Total Syntheses of Natural Products Containing Spirocarbocycles

The structures of natural products from a variety of sources contain spirocycles, two rings that share a common atom. The spiro motif is finding increasing inclusion in drug candidates, and as a structural component in several promising classes of chiral ligands used in asymmetric synthesis. Total syntheses of products containing all-carbon spirocycles feature several common methods of ring-closure which we examine in this review.


Introduction
Natural products have been an invaluable source of medicines and other resources throughout history. In recent years, the search for novel compounds has moved from the land to the sea, concentrating on marine organisms rather than terrestrial species. There are several ways of classifying such compounds: by genus of their source, by physiological effect, or by a common structural element such as their carbon skeleton. This review will focus on natural products containing the spiro motif. Spiro compounds possess two or more rings which share only one common atom, with the rings not linked by a bridge. 1 Spiro-containing compounds have been isolated from a wide range of biological sources, from plants to frogs to marine sponges. The shift from earth to water in search of compounds is mirrored here. Selected examples of such compounds are shown below (Figure 1). Spirocycles containing heteroatoms, such as the spirooxindoles, are well-known in nature, and there have been several reviews on these species. 2,3 One of the earliest isolated spiro natural products was β-vetivone, extracted from vetiver oil in 1939 by Pfau and Plattner. 4 However, for many years its structure was believed to be hydroazulenic (1.6) rather than a spiro[4.5]decane (1.7), an error which was only rectified by its total synthesis by Marshall et al. in 1968. 5,6 Figure 1 -Natural products and drugs containing spirocycles.
The spiro motif is becoming more prevalent as a template in drug discovery, and has been the subject of a recent review. 7 Spiro rings of different sizes convey both increased threedimensionality for potential improved activity, and novelty for patenting purposes. Two examples of marketed drugs containing spirocycles are spironolactone (1.2) and griseofulvin (1.5, Figure 1). Both are on the World Health Organisation's 2 | J. Name., 2012, 00, 1-3 This journal is © The Royal Society of Chemistry 20xx list of essential medicines; spironolactone is a diuretic and antihypertensive drug, whilst griseofulvin is an anti-fungal agent. An additional important application of spiro compounds is in asymmetric synthesis. Enantioselectivity in organic reactions can be achieved through the use of chiral ligands, such as BINOL (1.8) or cinchona alkaloids (1.9, Figure 2). Some spirocycles, such as the spirobiindane SPINOL (1.10), belong to a class of privileged chiral ligands which show particular promise 8 . The increased rigidity and restricted rotation at the spiro centre can assist in improving enantiomeric excess in asymmetric reactions. The asymmetric synthesis of quaternary carbon centres is extremely challenging, 9 with the stereocontrolled preparation of products containing such spiroatoms [10][11][12] conveying additional difficulty. Enantioselective syntheses of spirocyclic ligands utilise several construction methods, such as alkylation, radical cyclisation, rearrangements, cycloaddition reactions, cleavage of bridged ring systems and the use of transition metal catalysts. 13 Several ring-closing strategies are common in the synthesis of spirocarbocycle-containing natural products, and these will be discussed in this review. Examination of the syntheses of these compounds will be first by reaction type of the spirocycle-forming step, followed by increasing ring size, from [2.5] through to [5.5]. Not all ring sizes are represented in the natural world, nor have all spiro-containing natural products been the target of organic synthesis. Some reaction types have not been included in this article, for example the Wittig reaction, for the purposes of brevity.
Some of the earliest-discovered and best-studied compounds such as the aforementioned β-vetivone have been synthesised by several of these methods. The field has matured significantly since the 1970s, when many of the structures of these spiro compounds were elucidated. Syntheses of these compounds span the last fifty years, and so not all of the original articles were published in English, but where relevant, have been translated and summarised here. In general, newer spirocyclisation methods tend to be both more efficient and are conducted enantioselectively. For example, Diels-Alder spirocyclisations of the acoranes were first attempted in 1975 and re-examined in 2013, where one isomer was selectively synthesised rather than the full range of four isomers (Scheme 1).
Scheme 1 -Comparing the synthesis of acoranes by the Diels-Alder reaction.
Reports from the last five years indicate that new spiro natural products are still being isolated and synthesised. Total syntheses of some compounds isolated many years ago were achieved using the less sophisticated methodologies available at the time, but there has been much improvement of these older methods by the application of more modern processing techniques and understanding. It may also be beneficial to apply some of the approaches recently developed in the enantioselective synthesis of chiral ligands to the synthesis of natural products. Several new natural product compounds have been recently identified that would benefit from an efficient preparative synthesis. Cyanthiwigin AC (Figure 3), isolated from the marine sponge Myrmekioderma styx, 14 is such an example. The cyanthiwigins exhibit a broad range of biological activity, including inhibition of human immunodeficiency virus and Mycobacterium tuberculosis, and cytotoxicity against human primary tumour cells. 15 The first and only total synthesis of (+)-cyanthiwigin AC was achieved in 13 steps and 2% overall yield. 16 Other compounds have also shown interesting preliminary biological activity ranging from anti-bacterial to anti-fouling activity but further investigation is hampered by the inability to isolate or synthesise sufficient quantities. The ritterazines are such a class of compounds: they are a family of 26 steroidal alkaloids of which 11 contain a spirocarbocycle in addition to the spiroketal shared by all ritterazines. 17 They possess potent cytotoxic activity against apoptosis-resistant malignant cell lines but research has been limited by the small amounts of material successfully isolated from naturally occurring sources, so efforts have focussed on their total synthesis. There have been several published syntheses of the other 15 ritterazines and derivatives, but at the time of writing there has not yet been a published total synthesis of the spirocarbocycle-containing ritterazines. Taber et al. have come the closest to synthesising ritterazine N (1.20, Scheme 2) and related analogues 18,19 by assembling a ritterazine N precursor.
The final planned aldol cyclocondensation failed, but an alternative route via ozonolysis of an alkene followed by treatment with base successfully gave the related compound bis-18,18'desmethylritterazine N (1.23). 19,20 Scheme 2 -Attempted syntheses of ritterazine N. Bis-18,18'desmethylritterazine N was obtained as a single diastereomer in high yield.
As this review aims to summarise the progress in the field, but also to identify areas for improvement, where natural products have been synthesised by more than one strategy, the methods will be compared. Acorone (1975,1977,1978,1996,2007) [ Agarospirol (1970,1975,1980,1995,2006) [4.5] 2, 3, 6, 7 Table 1 -List of the natural products covered in this review, in order of increasing ring size then alphabetically.

