastereomer, likely due to the increased steric bulk of the proximal dithiane moiety, with the absolute configuration confirmed by XRD analysis. Unmasking the dithiane to the ketone gave pleurospiroketal A (9). Alternatively, the authors found that dithiane deprotection of 35, followed by oxidation to the 1,2-diketone 38, and subsequent acid-catalysed spiroketalisation, provided a mixture of pleurospiroketals A (9) and B (10), with A (9) as the major diastereomer formed. Both Ito and co-workers (Scheme 3)²⁶ and Kontham and co-workers (Scheme 4)²⁷ employed similar asymmetric strategies to pleurospiroketals A (9) and B (10) (intermediates 29 and 38 varying only by the choice of silyl protecting group), though only the spiroketalisation precursor dithiane 36 by Kontham and co-workers was able to afford stereoselective formation of the spiroketal core.

Scheme 4 Synthesis of pleurospiroketals A (9) and B (10) by Kontham and co-workers.²⁷

Angepubesins A–E (39–43) are [5,5]-spiroketal sesquiterpenes isolated from the roots of Angelica pubescens in 2021 by Tang and co-workers (Scheme 5).²⁸ In traditional Chinese medicine, A. pubescens roots, known as ‘Duhuo’, are used primarily as an analgesic.²⁹ Angepubesins B–D (40–42) demonstrated anti-inflammatory activity by inhibition of NO production (50 μM) in RAW 264.7 cells stimulated by LPS.²⁸

Scheme 5 The biosynthesis of angepubesins A–E (39–43), proposed by Tang and co-workers.²⁸

Biosynthetically, angepubesins A–E (39–43) are proposed to arise from γ-bisabolene (44) (Scheme 5).²⁸ Angepubesins A (39) and B (40) are proposed to arise from cyclisation of 46, arising from the oxidation of bisabolangelone (45). Alternatively, bisabolangelone (45) may undergo oxidative transformations to epoxide 47, and subsequently diol 48, which may then undergo cyclisation to angepubesins C–E (41–43). No total syntheses of angepubesins A–E (39–43) have been reported to date.

Phyllaemblicins G1 (49), G3–G8 (50–55), and H1–H14 (56–69) are recently-isolated phyllaemblic acid derivatives, isolated from Phyllanthus emblica in 2014 (ref. 30) and 2015 (ref. 31) by Zhang and co-workers (Scheme 6). In traditional Chinese medicine, P. emblica is used to treat sore throats, hypertension, and hepatitis B.³² Phyllaemblicin G6 (53) demonstrated anti-viral activity against hepatitis B antigen (IC₅₀ 8.53 ± 0.97 μM),³⁰ while phyllaemblicin H1 (56) showed cytotoxic activity against two cancer cell lines (A-549, IC₅₀ 4.7 ± 0.7 μM; SMMC-7721, IC₅₀ 9.9 ± 1.3 μM).³¹ Biosynthetically, the phyllaemblicin family are proposed to derive from farnesyl pyrophosphate (FFP) (15), which may undergo oxidative transformations to cyclohexene 70.³³ Further oxidation may give ketone 71, and subsequent spiroketalisation may then give key intermediate 72, which may then provide access to all phyllaemblic acid derivatives 49–69 in a divergent manner. No syntheses of phyllaemblicins G1 (49), G3–G8 (50–55), and H1–H14 (56–69) have been reported to date.

Scheme 6 Plausible biosynthetic origins of phyllaemblicins G1 (49), G3–G8 (50–55), and H1–H14 (56–69).

2.3 Diterpenoids

Leonuketal (73) (Scheme 7) is a complex 8,9-seco-labdane diterpenoid isolated from Leonurus japonicus in 2015 by Peng and co-workers, bearing a bridged [6,6]-spiroketal moiety.³⁴L. japonicus is known in traditional Chinese medicine as the source of Yi Mu Cao (Motherwort herb), a herbal remedy traditionally used for the treatment of gynaecological blood disorders.^34,35 Leonuketal (73) demonstrated strong vasorelaxant activity against KCl-induced contractions of rat aorta (EC₅₀ 2.32 μM).

Scheme 7 Biosynthesis of leonuketal (73) proposed by Peng and co-workers.³⁴

The biosynthesis of leonuketal (73) proposed by Peng and co-workers begins with Michael addition of water to common labdane precursor 74, to give 75 (Scheme 7).³⁴ Oxidative cleavage of the C-8/C-9 bond affords triketone 76 with the seco-labdane carbon framework furnished.³⁶ Spiroketalisation then affords 77, and final ketone reduction affords leonuketal (73).

