A unified strategy to prostaglandins: chemoenzymatic total synthesis of cloprostenol, bimatoprost, PGF_2α, fluprostenol, and travoprost guided by biocatalytic retrosynthesis

Abstract

Development of efficient and stereoselective synthesis of prostaglandins (PGs) is of utmost importance, owing to their valuable medicinal applications and unique chemical structures. We report here a unified synthesis of PGs cloprostenol, bimatoprost, PGF_2α, fluprostenol, and travoprost from the readily available dichloro-containing bicyclic ketone 6a guided by biocatalytic retrosynthesis, in 11–12 steps with 3.8–8.4% overall yields. An unprecedented Baeyer–Villiger monooxygenase (BVMO)-catalyzed stereoselective oxidation of 6a (99% ee), and a ketoreductase (KRED)-catalyzed diastereoselective reduction of enones 12 (87 : 13 to 99 : 1 dr) were utilized in combination for the first time to set the critical stereochemical configurations under mild conditions. Another key transformation was the copper(II)-catalyzed regioselective p-phenylbenzoylation of the secondary alcohol of diol 10 (9.3 : 1 rr). This study not only provides an alternative route to the highly stereoselective synthesis of PGs, but also showcases the usefulness and great potential of biocatalysis in construction of complex molecules.

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Introduction

Prostaglandins (PGs) are hormone-like lipid compounds often found in animals and human-beings,¹ and they are shown to display a multitude of biological functions. To date, more than 20 PG analogs have been developed as marketed medicines, such as the veterinary drugs cloprostenol (1) and fluprostenol (4), and antiglaucoma drugs bimatoprost (2) and travoprost (5) (Fig. 1).^1c In particular, bimatoprost has recently become a block-buster drug. Because of their valuable medicinal applications and unique chemical structures, tremendous efforts have been devoted to the efficient synthesis of PGs.^1,2 In fact, since Corey's landmark synthesis of prostaglandin F_2α (PGF_2α, 3) in the late 1960s,³ PGs have become one touchstone of state-of-the-art synthetic methodologies,⁴ as exemplified in the elegant synthesis recently reported by the groups of Aggarwal,^2,4a–c Hayashi,^4d–h Grubbs,⁴ⁱ Fletcher,^4j Nicolaou^4k,l and Baran.^4m Our group has developed an efficient and modular synthesis of PGs (Scheme S1†).⁵ Crucial to our success was the stereocontrolled synthesis of the key lactone intermediate 7avia a chiral spiro-phosphoric acid-catalyzed Baeyer–Villiger (B–V) oxidation of bicyclic ketone 6a (Scheme S1†).

Fig. 1 Structure of selected prostaglandins: cloprostenol (1), bimatoprost (2), PGF_2α (3), fluprostenol (4), and travoprost (5).

The past two decades have witnessed the rapid development of biocatalysis into a sophisticated technology, mainly thanks to the ever-increasing bioinformatics and protein engineering tools.⁶ Compared to chemocatalysis, biocatalysis usually offers unparalleled chemo-, regio-, and stereoselectivity pivotal for efficient synthesis of complex molecules like pharmaceuticals and bioactive natural products.^6e,f,7 To harness the synthetic potential of these extraordinary selectivities and other advantageous properties inherent to biocatalysis (e.g. mild reaction conditions) more productively, Turner introduced the concept of biocatalytic retrosynthesis to the scientific community about seven years ago,^6a which has since been widely and increasingly utilized when planning synthetic routes to target molecules.⁸ From a retrosynthetic viewpoint, a biocatalytic reaction can be adopted in two distinct ways: either being used as an alternative approach to replace an equivalent chemical step, or being employed to fulfill a disconnection which is impossible to realize by traditional chemical means.^6a As an example in the former scenario, ketoreductase (KRED)-catalyzed stereoselective reduction was demonstrated to be a milder and more sustainable process compared to (−)-DIP-Cl-mediated reduction, in the synthesis of a key chiral alcohol intermediate to the active pharmaceutical ingredient (API) montelukast.⁹ So far, the latter scenario has been less exploited.^6a Recently, a regio- and stereoselective transformation from prochiral ketoenones to 2,6-disubstituted piperidines, a formidable task for classical chemical methods, was accomplished via an ω-transaminase triggered intramolecular aza-Michael reaction and subsequent epimerization.^8a In continuation of our interest in the development of efficient, stereoselective, and green synthesis of PGs, as well as application of biocatalysis to organic synthesis,¹⁰ herein we report a tractable, chemoenzymatic total synthesis of prostaglandins guided by biocatalytic retrosynthesis.