Aldol Reaction
The aldol reaction is a powerful carbon-carbon bond forming reaction. 21 Stereochemical control is possible through the use of chiral aldehyde starting materials, or chiral auxiliaries. The asymmetric reaction can also be performed catalytically through the use of chiral Lewis acids. 22 This approach has been particularly effective in the synthesis of various natural product spirocarbocycles as illustrated below.

Vannusals A and B
Vannusals A and B are triterpenes with an unusual C 30 backbone, comprising seven rings and thirteen chiral centres, three of which are quaternary. They were isolated from the tropical zooplankton Euplotes vannus, and their biosynthesis is thought to proceed from a squalene-type precursor (Figure 4), via several possible routes. 23 The originally assigned structures for the vannusals were incorrect, and the true structures were only established following the total synthesis efforts of the Nicolaou group. 24 The fused polycyclic carbon framework was assembled by an intramolecular spirocyclisation via a Mukaiyama-type aldol reaction 25,26 (Scheme 3).
Scheme 3 -Mukaiyama aldol reaction in the synthesis of vannusal A.
The above methodology was used in the total syntheses of the original incorrect vannusal B structure, 27 as well as the resyntheses of the true vannusals A and B. 24,[28][29][30] 2.2 Acoranes: α-Acoradiene, Acorone, Isoacorone and Acorenone B The acoranes are a group of highly-oxygenated sesquiterpenes containing a spiro[4.5] core. They have been isolated from a variety of sources, [31][32][33] including sweet flag Acorus calamus, the essential oils of the woods of Juniperus rigida and J. chinensis, and Australian sandalwood oil from Santalum spicatum.