The total synthesis of leonuketal (73) was achieved in 2020 by Brimble and co-workers (Scheme 8).³⁷ The synthesis commenced with a titanocene radical cyclisation of geraniol-derived epoxide 78 to give ketone 79 as a single diastereomer. Epimerisation of the C-7 alcohol was affected over four steps using an oxidation–reduction sequence, to give 80. Shapiro-type fragmentation of the pyran ring then furnished the alkyne 81 over two steps, which was then elaborated in four steps to iodide 82, via an O-TIPS protection, hydroxymethylation, mesylation, and iodination sequence. Next, β-keto ester 83, upon treatment with NaH, was coupled to iodide 82 to give alkyne 84. Initially, β-ketoester 84 was subjected to catalytic AuCl·DMS, though effecting spirocyclisation proved challenging on this substrate. However, upon lactonisation of 84 to enol lactone 85, subsequent Au^I-catalysed cyclisation then effectively formed the spirocycle 86, with good diastereoselectivity (d.r. 9 : 1). The authors suggested the improved reactivity of enol lactone 85 over β-ketoester 84 in the Au^I-cyclisation event is likely due to the diminished conformational freedom of cyclic 85 over straight-chain 84, and hence reduced entropic loss during the cyclisation event. Diastereoselective reduction of the enol ether of 86 proved challenging; ultimately, Rh-catalysed hydrogenation gave an inseparable mixture of 1 : 1.2 α/β-face addition products (87 and 88, respectively), with concurrent thermodynamic C-7 epimerisation of the spiroketal observed for the β-addition product 88. A three-step lactone hydrolysis, alcohol oxidation, and acid-mediated acetalisation sequence furnished ketal-γ-lactone 89. From here, elaboration to leonuketal (73) was achieved in 5 steps, via a Grignard addition to complete the carbon skeleton, and oxidative transformations to complete the total synthesis of leonuketal (73).

Scheme 8 Synthesis of leonuketal (73) by Brimble and co-workers.

2.4 Triterpenoids

Harperspinoids A (90) and B (91) are a C-9 epimeric pair of [5,5]-spiroketal 16-nor limonoids, isolated from Harrisonia perforata in 2016 by Yan and co-workers (Scheme 9).³⁸ The roots of H. perforata are used in traditional Chinese medicine for the treatment of malaria, as well as wound healing.³⁹ Harperspinoid A (90) was shown to inhibit the cortisone reductase enzyme 11β-HSD1 (IC₅₀ 0.60 μM).³⁸

Scheme 9 Biosynthesis of harperspinoids A (90) and B (91) proposed by Yan and co-workers.³⁸

Yan and co-workers proposed a plausible biosynthetic pathway to harperspinoids A (90) and B (91) from known limonoid citriolide A (92) (Scheme 9).^38,40 Free-radical fragmentation via93 may give furan 94; subsequent oxidation and cyclisation transformations may then form harperspinoids A (90) and B (91). No synthesis of harperspinoids A (90) and B (91) has been reported to date.

2.5 Meroterpenoids

Ganoapplanin (97) is a dimeric meroterpenoid featuring a [6,6]-spiroketal moiety, isolated from Ganoderma applanatum in 2016 by Qiu and co-workers (Scheme 10).⁴¹ ‘Lingzhi’ (Ganoderma sp.) are fungi utilised in traditional Chinese medicine for their anti-tumour, anti-viral, and anti-fibrotic effects.^42,43 Ganoapplanin (97) was shown to inhibit T-type voltage-gated calcium channels (TTCC) with an IC₅₀ 51.5 μM ((+)-97) and 56.4 μM ((−)-97), maximum inhibition 80%. TTCC inhibition is important for the treatment of central nervous system disorders, such as Parkinson's disease.

Scheme 10 Biosynthesis of ganoapplanin (97) proposed by Qiu and co-workers.⁴¹

Qiu and co-workers traced the biosynthetic origins of ganoapplanin (97) back to gentisic acid (98) and lingzhilactone B (99) (Scheme 10).⁴¹ They propose intermolecular spiroketalisation between derivatives 100 and 101 may give key intermediate 102. From here, Qiu and co-workers propose diazotisation of 102 may give 103, which can undergo a Gomberg–Bachmann reaction to form the key C–C bond of intermediate 104. Although, it is plausible the biaryl coupling to give 104 may proceed via a P450 enzyme directly from a gentisic acid precursor, without the need for a diazo functionality.⁴⁴ From here, final lactonisation gives ganoapplanin (97).

The total synthesis of ganoapplanin (97) was achieved in 2024 by Magauer and co-workers (Scheme 11).⁴⁵ Synthesis of the southern fragment commenced with a Nozaki–Hiyama–Kishi reaction between known aldehyde 105 and vinyl iodide 106; which, following silyl protection, gave 107. A diastereoselective 5-exo-trig cyclisation in the presence of excess iodine afforded bicyclic lactone 108 as a single diastereomer. Removal of the methyl ester, allylation, and subsequent ozonolysis then afforded aldehyde 109 in three steps. Synthesis of the northern fragment began with elaboration of carboxylic acid 110 to bromide 111 in three steps. O-Alkylation of 1,4-hydroxyquinone with primary bromide 111 gave 112, which was then subjected to oxidative dearomatisation with phenyliodine(III) diacetate (PIDA) to give acetal 113. An efficient one-pot 1,4-radical cycloaddition/aldol reaction cascade between iodide 113 and aldehyde 109, furnished 114 with the core carbon scaffold of ganoapplanin (97) assembled. Subsequent protecting group and oxidative manipulations then afforded aldehyde 115. Treatment of 115 with phenyliodine bis(trifluoroacetate) (PIFA) afforded quinone 116; 116 was then converted to the hydroquinone using sodium dithionate, and acid-catalysed spiroketalisation afforded 117 in 20–53% yield over three steps, as a single diastereomer. Next, acetylation of the free phenol gave 118, and Cu^I-mediated benzylic oxidation in the presence of TBHP afforded the lactone 119. Final deacetylation then afforded ganoapplanin (97) in 95% yield.