Results and discussion

In our biocatalytic retrosynthesis (Fig. 2), cloprostenol (1), bimatoprost (2), PGF_2α (3), and fluprostenol (4), prostaglandins containing an allylic alcohol moiety on the ω-side-chain, could be synthesized from lactones 13a–13d through a three-step sequence of p-phenylbenzoyl (PPB) ester hydrolysis, DIBAL-H reduction, and Wittig olefination. We envisioned that the crucial stereogenic center of the ω-side-chain of 13a–13d could be installed via a KRED-catalyzed diastereoselective reduction of enones 12a–12d, which were prepared from C₁₁-OH-PPB-protected Corey alcohol 11 (prostaglandin numbering) through primary alcohol oxidation, followed by Horner–Wadsworth–Emmons (HWE) olefination. It was hoped that 11 might be furnished via a catalyst-controlled regioselective p-phenylbenzoylation of the C₁₁-OH of the known diol 10,¹¹ which was synthesized from lactone 7avia a three-step sequence of dechlorination, the Prins reaction, and deformylation. At this stage, the key lactone 7a was envisioned to be accessible through a Baeyer–Villiger monooxygenase (BVMO)-catalyzed stereoselective oxidation of bicyclic ketone 6a. Finally, travoprost (5) can be synthesized from fluprostenol (4) via a late-stage i-propyl ester formation reaction.^4b

Fig. 2 Biocatalytic retrosynthesis of prostaglandins 1–4.