Spirovetivanes: Agarospirol, β-Vetivone, α-Vetispirene and Anhydro-β-rotunol
The spirovetivanes are sesquiterpenes with a spiro[4.5] core. They include metabolites and phytoalexins, compounds produced when a species is subjected to stress such as pathogenic attack. 40 Vetiver oil from Vetiveria zizanioides contains several spirovetivanes, including β-vetivone, 4 αvetispirene and β-vetispirene. 41 As stated above, for many years, the structure of β-vetivone was thought to be hydroazulenic but this was revised to the spiro[4.5]decane in 1968. 5,6 Following on from this work by Marshall and Johnson, Wenkert et al. used methanolic KOH to effect a basic aldol cyclisation of a key intermediate in the synthesis of βvetivone. 42 The reaction proceeded in 95% yield, as part of a formal total synthesis requiring 8 steps to reach the preliminary compound.
Scheme 11 -One-pot deacetalisation-Aldol condensation in the synthesis of (-)vitrenal. Intermediate 2.42 was present as a 1:1 mixture of geometrical isomers, which were only separable by gas chromatography. Both isomers gave the same acid (2.43).

Cannabispirans: Cannabispirenones A and B and Cannabispirone
A variety of spiroindane products have been isolated from the marijuana plant, Cannabis sativa. [56][57][58][59][60][61][62] They are structurally related to some synthetic oestrogenic-potentiating agents and so may show similar oestrogenic activity to marijuana. 59 Crombie et al. used KOH to intramolecularly close the spiro ring of several cannabispirans 63,64 (Scheme 12).  Similarly, cannabispirenone B was synthesised by Novak et al. in 1982. 67 Aldehyde 2.51 was transformed to the piperidine enamine, before it was reacted with methyl vinyl ketone followed by hydrolysis and aldol cyclisation. The increased difficulty in the synthesis of cannabispirenone B over A lay in the selective ether cleavage of the less sterically hindered aryl methoxy group. Previously this had only been possible using BBr 3 , which gave low yields and led to the formation of insoluble tars. Ten reagents were considered by the authors, of which only LiI in 2,4,6-trimethylpyridine was successful in 56% conversion and 82% yield.

Chamigrenes: Majusculone and Laurencenone C
The chamigrene family is a group of over 100 sesquiterpene natural products that contain a spiro [5.5]undecane core. Examples include majusculone and laurencenone C. 68,69 Some chamigrenes are halogenated, and the group shows diverse biological activity.
Zhu et al. synthesised racemic laurencenone C in 2010, and the key step was a base-catalysed aldol condensation 70 (Scheme 15). The full sequence involved 11 steps with an overall yield of 17%. The key step in this particular synthesis was the tandem oxidation-aldol condensation of the diol to the keto aldehyde which underwent base-mediated cyclisation. Scheme 16 -Oxidation followed by cyclisation in the synthesis of (±)majusculone.

Acid and Base Promoted Spirocyclisation
Acids or bases can be used in a wider range of cyclisation reactions than just to promote the aldol reaction. For example, the related Robinson annulation and Prins cyclisation have been used to synthesise natural products containing spirocarbocycles as described below.

Acoranes: Acorenone, Acorenone B and α-Acorenol
(±)-Acorenone B has been synthesised from (±)camporone. 72 Formic acid was used to activate the allylic No mention was made of stereochemistry or diastereoselectivity in the original 1975 paper and thus this method may be less effective than the cyclisation in base which was described above.
In 1976, White et al. started from (R)-(+)-limonene to synthesise (-)-acorenone B in 11 steps. 73 The key step was an acid-catalysed ring-opening of a cyclopropane to reveal the spiro[4.5]decanone.  The reaction gave the key spiro intermediate in low yield, with the stereoselectivity of the reaction controlled by the isopropyl and methyl groups, which direct the electrophilic addition of the methyl vinyl ketone to the less hindered face.
More recently, Sakai et al. used a Lewis acid to effect a one-pot transformation to a spiro compound en route to racemic acorenone B. 75 The mechanism is thought to proceed via an aldol condensation, followed by hemiacetalisation and finally a Grob-type fragmentation to realise the intermediate.