Scheme 11 Synthesis of ganoapplanin (97) by Magauer and co-workers.⁴⁵

3. Polyketide spiroketals

Polyketides are a diverse class of natural products constructed from the condensation of simple acyl thioesters by polyketide synthases.⁴⁶ These give rise to an impressively diverse array of natural scaffolds including polyphenols, macrolides, polyenes, and polyethers.⁴⁷

3.1 Cephalosporolide B derived

Tenuipyrone (120) is a [6,5]-spiroketal polyketide, isolated from Isaria tenuipes in 2012 by Oshima and co-workers (Scheme 12).⁴⁸I. tenuipes is a fungus known in traditional Chinese medicine for its anti-bacterial, anti-aging, anti-cancer, and anti-oxidant activity.⁴⁹ To date, the bioactivity of tenuipyrone (120) remains unknown.

Scheme 12 Biosynthesis of tenuipyrone (120), opaliferin (121), and cordycicadin F (122) from cephalosporolide B (123).

Opaliferin (121) is a polyketide natural product bearing a [5,5]-spiroketal motif, isolated from Cordyceps sp. NBRC 106954 in 2013 by Grudniewska and co-workers (Scheme 12).⁵¹Cordyceps sp. are fungi well established in traditional Chinese medicine for broad-sweeping biological effects such as anti-aging, anti-cancer, anti-inflammatory, anti-oxidant, and immune-boosting effects.^52,53Cordyceps sp. were officially listed as a drug in Chinese Pharmacopoeia in 1964.⁵³ Opaliferin (121) was found to demonstrate weak anti-cancer activity, inhibiting proliferation of three cancer cell lines (at 100 μM: HSC-2, 60% inhibition; HeLa, 30% inhibition; RERF-LC-KJ, 20% inhibition).⁵¹ To date, no total synthesis of opaliferin (121) has been achieved.

Cordycicadin F (122) is another [5,5]-spiroketal polyketide arising from a Cordyceps fungus, isolated from C. cicadae JXCH1 in 2023 by Chen and co-workers (Scheme 12).⁵⁴ Cordycicadin F (122) was evaluated for anti-tumour activity in A549 human cells, and anti-inflammatory activity in murine microphages, but was found to be inactive. No total synthesis of cordycicadin F (122) has been reported to date.

Biosynthetically, tenuipyrone (120), opaliferin (121), and cordycicadin F (122) are all proposed to arise from the common macrolide precursor cephalosporolide B (123) (Scheme 12). Additional spiroketal polyketides (isolated before 2010 and hence beyond the scope of this review) derived from cephalosporolide B (123) have also been documented,⁵⁵ including cephalosporolides E, F,⁵⁶ H, and I,⁵⁷ penisporolides A and B,⁵⁸ and ascospiroketals A and B;⁵⁹ of these, cephalosporolides E and F have been isolated from traditional Chinese medicine sources (Cordyceps militaris BCC 2816).⁶⁰ A full biosynthetic route to cephalosporolide B (123) itself has not been proposed.

Tenuipyrone (120) is proposed to arise from a Michael addition of 4-hydroxy-6-methylpyrone-derived 124 and (E)-cephalosporolide B (E-123).⁴⁸ Intramolecular Claisen condensation of the resultant 125 gives 126, and final dehydrative cyclisations gives tenuipyrone (120).

Opaliferin (121) purportedly arises from Michael addition of intermediate 127 and (Z)-cephalosporolide B (Z-123), and decarboxylation, to make 128.⁵¹ From here, intramolecular Claisen condensation of 128 may give 129, and further rearrangement gives 130. Dehydration to the enone, and sequential spiroketalisation, gives opaliferin (129).

The biosynthesis of cordycicadin F (122) has not been proposed, but structurally it appears to represent a cyclised pyran derivative of opaliferin (121).

Synthetic access to tenuipyrone (120) was achieved in 2013 by Tong and co-workers (Scheme 13).⁵⁰ Known cyclopentenone 131 was elaborated to the phenylselenium derivative 132, which underwent a reductive deselenation aldol coupling with aldehyde 133 to give alcohol 134 in 48–65% yield. After oxidation to β-keto-enone 135, a bioinspired cascade furnished the core scaffold. First, Michael addition with 135 and 4-hydroxy-6-methyl-2-pyrone (136) gave β-ketolactone 137, and subsequent spiroketalisation afforded spiroketal 138. Final tandem silyl deprotection/isomerisation of the spiroketal core under wet acidic conditions afforded tenuipyrone 120, in 47% yield over two steps.

Scheme 13 Synthesis of tenuipyrone (120) by Tong and co-workers.⁵⁰

3.2 Polycyclic polyprenylated acylphloroglucinols

Hyperacmosins K (139), L (140), and R (141) are polycyclic polyprenylated acylphloroglucinols (PPAPs) isolated from Hypericum acmosepalum (Scheme 14). Hyperacmosins K (139) and L (140) feature a [5,5]-spiroketal moiety, isolated in 2021 by Ji and co-workers.⁶¹ In 2022, Ji and co-workers reported the isolation of hyperacmosin R (141). Bearing an unusual [5,8]-spiroketal core, from H. acmosepalum.⁶² In traditional Chinese medicine, H. acmosepalum is used as an anti-inflammatory, as well as to treat hepatitis.⁶³ Hyperacmosin R (141) demonstrated weak hepatoprotective activity against paracetamol-induced HepG2 cell damage at 10 μM.⁶²

Scheme 14 Biosynthesis of hyperacmosins K (139), L (140), and R (141) proposed by Ji and co-workers.^61,62

Biosynthetically, hyperacmosins K (139), L (140), and R (141) all originate from acylphloroglucinol 142, which undergoes a series of prenylation and cyclisation reactions to form key PPAP intermediate 143 (Scheme 14). Subsequent retro-Claisen and decarboxylation gives intermediate 144, from which the biosyntheses of hyperacmosins K (139) and L (140), and R (141) diverge. For hyperacmosin R (141), oxidative transformations give 145, which undergoes sequential cyclisations to first give 146, then 147. From here, epoxidation gives 148, and a final cyclisation then furnishes the [5,8]-spiroketal hyperacmosin R (141). Similarly, hyperacmosins K (139) and L (140) could arise from oxidation of 144 to 149, which can cyclise via keto–enol tautomer 150 to give the [5,5]-spiroketal natural products 139 and 140. To date, no studies towards the synthesis of hyperacmosins K (139), L (140), or R (141) have been reported.