Bicyclo[3.2.0]hept-2-en-6-one (14), a bicyclic ketone lacking the dichloro functionality in comparison to 6a, was routinely employed as the model substrate in BVMO-catalyzed oxidation reactions (Scheme S2†).¹² A regiodivergent conversion of 14 has been observed for most BVMOs, resulting in the formation of the enantioenriched normal lactone (NL) 15 and abnormal lactone (AL) 16 in similar amounts (Scheme S2†). Under such circumstances, the separation of 15 and 16 is troublesome. On the other hand, our previous study on chiral spiro-phosphoric acid-catalyzed B–V oxidation suggested that it was beneficial to introduce the dichloro functionality into the cyclobutanone ring of 6a. Firstly, formation of NL 7a becomes more favored because of the electron-withdrawing effect of the dichloro group (Scheme S1†). Secondly, the formed AL 8 could be readily converted to the easily removable cyclopentene dicarboxylic acid 9 during the reaction workup, thus facilitating the isolation of the desired 7a (Scheme S1†). Therefore, we were interested in the development of BVMO-catalyzed oxidation of dichloro-containing bicyclic ketone 6a to lactone 7a, which has not been reported, to the best of our knowledge. In the present study, three “regiodivergent” BVMOs, namely CHMO_Rhodo1,¹³ CHMO_Arthro,¹³ and CHMO_Brevi1,¹⁴ as well as the unique “regioselective” enzyme BVMO-MO14 originating from Rhodococcus jostii,¹⁵ are examined in the oxidation of 6a. On one hand, moderate to high conversion of 6a and formation of trace-amounts of the desired 7a (≤5%) were observed in the CHMO_Arthro-, CHMO_Brevi1-, and BVMO-MO14-catalyzed reactions (Table S2†). On the other hand, 6a was completely consumed in the CHMO_Rhodo1-catalyzed reaction, but the yield of 7a was only 25% (Table 1, entry 1, also see Table S2†), much lower than the 50% maximum theoretical yield for a resolution event. To account for this mass imbalance, we investigated the stability of 6a and racemic-7a. In the absence of BVMO, it was found that 6a and racemic-7a decomposed readily, with only 58% and 66% remaining, respectively, after incubation for 90 minutes (Fig. S5†). Presumably, the electron-withdrawing dichloro group makes the carbonyl more electron-deficient, hence rendering 6a and 7a more susceptible to hydrolysis as previously suggested.¹⁶ To improve the yield of 7a, CHMO_Rhodo1-catalyzed oxidation reactions were carried out at different pHs (Table 1, entries 2 and 3) or at different temperatures (Table 1, entries 4 and 5). No significant improvement was achieved. Next, five organic solvents were screened in an effort to increase the product yield (Table 1, entries 6–10). To our delight, the use of methyl tert-butyl ether (MTBE) and 2-methoxyethanol (MME) significantly increased the yield of 7a to 38% and 36%, respectively. Not only the yield, but also the enantiomeric purity of 7a was revealed to be dependent on the co-solvents employed. While the use of MTBE resulted in an inferior enantiomeric purity (73% ee), no adverse effect was observed when using MME. Hence, the latter solvent was chosen for the remaining study. Gratifyingly, a semi-preparative-scale (0.5 mmol) oxidation of 6a under the above optimized conditions furnished the desired 7a in 38% isolated yield with 99% ee (Table 1, entry 11). Hence, we have realized the BVMO-catalyzed oxidation of 6a, which is prone to hydrolysis, to normal lactone 7a under buffer conditions in good yield and excellent stereoselectivity for the first time. The chemical structure and the stereochemical assignment of 7a were unambiguously established by X-ray crystallography (Fig. 3). Cyclopentene dicarboxylic acid 9, resulting from the hydrolysis of abnormal lactone 8, was isolated in 35% yield with 82% ee (determined upon derivatization, see Scheme S6† for details).

Table 1 CHMO_Rhodo1-catalyzed oxidation of 6a^a

Entry	Co-solvent	pH	6a remaining^b (%)	Yield of 7a^b (%)	ee of 7a^c (%)
a Reaction conditions (1 mL): 6a (10 mM), glucose (60 mM), NADP^+ (0.2 mM), FAD (0.05 mM), 0.79 mL of 20% (w/v) cell-free extract (CFE) of CHMORhodo1 in NaPi buffer (50 mM), 0.016 mL of 15% (w/v) CFE of glucose dehydrogenase (GDH) in NaPi buffer (50 mM, pH 7.0), and 0.1 mL co-solvent (if applicable) in NaPi buffer (50 mM). Reaction mixtures were incubated at 25 °C with 200 rpm shaking for 90 min. b Determined by GC analysis using undecane as the internal standard. c Determined by SFC analysis. d Not applicable (N.A.). e Not determined (N.D.). f The reaction temperature was 30 °C. g The reaction temperature was 35 °C. h The reaction volume was 50 mL (0.5 mmol scale). i Isolated yield.
1	N.A.^d	7.5	0	25	99
2	N.A.	7.0	0	26	N.D.^e
3	N.A.	6.5	6	26	N.D.
4^f	N.A.	7.5	0	22	N.D.
5^g	N.A.	7.5	0	22	N.D.
6	Cyclohexane	7.5	23	23	N.D.
7	DMSO	7.5	0	28	N.D.
8	MTBE	7.5	0	38	73
9	Dioxane	7.5	0	32	82
10	MME	7.5	0	36	99
11^h	MME	7.5	0	40 (38^i)	99

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Fig. 3 Substrate scope of CHMO_Rhodo1-catalyzed B–V oxidation of bicyclic cyclobutanones.