Spirovetivanes: Agarospirol, Hinesol, β-Vetivone, α-Vetispirene and β-Vetispirene
Acetal formation followed by reduction and treatment with acid was used to form the molecules hinesol, β-vetivone, and αand β-vetispirenes 77  Of the four possible diastereomers, the two shown in Scheme 23 constituted 97% of the reaction mixture. The mechanism is thought to proceed via a 6-membered ring transition state.
Deslongchamps et al. have synthesised racemic agarospirol and hinesol from a keto ester. 80,81 The spirocycle was formed by treatment of a cyclopropane intermediate with acid (Scheme 24).  Acidic or basic conditions are commonly employed in the key cyclisation step of (±)-hinesol. 85
Racemic solavetivone was synthesised through a Diels-Alder reaction followed by π-cyclisation 85 (Scheme 29) by . The starting material was 3,5dimethylanisole and the procedure required 12 steps to give the product in 3% overall yield. The Murai group used this methodology to synthesise lubimin and oxylubimin in a series of follow-up articles. 94,95 Scheme 29 -Oxalic acid-induced cyclisation in the synthesis of (±)-solavetivone.
Other conditions attempted for the π-cyclisation were unsuccessful, including using SnCl 4 in DCM at -78 °C, acetic acid at 110 °C and formic acid at 50 °C.
Premnaspirodiene and hinesene have both been synthesised

Vitrenal
Takahashi synthesised racemic vitrenal in 12 steps and 7% overall yield from the monoterpene piperitenone 97 in 1982. The key step was an acid promoted acetal hydrolysis and enol ether trapping (Scheme 31).
The first enantioselective synthesis of (+)-majusculone was achieved by alkylidene carbene insertion promoted by KHMDS 102
α-Elimination generated the carbene which inserted into a C-H bond shown in red to give the spirocycle. An alkyne by-product from the 1,2-rearrangement was also observed, shown in blue. Concerns over the ability of the carbene to insert into the deactivated, congested methine were not realised.

Spirocurcasone
Spirocurcasone is a diterpenoid with a novel carbon skeleton, isolated from Jatropha curcas. 103 A high-yielding one-pot semisynthesis of (+)-spirocurcasone from the natural products curcasones A and B has been reported very recently In the absence of a metal, the TMEDA is thought to act as a base. Various conditions were screened, and MeOH was found to give higher conversions of the starting material over toluene or DCM, and the relatively mild temperature of 50 °C was more effective than 90 °C or reflux. Spirocarbocycles can be synthesised using a variety of metal complexes. 105 Ring-closing metathesis 106,107 typically uses a ruthenium-based catalyst such as Grubbs' I or Grubbs' II to react two terminal alkenes intramolecularly to give an alkene and ethene. Kotha

Illudins
The illudins are sesquiterpenoids isolated from fungi, including the highly poisonous Jack-o' lantern mushroom, Omphalotus illudens. [109][110][111] The illudins themselves are extremely toxic. Illudins M and S show potent cytotoxic activity in vitro, but are much less effective in vivo. 112,113 There has been considerable work towards developing anti-cancer derivatives, which are now in clinical trials. These include bicyclic and isomeric analogues of illudin M, 114 acylfulvene and hydroxymethylacylfulvene 115,116 (Figure 5). The latter is now in Phase II clinical trials against ovarian, prostate, and gastrointestinal cancers. 117  With Grubbs' II catalyst, conversion to the desired product was 90% by 1 H NMR spectroscopy, whereas with Grubbs' I catalyst the conversion was only 25% to an unwanted side-product that contained a five-membered ring.
Scheme 37-Enyne ring-closing metathesis in the synthesis of the illudin core to give an unwanted side-product.
Illudins M and S have also been previously synthesised by Matsumoto et al., using a basic aldol ring-closing reaction. 119,120
The stereochemistry of the product formed did not match the stereochemistry of the broad-horned beetle pheromone, thus disproving the originally proposed structure and identifying the pheromone as (1S,4R,5R)-α-acoradiene. 38