3.3 Other phenolic polyketides

Virgatolides A–C (151–153) are benzannulated [6,6]-spiroketals isolated from the endophytic fungus Pestalotiopsis virgatula on the leaves of Dracontomelon duperreanum, in 2011 by Che and co-workers (Scheme 15).⁶⁴ The leaves of D. duperreanum are used in traditional Chinese medicine for wound healing, especially bedsores, skin ulcers, and topical infections.⁶⁵ Virgatolides A–C (151–153) demonstrated weak cytotoxic bioactivity against HeLa cell lines (IC₅₀ = 19.0 μM, 22.5 μM, 20.6 μM, respectively).

Scheme 15 Biosynthesis of virgatolides A–C (151–153) proposed by Che and co-workers.⁶⁴

The biosynthesis of virgatolides A–C (151–153) is thought to arise from 4-hydroxy-6-methyl-2-pyrone (136) (Scheme 15).⁶⁴ Addition of malonyl-SCoA gives 154, which can cyclise with either 6-desmethylpestaphthalide A (155) or 6-desmethylpestaphthalide B (156) to give [6,6]-spiroketals 157 or 158, respectively. From here, reductive transformations give virgatolides B (152) or C (153), respectively. Virgatolide B (152) can undergo further elaboration to 159, and final dehydrative cyclisation gives virgatolide A (151).

The enantioselective total synthesis of virgatolide B (152) was achieved in 2014 by Brimble and co-workers (Scheme 16).^66,67 The synthesis commenced with the elaboration of 3,5-dihydroxybenzoic acid (160) to aryl bromide 161 in 6 steps. This was then coupled with known enantioenriched β-trifluoroboratoamide 162 to give amide 163; reaction with MeLi then gave methyl ketone 164. Sharpless asymmetric dihydroxylation then afforded diol 165 as a single diastereomer. Next, iodination gave aryl iodide 166, which underwent a tandem carbonylation/intramolecular alkoxylation reaction to give phthalide 167. The key aldol transformation to join phthalide 167 with aldehyde 168, via the vinyl triflate, gave phthalide 169 as a single diastereomer over three steps. Global deprotection, and final acid-catalysed spiroketalisation, furnished virgatolide B (152).

Scheme 16 Synthesis of virgatolide B (152) by Brimble and co-workers.⁶⁷

Pestalospiranes A (170) and B (171) are dimeric spiroketals isolated from the endophytic fungus P. virgatula cultures found in the plant Terminalia chebula, in 2011 by Jaroszewski and co-workers (Scheme 17).⁶⁸ The fruits of T. chebula are used in traditional Chinese medicine to treat laryngitis, asthma, bronchitis, and tonsilitis.⁶⁹ The biological activity of pestalospiranes A (170) and B (171) have not been determined. The full biosynthetic origins of pestalospiranes A (170) and B (171) are not yet known; though, benzo[c]oxepin 172 was also isolated alongside 170 and 171, and likely a dimerisation/spiroketalisation event of 172 is able to form pestalospiranes A (170) and B (171) (Scheme 17).⁷⁰

Scheme 17 Plausible biosynthetic origins of pestalospiranes A (170) and B (171), from benzo[c]oxepin 172.

The total synthesis of pestalospirane B (171) was achieved in 2017 by Brimble and co-workers (Scheme 18).⁷⁰ Known alkynylsilane 173 was converted into alkyne 174 in four steps by protecting-group and reduction manipulations. Coupling of 174 with Weinreb amide 175 then gave 176, which was then subjected to hydrogenation to give ketone 177. Acid-catalysed cyclisation then afforded ketal 178, and treatment with DDQ resulted in the desired dehydrogenation, albeit with undesired oxidation of the chiral alcohol to the achiral ketone 179. Nevertheless, enantioselective reduction with (R)-Me-CBS resulted in 180 with the desired stereochemistry, and this was subsequently subjected to the bioinspired acid-catalysed tandem dimerization/spiroketalisation reaction, to give pestalospirane B (171) as the major diastereomer. The authors noted that minor amounts of pestalospirane A (170), though not isolated, were also observed by crude ¹H NMR.

Scheme 18 Synthesis of pestalospirane B (171) by Brimble and co-workers.⁷⁰

In 2021, Oh and co-workers reported the isolation of rugonidines A–F (181–186), three semi-stable epimeric pairs of spirocyclic alkaloids isolated from Alchornea rugosa leaves (Fig. 3).⁷¹ The leaves of A. rugosa are known in traditional Chinese medicine for their wound-healing properties.⁷² In addition, A. rugosa leaves are well documented across South East Asian folk medicine for the treatment of fever and malaria.⁷³ Of the six natural products isolated, rugonidines A–C (181–186) demonstrated promising antidiabetic activity, by increasing glucose uptake levels in differentiated 3T3-L1 adipocytes.⁷¹ Rugonidines A–F (181–186) possess an unprecedented 1,6-dioxa-7,9-diazaspiro[4.5]dec-7-en-8-amine bicyclic moiety, fused onto a (+)-catechin (187) scaffold. A plausible biosynthetic pathway has not been proposed, although the natural products appear to be comprised of subunits arising from (+)-catechin (187), and one of either galegine (188) or pterogynidine (189), which have been previously isolated from the genus Alchornea.^74–76 No synthetic studies towards rugonidines A–F (181–186) have been reported to date.