Other bicyclic cyclobutanones were employed as substrates to evaluate the feasibility of the CHMO_Rhodo1-catalyzed oxidation reaction (Fig. 3). Firstly, ketone 6b without an olefin moiety was well recognized by CHMO_Rhodo1, furnishing the desired normal lactone 7b in 30% isolated yield with 99% ee. Secondly, the size of the fused ring seemed to play an important role in this enzyme-catalyzed B–V oxidation process. For instance, the oxidation of substrates containing a 5-membered or 6-membered fused ring (6b–6e) occurred smoothly, affording the corresponding lactones 7b–7e in 22–34% isolated yield with 91–99% ee. However, ketone 6f with an 8-membered fused ring could not be converted by CHMO_Rhodo1 and no desired lactone 7f was detected. It is possible that the overlarge size of this fused ring prevents a proper binding of 6f to CHMO_Rhodo1.

The transformation of 7a into the known diol 10 was realized via a three-step sequence in flow chemistry, which was significantly more time-economical compared to batch reactions (Fig. 4).⁵ Firstly, a continuous flow dechlorination of 7a was accomplished in a packed bed reactor (Shenzhen E-Zheng Tech Co., Ltd) filled with zinc powder at 70 °C under 7 bar back-pressure with 10 min residence time, giving NL 15 in 90% yield. Then NL 15 was dissolved in a 10 : 1 HCOOH/H₂SO₄ solution containing pre-dissolved paraformaldehyde, and the mixture was pumped into a 0.5 mL PTFE reactor coil (i.d. = 0.8 mm) at 70 °C under 17 bar back-pressure with 15 min residence time to afford crude 17 as the major product with full conversion by the Prins reaction. After neutralization and removing the inorganic salt, the crude 17 in MeOH combined with NaOMe was pumped into a 0.5 mL PTFE reactor coil via a T-junction to complete deformylation followed by quenching with AcOH to give diol 10 smoothly in 81% yield over two steps (Fig. 4).

Fig. 4 Conversion of lactone 7a into diol 10 in continuous flow.

With access to diol 10, the site-selective protection of C₁₁-OH could now commence. In the traditional total synthesis of PGs, the three-step choreography of protecting group manipulations of diol 10 would establish a C₁₁-OH-protected Corey alcohol motif present within PGs, aiding purification and imparting chemoselectivity in subsequent transformations.¹⁷ However, this strategy is not step-economic for PG synthesis. To circumvent this limitation, we recently developed a directly regioselective oxidation of the primary alcohol of 10 by using a modified TEMPO/PhI(OAc)₂ protocol to C₁₁-OH non-masked Corey aldehyde for joining the ω-side-chain via Horner–Wadsworth–Emmons (HWE) olefination.^5,18 While seemingly straightforward, successful implementation of our strategy would not be attractive and practical in the large-scale synthesis of prostaglandins, because most of the oil prostaglandin intermediates obtained in this protecting-group-free chemistry are unstable. Therefore, designing a one-step method for catalyst-controlled site-selective p-phenylbenzoylation of C₁₁-OH in 10 would represent a valuable goal that promises streamlined access to the stable and nicely crystalline C₁₁-OH-PPB-protected Corey alcohol 11 (Fig. 2).¹⁹