Spiroaxanes: Axenol and Gleenol
Axenol and gleenol are enantiomeric sesquiterpenes sharing the spiroaxane skeleton. They have both been isolated from the Eurypon marine sponges, but (-)-gleenol has also been found in a variety of coniferous trees, including Picea glehnii, P. koraiensis, Criptomeria japonica and Juniperus oxycedrus. Axenol has not been found to exhibit biological activity, but (+)-gleenol possesses termiticidal and anti-helmintic activities, as well as regulating the growth of plant seeds.
(+)-Axenol and (-)-gleenol have been synthesised by Spitzner et al., using an organometallic domino reaction, with an unusual combination of methylenation followed by ringclosing metathesis 125,126 (Scheme 41). The first step in the synthesis was a Michael addition with methyl vinyl ketone, 127 followed by selective methylenation with the Kauffmann molybdenum complex which was generated in situ from MoOCl 3 (THF) 2 . 128 A mixture of inseparable isomers was formed in the ratio 2:1, of which the major (R)-isomer (4. 22) was the desired product. Grubbs' I catalyst was added to the crude reaction mixture in a one-pot reaction to give epimers of the spiroketone 4.23.
A McMurry coupling was also directly attempted on the tricarbonyl, but the spiroketone product was not observed by GC-MS analysis.
Kobayashi et al. synthesised (-)-axenol in 2015 using ringclosing metathesis. 129 The key stereochemistry of the quaternary carbon in the intermediate was built using allylic substitution starting from a picolinate derived from menthol. The TBS-protected alcohol was also considered, but gave lower conversions of substrate.

Spirocurcasone
The total synthesis of (-)-spirocurcasone was achieved by ring-closing metathesis of a pentaene 130 by Ito et al. in 2013 (Scheme 43). No protecting groups were required in the 9-step procedure starting from (R)-perillaldehyde.

Sequosempervirin A
Sequosempervirin A is a novel spiro-norlignan compound isolated in 2004 from the branches and leaves of the California redwood Sequoia sempervirens. 131 Three years later, the first total synthesis of sequosempervirin A used ring-closing

Chamigrenes: α-Chamigrene, β-Chamigrene, Laurencenones B and C and Elatol
Other members of the chamigrene natural product family not previously mentioned include β-chamigrene, laurencenone B and elatol. The potent and varied biological activity of elatol makes it a compound of significant interest. It has shown antibiofouling activity, 133 anti-bacterial activity, [134][135][136] anti-fungal activity 137 and cytotoxicity. 138 It was isolated from red algae Laurencia sp., as were laurencenones B and C. Like αchamigrene, β-chamigrene has been isolated from Schisandra chinensis, although it was first isolated from the leaf oil of the cypress Chamaecyparis taiwanensis. 139 (±)-α-Chamigrene was synthesised from cyclohexenecarboxylic acid 140 by Srikrishna et al. in 2006. The procedure used an Ireland-Claisen rearrangement and ringclosing metathesis as previously demonstrated by the same group in the synthesis of (±)-isoacorone to give the product in 11 steps and 32% overall yield (Scheme 45). Racemic β-chamigrene and laurencenone C have been synthesised by an Ireland-Claisen rearrangement, followed by either ring-closing metathesis 140 or an intramolecular type II carbonyl ene reaction 141 by Srikrishna et al.. The metathesis method required 11 steps to give the product in 32% overall yield starting from a cyclohexenecarboxylic acid. For laurencenone C, the initial method required 10 steps to give a 17% overall yield.
This catalytic asymmetric method was generalised to the chamigrene product family, using a palladium-based enantioselective decarboxylative allylation followed by ringclosing metathesis. 143

Diels-Alder Reaction
The Diels-Alder reaction is a valuable tool in total synthesis, 144 since it forms two carbon-carbon bonds and up to four stereocentres. Its application to the synthesis of spirocarbocycle-containing natural products is described below.