Fig. 3 Rugonidines A–F (181–186).

4. Polyacetylene spiroketals

Polyacetylene spiroketals are a large class of natural products, biosynthesised from fatty acids.^77,78 Recently isolated members of this family are shown in Scheme 19. Lactiflodiynes A–F (190–195) were isolated from Artemisia lactifloria by Ye and co-workers in 2011.⁷⁹ Also known as ‘Bai Bao Hao’, A. lactifloria is known in traditional Chinese medicine for anti-inflammatory effects.⁸⁰ Chrysindins C (196) and D (197) were also isolated in 2011, from the flowers of Chrysanthemum indicum, by Shi and co-workers.⁸¹ In traditional Chinese medicine, C. indicum is known for its anti-inflammatory and analgesic properties.⁸² Artemiselenols A–C (198–200) were isolated from Artemisia selengensis in 2016 by Luo and co-workers.⁸³ The plant A. selengensis is used in traditional Chinese medicine for haemostasis, and as an anti-inflammatory agent.⁸⁴ Artemiselenols B (199) and C (200) showed modest neuroprotective activity at 20 μM against H₂O₂-injured PC12 cells, though the exact cell viability was not explicitly stated. 199 and 200 also showed inhibition of human monoamine oxidase (hMAO) (IC₅₀ = 90 μM (199) and 98 μM (200) in hMAO-A, and 77 μM (199) and 87 μM (200) in hMAO-B). In 2023, rupesdiynes A (201) and B (202) were isolated form Artemisia rupestris, by Mu and co-workers.⁸⁵ Another member of the Artemisia genus, A. rupestris is used in traditional Chinese medicine for its analgesic and anti-inflammatory properties.⁸⁶ Rupesdiynes A (201) and B (202) showed synergistic anti-bacterial activity with oxacillin against oxacillin-resistant EMRSA-16 (MIC 64 mg L⁻¹).⁸⁵

Scheme 19 Recently isolated polyacetylene spiroketals, and the general biosynthetic origins of the polyacetylene spiroketal scaffold proposed by Greger.⁷⁸

Biosynthetically, Greger proposes that polyacetylene spiroketals derive from oleic acid (203); subsequent dehydrogenative transformations give the polyacetylene 204, and further oxidative transformations give ketoalcohol 205, which upon cyclisation gives the general polyacetylene spiroketal scaffold 206 (Scheme 19).⁷⁸ No total syntheses of any of 190–202 have been reported.

5. Glycoside spiroketals

Glycosides are a large family of natural products, consisting of at least one glycosidic bond.⁸⁷ Natural products wherein the spiroketal motif arises from a sugar-based biosynthetic precursor are discussed below.

5.1 Acylphloroglucinol glycosides

Lysidicin J (207) is a benzannulated [6,5]-spiroketal isolated from the roots of Lysidice rhodostegia by Yu and co-workers in 2011 (Scheme 20).⁸⁸ The roots of L. rhodostegia are used in traditional Chinese medicine to treat haemorrhage, fractures, as well as for their analgesic effects.^88,89 Lysidicin J (207) demonstrated modest anti-oxidant activity (100% inhibition at 0.1 μmol mL⁻¹; no inhibition at 0.01 μmol mL⁻¹). Biosynthetically, lysidicin J (207) arises from the twice-dehydrative cyclisation between isovalerylphloroglucinol (208) and D-fructose (209). No synthesis of lysidicin J (207) is yet reported.

Scheme 20 Biosynthesis of lysidicin J (207) proposed by Yu and co-workers.⁸⁸

Melicospiroketals A–E (210–214) are diastereomeric [5,5]-spiroketal acetophenones isolated from the leaves of Melicope pteleifolia in 2016 by Oh and co-workers (Scheme 21).⁹⁰ In traditional Chinese medicine, M. pteleifolia is used to treat sore throats, eczema, dermatitis, and snake bites.^91,92 Melicospiroketals A–E (210–214) were assessed for anti-viral activity against H1N1 influenza, but were found to be inactive. The biosynthesis may proceed via Friedel–Crafts-type reaction between 2,4,6-trihydroxyacetophenone (216) and D-glucose (215), to form C-glycoside 217. From here, dehydrative cyclisation with the relevant sugar 218 would give the corresponding natural product 210–214. The total synthesis of melicospiroketals A–E (210–214) has not been achieved to date.

Scheme 21 Plausible biosynthesis of melicospiroketals A–E (210–214).

5.2 Flavonoid glycosides

Saffloquinoside A (219) is a flavonoid with a sugar-derived [6,5]-spiroketal moiety, isolated in 2010 by Zhang and co-workers from Carthamus tinctorius florets (Scheme 22).⁹³ Zhang and co-workers isolated Saffloquinoside C (220) three years later from the same plant.⁹⁴C. tinctorius florets are known in traditional Chinese medicine as ‘Honghua’, and are used to treat blood stasis and promote blood circulation.⁹⁵ Saffloquinoside A (219) demonstrated anti-inflammatory activity by inhibiting the release of β-glucuronidase from rat polymorphonuclear neutrophils (54.3% inhibition at 10⁻⁵ mmol) induced by platelet-activating factor.⁹³ The syntheses of saffloquinosides A (219) and C (220) have yet to be achieved.