With this assumption in mind, the p-phenylbenzoylation of diol 10 was first conducted under typical acylation conditions as reported by the Dong group.²⁰ As summarized in Table 2, monoesters 11 and 18 were both formed in 8% and 16% yields, respectively (entry 1). When the reaction temperature was lowered to −20 °C, only a trace amount of 18 (<5%) was obtained (entry 2). The use of copper salt and appropriate ligands/additives was previously demonstrated to promote regioselective acylation of substrates containing multiple hydroxyl groups.¹¹ In the present study, we found that CuCl₂ alone could accelerate the reaction, and importantly, the desired 11 was afforded in 48% yield as the major product (entry 3). To further improve the regioselectivity, several additives were tested. When chiral bisoxazoline compounds Ad1 and Ad2, the two commonly used ligands/additives previously in regioselective acylation reactions,¹¹ were examined first, the undesired monoester 18 was predominantly generated (entries 4 and 5). To our delight, the use of additives Ad3–Ad6 containing a pyridine moiety favoured the formation of 11 (entries 6–9). In particular, the ratio of 11 to 18 was improved to 4.3 : 1 when Ad4 was attempted (entry 7, also see Fig. S6 and S7†). Encouraged by these results, we further designed and synthesized compounds Ad7–Ad12 based on the structure of Ad4. Upon screening (entries 10–15), a regioisomeric ratio of 6.4 : 1 was accomplished by using compound Ad9 containing a fluorene moiety as the additive (entry 12). Optimization of reaction conditions, including temperature, time, and the amount of DIPEA, further increased the regioisomeric ratio to 9.3 : 1, furnishing the desired monoester 11 in 73% isolated yield (entry 17). Taken together, by using a cheap copper(II) salt and an achiral additive Ad9, the regioselective p-phenylbenzoylation of diol 10 to the desired monoester 11 was realized in good yield and regioisomeric ratio. We believe that this regioselective p-phenylbenzoylation reaction will find wide application in the synthesis of prostaglandins. The detailed mechanism of this copper(II)-catalyzed regioselective acylation reaction warrants investigation in the future.

Table 2 Regioselective p-phenylbenzoylation of diol 10^a

Entry	Metal	Additive	Yield of 11^b (%)	Yield of 18^b (%)	Yield of 19^b (%)	Regioisomeric ratio (rr), 11 : 18
a Reaction conditions (1.5 mL): 10 (0.1 mmol), PPBCl (0.1 mmol), DIPEA (0.1 mmol), CuCl2 (0.1 equiv., if applicable), additive (0.1 equiv., if applicable) in MeCN (1 mL) and DCM (0.5 mL). Reaction mixtures were stirred at −20 °C for 14 h. b Determined by ^1H NMR analysis of the reaction crude using 1,3,5-trimethoxybenzene as the internal standard. c Reaction run at room temperature. d Not applicable (N.A.). e Stirred at −10 °C for 24 h. f Reaction scale (1 mmol), DIPEA (1.2 equiv.), stirred at −10 °C for 60 h. g Isolated yield.
1^c	N.A.^d	N.A.	8	16	1	1 : 2
2	N.A.	N.A.	0	<5	0	N.A.
3	CuCl2	N.A.	48	18	4	2.7 : 1
4	CuCl2	Ad1	13	63	0	1 : 4.8
5	CuCl2	Ad2	32	45	3	1 : 1.4
6	CuCl2	Ad3	54	14	2	4.0 : 1
7	CuCl2	Ad4	57	13	2	4.3 : 1
8	CuCl2	Ad5	40	18	2	2.2 : 1
9	CuCl2	Ad6	57	20	2	2.9 : 1
10	CuCl2	Ad7	59	19	2	3.0 : 1
11	CuCl2	Ad8	55	15	3	3.6 : 1
12	CuCl2	Ad9	58	9	2	6.4 : 1
13	CuCl2	Ad10	49	18	3	2.8 : 1
14	CuCl2	Ad11	52	22	2	2.4 : 1
15	CuCl2	Ad12	50	17	2	3.0 : 1
16^e	CuCl2	Ad9	66	8	2	8.1 : 1
17^f	CuCl2	Ad9	83 (73^g)	9	2	9.3 : 1