Acoranes: Colletoic acid, β-, γ-, δ-Acoradienes, Acorone and Isoacorone
For the avoidance of confusion, it should be noted that γ-and δacoradiene are also known as α-and β-alaskene respectively, due to their isolation from the Alaskan cedar, Chamaecyparis nootkatensis. 145,146 Retrosynthetic analysis of the acorane carbon skeleton reveals that the cyclohexene can be broken into two units, of which one is isoprene ( Figure 6). γ-and δ-Acoradiene were synthesised by Marx et al. as early as 1973 147 using a method which was extended to βacoradiene, acorone, and isoacorone two years later 148 (Scheme 47). The starting material was (R)-pulegone, and a SnCl 4catalysed Diels-Alder reaction was used. The major products were those formed from the electronically favourable para orientation of the reagents, whilst the minor products were formed from the meta orientation. The Lewis acid increased selectivity for the products of para cycloaddition. The reaction had been performed without SnCl 4 , with heating to 100 °C and gave the same products, in the ratio 45:25:20:10. The transition state of the major isomer was formed by attack of the isoprene unit on the opposite face to the methyl group, reducing unfavourable steric interactions, and eventually leading to the formation of γ-acoradiene. The second isomer was formed by attack of the isoprene on the same face as the methyl group, which was then through further functionalisation converted to δ-acoradiene.
(+)-Colletoic acid is a sesquiterpenoid also from the acorane family and a potent inhibitor of the 11β-hydroxysteroid type 1 enzyme, making it a therapeutic target for metabolic syndrome. 149 This compound was generated as a single enantiomer in 2013 by a diastereoselective intramolecular 5exo-Heck reaction. 150 At the same time, Nakada et al. used a stereoselective Lewis-acid catalysed Diels-Alder reaction with isoprene to create the spirocentre 151,152 (Scheme 48). It was found that the starting material for the Diels-Alder reaction decomposed at high temperatures, but when run at -20 °C in the presence of the Lewis acid AlCl 3 , the reaction gave the desired product as a single isomer.

Ainsliadimers A and B and Gochnatiolides A-C
The ainsliadimers and gochnatiolides are a related set of dimeric sesquiterpene lactones, biosynthesised from the guaianolide dehydrozaluzanin C.
During an investigation into hydrogen bond donor catalysis, hydrogen-bonding solvents were used but only chloroform gave a trace amount of the dimer when the monomer was heated at 35 °C for 12 h. Additives including triethylamine or hydrochloric acid did not improve conversion, and longer reaction times and higher temperatures led to increased decomposition. Hydrogen bond donor catalysts were studied, and racemic BINOL was found to be particularly effective at catalysing the dimerisation. More recently, Qin et al. reported the total syntheses of ainsliadimer B and gochnatiolides A and B which proceeded in 25 steps and 1% overall yield using a cross Diels-Alder cycloaddition 154 (Scheme 50). The Diels-Alder reaction could proceed via either an endo or exo transition state, but only the product arising from the endo transition state was observed. The exo transition state would experience a high degree of steric interaction between the silyl groups on the bottom face of the diene and the seven-membered ring of the dehydrozazulin C. When attempted in a variety of solvents, the reaction proceeded slowly to give the Diels-Alder product in less than 15% yield and so the reaction was run neat. Addition of acids such as ceric ammonium sulfate, pyridinium p-toluenesulfonate, tosic acid and acetic acid failed to catalyse the reaction, instead causing the decomposition of the dehydrozaluzanin C product.