Scheme 22 Biosynthesis of flavonoid glycoside spiroketals saffloquinosides A (219) and C (220), pinnatifine E (221), and heteryunine A (222).

Pinnatifine E (221), a flavonoid glycoside with a benzannulated [5,5]-spiroketal moiety, was isolated from Vaccaria segetalis in 2015, also by Zhang and co-workers (Scheme 22).⁹⁶ Also called ‘Wang Bu Liu Xing’, V. segatalis was first established in traditional Chinese medicine over 2000 years ago, used to treat amenorrhea, dysmenorrhea, carbuncles, and aid in lactation.⁹⁷ Pinnatifine E (221) demonstrated weak anticoagulant activity, by inhibition of coagulation Factor Xa (IC₅₀ = 165 μM).⁹⁶ No total synthesis of pinnatifine E (221) has been reported to date.⁹⁸

Heteryunine A (222) is a [5,5]-benzannulated spiroketal natural product isolated in 2024 by Zhang and co-workers from Heterosmilax yunnanensis (Scheme 22).⁹⁹ In traditional Chinese medicine, H. yunnanensis, known as ‘Baituling’, is one of two main ingredients in the preparation called “Compound Kushen Injection”, used clinically today for its anticancer effects.^100,101 Heteryunine A (222) was shown to possess antifibrotic activity by inhibition of Ras homolog family member A (RhoA) activity.⁹⁹ To date, no total synthesis of heteryunine A (222) has been reported.

The biosynthesis of spiroketals saffloquinosides A (219) and C (220), pinnatifine E (221), and heteryunine A (222) can all be traced back to naringenin chalcone (223) (Scheme 22). From here, oxidation to the tetrahydroxyphenyl derivative and subsequent incorporation of two D-glucose (215) subunits may provide either of saffloquinosides A (219) or C (220).⁹³ Alternatively, cyclisation of 223 to naringenin (224), elimination to apigenin (225), and subsequent nucleophilic addition to D-glucose (215) could give pinnatifine E (221). In the case of heteryunine A (222), oxidation of 224 to quercetin (226), reduction to (+)-catechin (187), nucleophilic addition to 3-deoxy-D-glucosone (227), and further elaboration with L-tryptophan (229) and malonic acid (230) subunits, gives heteryunine A (222).⁹⁹

5.3 Pyrrolomorpholine spiroketal alkaloids

The pyrrolomorpholine spiroketal alkaloid (PSA) natural products (231–236) are a family of natural products bearing 2-formyl pyrrole and spiro-fused sugar-morpholine spiroketal subunits.^102–107

All members of the pyrrolomorpholine spiroketal family are derived from traditional Chinese medicine sources, isolated sporadically from six different sources from 2010 to 2015.^102–107 Acortatarins A (231) and B (235) were isolated in 2010 from Acorus tatarinowii rhizomes, which were first recorded in c.a. 200 AD in the first Chinese materia medica book “Shen Nong Ben Cao Jing”.^102,108A. tatarinowii rhizomes have been incorporated in the Pharmacopoeia of the People's Republic of China since 1963 for the treatment of central nervous system disorders.^102,108 Concurrently, in 2010 pollenopyrrosides A (234), and acortatarin A (231) were isolated from bee-collected Brassica campestris pollen.¹⁰³ In traditional Chinese medicine, this pollen extract is renowned for its anti-inflammatory, anti-oxidant, anti-allergic, and cardioprotective activity.^103,109 Also in 2010, acortatarin A (231) and shensongine A (233) were isolated from Capparis spinosa fruits.¹⁰⁴C. spinosa has long been used in traditional Chinese medicine, especially in the Xinjiang Uygur Autonomous Region, mostly for its anti-inflammatory effects.^104,110 In April 2015, shensongines A (233), B (232), and C (236), along with acortatarin A (231), were isolated from Shensong Yanxin capsules, a traditional Chinese medical preparation consisting of 12 ingredients from Chinese materia medica.¹⁰⁶ Shensong Yanxin capsules are widely implemented in traditional Chinese medicine for anti-arrhythmic effects.^106,111 In December 2015, shensongines A (233) and B (232) were isolated from Xylaria nigripes, a medicinal fungus also known as Wu Ling Shen in traditional Chinese medicine to treat insomnia, trauma, and depression.^107,112,113

Due to their privileged scaffold and prevalence in traditional Chinese materia medica, several studies have focused on the bioactivity of the PSAs. In their initial 2010 report, Cheng and co-workers determined that acortatarin A (231) inhibited the production of reactive oxygen species (ROS) in high-glucose-stimulated mesangial cells in a dose- and time-dependent manner.¹⁰² This bioactivity was further elaborated by Nie and co-workers in 2013, who determined that acortatarin A (231) increases phosphorylation of p47^phox, causing inhibition of high-glucose-induced activation of NADPH oxidase, thereby inhibiting ROS production in mesangial cells.¹¹⁴ Verano and Tan built on this in 2017, evaluating the ROS-inhibition potential of all pyrrolomorpholine spiroketal alkaloids.¹¹⁵ Acortatarin A (231) and shensongine C (236) were found to be the most potent ROS-inhibitors (IC₅₀ 4.6 μM and 4.8 μM, respectively; maximum ROS inhibition 100%), although all six natural products 231–236 demonstrated antioxidant activity by inhibiting high-glucose-induced ROS generation in a dose-dependent manner, to some extent.¹¹⁵ Development of ROS-inhibiting agents like the pyrrolomorpholine spiroketal alkaloids could prove crucial for the treatment of diabetic neuropathy, a common and often fatal complication of both type I and type II diabetes.¹¹⁶ In addition to antioxidant effects, Yao and co-workers found that shensongines A (233) and C (236) shortened the action potential duration in rat myocardial cells, possibly due to either the inhibition of L-type calcium channels, or the facilitation of potassium channels.¹⁰⁶