Claisen Rearrangement
The Claisen rearrangement is a [3,3]-sigmatropic rearrangement where an allyl vinyl ether is converted to an unsaturated carbonyl. 155,156 Its use in the synthesis of spiro sesquiterpenoids has been recently reviewed by Kobayashi et al.. 157

Spiroaxanes: Axenol, Axisonitrile-3 and Gleenol
Axisonitrile-3 is a nitrogenous sesquiterpenoid of interest due to its significant biological activity, including anti-fouling activity against barnacle larvae, 162 cytotoxicity 163 and potent anti-malarial activity. 164,165 It possesses an unusual isonitrile group. Axenol is a key intermediate in its synthesis.
Racemic axenol and gleenol have been synthesised by the Kobayashi group in 2007, 166,167 applying the same methodology as for the synthesis of other spiro[4.5]decanes described above.

Erythrodiene and Spirojatamol
Eilbracht et al. reported a formal synthesis of erythrodiene and spirojatamol which used a rhodium-based catalyst for a tandem Claisen rearrangement-hydroacylation of an allyl vinyl ether 169 (Scheme 54). This method was also applied in 1996 for the synthesis of acoradienes and spirovetivanes as they share the spiro[4.5]decanone skeleton. 170 Scheme 54 -Rhodium-catalysed Claisen rearrangement in the synthesis of erythrodiene.
The acoradiene and spirovetivane procedure was performed in one pot, 171 and the particular rhodium catalyst chosen was selected because it did not decompose at the required higher temperatures and thus could catalyse the Claisen and hydroacylation reactions. 172

Photochemical Reaction
Photochemistry is a powerful tool used in total synthesis to access products that can be otherwise difficult to form. Its use in natural product synthesis has been extensively reviewed. [173][174][175]

Acutumine
Acutumine is a tetracyclic alkaloid which shows selective T-cell cytotoxicity and anti-amnesic properties. 176 Castle et al. reported the first total synthesis of (-)acutumine in 2009, which utilised a photochemical radicalpolar crossover reaction to form the spirocycle 177 (Scheme 55).
The cascade process was observed at 0 °C with a sun-lamp. Optimisation experiments identified Et 3 Al as a more effective promoter than Et 3 B or Et 2 Zn, and 3-phenyl-2-(phenylsulfonyl)oxaziridine as a better hydroxylating agent than oxygen, DMDO or t BuOOH. Side-products were observed, with the alcohol group replaced by an iodide or hydrogen. Attempts were made to convert the iodide into the desired product by treatment with diethyl zinc, oxygen and the oxaziridine gave the desired product in 62% yield, thus increasing the overall yield to 66%. Samarium diiodide/HMPA was considered as a replacement for (Bu 3 Sn) 2 , but only gave a low yield of the desired product, and led to the formation of many side-products. This was attributed to the presence of functional groups in the starting material that could react with SmI 2 . The mechanism still requires further work to develop a better understanding.
Reisman et al. have attempted the synthesis of (-)acutumine via a photochemical [2+2]-cycloaddition, but were only successful in constructing the aza-propellane core. 178   The key photochemical step was highly regio-and stereospecific, with the furanone reacting only in a head-to-tail manner. (-)-Acorenone was synthesised in a total of 13 steps in approximately 8% overall yield.
Irradiation of an enone was also used as the key step by Oppolzer et al. in 1983 in the synthesis of racemic αacoradiene 182 (Scheme 58).
Scheme 58 -Photochemical cyclobutane formation followed by reductive fragmentation in the synthesis of racemic α-acoradiene. The configurations of 7.13 and 7.14 were not determined because this chirality was lost by reduction.
The low yield was ascribed to the difficulty in separating the various isomers produced by the radical reaction. Formation of the diradical led to a loss of stereochemical information, and resulted in the formation of two enantiomers, of which reductive fragmentation of only one gave the desired product.
The reaction proceeded via a Dowd-Beckwith type ring expansion to give the spirocyclic intermediate. Takeshita et al. used photocycloaddition followed by a base-catalysed benzilic acid rearrangement in 1995 to synthesise racemic hinesol and agarospirol 189 (Scheme 60).
Scheme 60 -Photochemical reaction in the synthesis of racemic hinesol and agarospirol.
As the rearrangement was not triggered using triethylamine as a base, it was postulated that formation of a sodium chelated intermediate could assist in the rearrangement.
(-)-β-Vetivone has been synthesised by a dienone ring contraction of a spiro starting material with irradiation in acid to form a spiroketone. 45   Glacial rather than aqueous acetic acid gave a dramatic improvement in yield (28% to 89%), and a reduction in sideproduct formation. The absence of water prevented hydrolysis of the enol ether.