Biosynthetically, they are thought to arise from the intermolecular condensation of 3-deoxy-D-glucosone (227) with one of either 1-amino-1,3-dideoxy-D-fructose (237) or 1-amino-1-deoxy-D-fructose (239) (Scheme 23).¹¹⁷ Condensation of 227 with 237 gives rise to the assumed oxocarbenium precursor 238, from which two anomeric pairs of regioisomeric kinetic [6,5]- and thermodynamic [6,6]-spiroketal products are formed (acortatarin A (231), shensongine B (232), shensongine A (233), and pollenopyrroside A (234)), depending on which hydroxy group participates in the cyclisation event. Likewise, condensation of 227 with 239 gives 240, although only the [6,5]-spiroketal regioisomers have been found in nature (acortatarin B (235), and shensongine C (236)).

Scheme 23 Biosynthetic origins of the pyrrolomorpholine spiroketal alkaloid family.

There are 11 individual reports on the total synthesis of members of the PSA family, and these syntheses have been reviewed extensively.^117–121 Hence, these syntheses will not be reiterated in this review.

5.4 Lycibarbarines A–C

Lycibarbarines A–C (241–243) are spiroketal alkaloids isolated in 2021 by Chen and co-workers from the fruits of Lycium barbarum, commonly known as goji berries (Scheme 24).¹²² Goji berries have been used in traditional Chinese medicine for more than 2000 years for the treatment of age-related conditions, including improved eyesight, improved liver and kidney functions, as well as antioxidant, immunomodulating, and antitumor activity.¹²³ Lycibarbarines A (241) and C (243) demonstrated neuroprotective activity by reduction of apoptosis in corticosterone-injured PC12 cells (cell viabilities 79% and 69%, respectively, at 20 μM).¹²² Further investigation revealed that 241 and 243 reduced the expression levels of cleaved caspase-3 and caspase-9, both marker proteins for apoptosis.

Scheme 24 Biosynthesis of lycibarbarines A–C (241–243) proposed by Chen and co-workers.¹²²

Lycibarbarines A–C (241–243) possess a unique tetracyclic scaffold composed of tetrahydroquinoline and spiro-fused oxazine-sugar spiroketal subunits. Biosynthetically, they are proposed to arise from nucleophilic attack of 3-deoxy-D-fructose (244) with the key tetrahydroquinoline intermediate 245; subsequent dehydration then gives the key oxocarbenium reactive intermediate 246. 246 can then undergo cyclisation to afford either of the kinetic [6,5]-spiroketal natural products 241 and 242, or the thermodynamic [6,6]-spiroketal 243.

In April 2023, Brimble and co-workers reported the first total synthesis of lycibarbarines A–C (241–243) (Scheme 25).¹²⁴ The synthesis commenced with known epoxide 247, which was accessed in four steps from 2-deoxy-D-ribose. Ring-opening of the epoxide and subsequent oxidation gave α-bromoketone 248. N-Alkylation of tetrahydroquinoline 249 with 248 gave the unstable hemiketal intermediate 250; subsequent acid-mediated cyclisation gave [6,5]-spiroketals 251 and 252 (d.r. 1.4 : 1, respectively), which were separated for the remainder of the synthesis. Late-stage formylation and final silyl deprotection then separately afforded lycibarbarines A (241) and B (242). Attempts to cyclise hemiketal 250 to a [6,6]-spiroketal scaffold, via silyl-deprotection and subsequent acid-mediated cyclisation, instead afforded the [6,5]-spiroketal 253 exclusively. Likewise, lycibarbarines A (241) and B (242) were unable to undergo acid-mediated isomerisation to the thermodynamic [6,6]-spiroketal lycibarbarine C (243). Instead, a revised protecting group strategy involving elaboration of known alkene 254 to α-bromoketone 255 was undertaken. Analogous N-alkylation of tetrahydroquinoline 249 with 255, and subsequent acid-mediated cyclisation, gave the desired [6,6]-spiroketal 256 as the major diastereomer; late-stage formylation and final silyl deprotection then gave lycibarbarine C (243).

Scheme 25 Synthesis of lycibarbarines A–C (241–243) by Brimble and co-workers.¹²⁴

In June 2023, Ghosh and co-workers reported the second total synthesis of lycibarbarines A (241) and B (242) (Scheme 26).¹²⁵ Known alkene 257, derived from 2-deoxy-D-ribose, was selected as the starting material. Subsequent benzyl protection, and dihydroxylation of the alkene, then afforded glycol 258. Selective oxidation of the secondary alcohol using NBS in the presence of bis(tributyltin) oxide afforded α-hydroxyketone 259, which was further elaborated to tosylate 260. N-Alkylation of tetrahydroquinoline 261 with tosylate 260 in the presence of NaI, and subsequent acid-mediated cyclisation (d.r. 2.5 : 1), furnished the tetracyclic scaffold. The resultant [6,5]-spiroketals 262 and 263 were separated, and individual O-Bn deprotection afforded lycibarbarines A (241) and B (242).

Scheme 26 Synthesis of lycibarbarines A (241) and B (242) by Ghosh and co-workers.¹²⁵

In July 2023, Du and co-workers reported their total synthesis of lycibarbarines A–C (241–243), which features a concise 4-step (longest linear sequence) divergent synthetic route to all three natural products (Scheme 27).¹²⁶ The synthesis commenced with formation of known lactone 264,¹²⁷ from 2-deoxy-D-ribose. An efficient one-pot procedure combined the two fragments together; firstly, addition of methylene iodide and methyl lithium to 264 in the non-coordinating solvent toluene allowed for in situ formation of the LiCH₂I species, which underwent nucleophilic addition to provide iodide 265. Addition of the tetrahydroquinoline 261 into the reaction mixture allowed for N-alkylation with the iodide 265, to provide hemiketal 266.

Scheme 27 Synthesis of lycibarbarines A–C (241–243) by Du and co-workers.¹²⁶

Acid-mediated tandem acetonide-deprotection/spiroketalisation of 266 would then furnish the natural product scaffolds. Initially, treatment of hemiketal 266 with CSA in dichloromethane afforded a mixture of 241, 242, and 243 in approximately 3 : 3:2.5 ratio. Under these conditions, it appears the formation of the kinetic [6,5]-spiroketal scaffolds of 241 and 242 was favoured over the [6,6]-spiroketal framework of 243. The authors noted that once formed, the spiroketal natural products were unable to undergo acid-mediated equilibration to the thermodynamic product lycibarbarine C (243). Nonetheless, further optimisation allowed for the selective, and divergent, formation of either the [6,5]-spiroketals lycibarbarines A (241) and B (242), or [6,6]-spiroketal lycibarbarine C (243). Treatment of hemiketal 266 with 3 N HCl in MeOH/EtOAc afforded lycibarbarines A (241) and B (242) in a 1 : 1 diastereomeric mixture, without formation of lycibarbarine C (243). Careful treatment of 266 with stoichiometric p-TSA in acetone afforded [6,6]-spiroketal 267 as a single diastereomer with the acetonide protecting group intact; subsequent acid-mediated acetonide deprotection afforded lycibarbarine C (243).

Brimble and co-workers (Scheme 25),¹²⁴ Ghosh and co-workers (Scheme 26),¹²⁵ and Du and co-workers (Scheme 27),¹²⁶ all employed similar strategies to lycibarbarines A–C (241–243), involving the N-alkylation of a tetrahydroquinoline substrate with a halide/pseudohalide derived from 2-deoxy-D-ribose. Both Brimble and co-workers, and Du and co-workers, commented on the unique resistance of the lycibarbarine oxazine-spiroketal core to acid-promoted thermodynamic isomerisation; an unusual physicochemical property likely due to the presence of the proximal basic nitrogen atom.

6. Summary and perspectives

Traditional Chinese medicine denotes an invaluable knowledge base of therapeutic preparations; as such, the isolation of natural products from traditional Chinese medicine sources explores an area of chemical space pre-dispositioned towards biologically relevant natural scaffolds. Among these are spiroketal-containing natural products, which originate from diverse phylogenetic and biosynthetic origins, implicating a convergent evolutionary drive towards this privileged scaffold.

While plausible biosynthetic pathways to most spiroketal natural products discussed in this review have been proposed, a deeper biochemical understanding of these biosynthetic pathways are not yet well understood. Further analysis in this area, including the identification of key metabolic enzymes and ensuant potential for genome-mining of therapeutic plants, may allow for the discovery of a larger array of spiroketal natural products. Given their propensity to possess potent bioactivity, further studies towards the biochemical origins of spiroketal natural products may prove highly beneficial to future drug discovery and development efforts.

Furthermore, many spiroketal natural products discussed herein remain to be accessed synthetically. The total chemical synthesis of spiroketal natural products is imperative to their full biological evaluation and pre-clinical development for modern drug discovery efforts, as it allows for access to clinically meaningful quantities of the natural product. Additionally, chemical synthesis also enables the synthesis of synthetic derivatives, to further probe the implementation of natural pharmacophores for drug discovery.

One significant characteristic of the spiroketal moiety is the tendency of the chiral spirocentre to undergo epimerisation under mildly acidic conditions, potentially signalling a challenge for drug design and synthesis. Because the spiroketal core is often involved in key target-binding interactions, as well as the overall three-dimensional architecture of the molecule, control over the stereochemistry of the spiroketal core is crucial to its success as a potential drug candidate. To this end, the recent isolation and synthesis of N-heterocyclic spiroketal alkaloids, most especially lycibarbarines A–C (241–243), denote a notably more acid-stable spiroketal scaffold which may prove useful for future endeavours in drug design.

We anticipate that future efforts towards the isolation, biological evaluation, and synthesis of spiroketals from traditional Chinese medicine will have important implications for drug design efforts. As drug discovery efforts shift towards targets with greater three-dimensional complexity,^128,129 privileged pharmacophores emergent from traditional medicine sources, such as the spiroketal moiety, may prove instructive towards the design of new drug targets.

7. Conflicts of interest

There are no conflicts to declare.

8. Acknowledgements

The authors thank the University of Auckland for a Doctoral Scholarship Award (EGY). The table of contents image was partly generated using Adobe Express.

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