Proaporphines
The proaporphines are a major group of isoquinoline alkaloids. They are biosynthetic precursors to aporphine alkaloids which exhibit a broad range of biological activity.
Photolysis has been used by the Iwata and Fukumoto groups. to synthesise pronuciferine, 191,192 orientalinone, 193 Omethyl-orientalinone, 194 and O-methyl-kreysiginone 195  Pronuciferine has been synthesised using methyl vinyl ketone in base, 196 and more recently pronuciferine and stepharine have been synthesised by aromatic oxidation with a hypervalent iodine reagent. 197 This led to the unprecedented C-C bond formation between the para position of a phenol and an enamide carbon.
Irradiation of the neat starting material as a solid state thin-film produced a mixture of products, including linderaspirone A, bilinderone, epi-bilinderone and an additional side product containing an unusual spiro[3.4] system. Photolysis in various solvents, including methanol, acetone, THF, acetonitrile, toluene and benzene all gave yields of linderaspirone A and bilinderone in less than 20%.
In 2013, Hu et al. synthesised linderaspirone A in 3 steps as well as bilinderone using a Darzens-type cyclopentadione reaction. 201 The starting material was the same as was used above. When performed under oxygen rather than argon, the yield of linderaspirone A increased from 38% to 50%.

Conclusions
Spirocarbocycles are important structural motifs found in drugs, ligands and natural products, and so strategies towards their challenging synthesis are key. This review has examined several common reactions used to form these quaternary centres, including the use of acid and base, particularly in the aldol reaction. Ring-closing metathesis, the Diels-Alder reaction, Claisen rearrangements and photochemistry have also been used to synthesise these spirocarbocycles.
As seen above, the aldol reaction has been successful on a wide range of carbon skeletons, although it lacks stereoselectivity unless the substrate already contains the desired chirality. High yields are achievable with suitable starting materials but if a mixture of products are obtained, the efficiency decreases. This is similarly true for those reactions effected using acid or base, but since these do not operate under one mechanism, it is difficult to generalise. Stereoselectivity depends on both the reagents and the reaction conditions: for example, the possibility of a 6-membered chair transition state can increase selectivity. It is well-known that ring-closing metathesis typically proceeds in excellent yields and, like the Claisen rearrangement, is highly atom efficient. This also applies to the Diels-Alder reaction, which can be chemo-, regio-, diastereo-and enantioselective. It is important to note, however, that a substrate's suitability for these methods depends on its carbon skeleton and structure. Finally, photochemical reactions are often not the obvious first choice for many organic chemists, and as shown above, their use is again highly substrate-dependent. It can be possible to get complex mixtures of products, but on occasion high yields are achievable, as shown by several examples above.
More work is required to develop new and improved routes to these compounds and others in the literature that are yet to be synthesised. As highlighted above, some natural products with biological applications still require syntheses. Examples of compounds discovered in the last ten years with biological activity which are yet to be synthesised include biyouyanagiol, 202 spiromamakone A, 203 and dibelamcandal A 204 (Figure 7). Other natural products would benefit from more efficient routes, for example by the application of modern techniques to improve selectivity in older methods. Whilst progress over the last fifty years has been substantial, there is potential for further synthesis and exploitation of natural products containing spirocarbocycles.