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With the key C₁₁-OH-PPB-protected Corey alcohol 11 in hand, we then turned our attention to efficiently install the ω-side-chain of PGs (Fig. 5). Oxidation of 11 with TEMPO and trichloroisocyanuric acid (TCCA) in ethyl acetate and dimethyl carbonate at 0 °C provided the desired crude C₁₁-OH-PPB-protected Corey aldehyde^20,21 and set the stage for the C₁₃–C₁₄ HWE olefination. Addition of the crude aldehyde 20 to the solutions of phosphonates 21a–21d in dichloromethane in the presence of 30% aq. NaOH at 0 °C afforded the corresponding enones 12a–12d in good isolated yields (71–75% over two steps) as a single geometric isomer with an (E)-configuration at the newly formed C₁₃–C₁₄ double bond judged by the ¹H NMR spectral analysis.²²

Fig. 5 Completion of the chemoenzymatic total synthesis of cloprostenol (1), bimatoprost (2), PGF_2α (3), fluprostenol (4), and travoprost (5).

Stereoselective reduction of the keto functionality of the prostaglandin's ω-side-chain is dominated by chemical approaches, with biocatalytic reduction being rarely exploited. Recently, the Romano group reported an efficient and highly diastereoselective synthesis of a key allylic alcohol-containing intermediate to bimatoprost via the yeast Pichia anomala-mediated reduction.²³ However, careful optimization of the biotransformation conditions was necessary in order to suppress the competing reduction of the carbon–carbon double bond by the enoate reductase present in the same yeast. To alleviate this inconvenience, use of KREDs recombinantly over-expressed in E. coli should be a viable option. Due to their excellent stereoselectivity, broad substrate spectrum, good stability, and high volumetric productivity, such over-expressed KREDs either in isolated form (purified enzyme or crude-cell lysate) or in whole-cell form, have been widely adopted in the synthesis of pharmaceuticals and bioactive molecules.^9,10b,24 The Pietruszka group has reported an elegant chemoenzymatic total synthesis of travoprost.²⁵ Ketoreductase was employed to stereoselectively prepare the chiral alcohol containing the ω-side-chain, which was then attached to the cyclopentenone ring via a three-component coupling strategy. Recombinantly over-expressed KRED-catalyzed stereoselective reduction of pre-assembled enone intermediates to prostaglandins, such as enones 12, has not been reported, to the best of our knowledge.

To this end, we examined the bioreduction of 12a using a small library of in-house preserved KREDs (Table S3†). Out of 15 enzymes tested, ten could not transform 12a at all. The conversion of the rest five enzymatic reactions was in the range of 4–43%, with ChKRED20 giving the best performance (Table 3, entry 1, also see Table S3†).²⁶ It was noticed that the low conversion of 12a was to some extent due to its apparently poor solubility under current reaction conditions, which probably arises from its hydrophobic structure. To help solubilize 12a and improve the conversion, reactions with a larger amount of the co-solvent DMSO were attempted. Indeed, the conversions of 12a were increased to 58% and 71% (Table 3, entries 2 and 3), respectively, when 5% and 10% of DMSO were used. Although using 20% of DMSO further improved the conversion to 83%, an inferior diastereoselectivity (93.9 : 6.1 dr) was obtained (Table 3, entry 4). On the other hand, decrease of the substrate concentration from 10 mM to 5 mM boosted the reaction conversion to 90%, and importantly the excellent diastereoselectivity (97.9 : 2.1 dr) was retained (Table 3, entry 5). We believe that efficient reduction of the higher concentration of 12a would become possible by optimization of the reaction and protein engineering in the future.^6d,27 Pleasingly, ChKRED20-catalyzed reduction of 12a under the optimized conditions at a semi-preparative-scale occurred smoothly, delivering the desired allylic alcohol 13a in 91% isolated yield with 97.9 : 2.1 dr (Table 3, entry 6). The configuration of the newly generated stereogenic center (C-15, prostaglandin numbering) was assigned as α by comparing to that of 13a prepared using (−)-DIP-Cl-mediated reduction.²⁸ From 13a, hydrolysis of the PPB ester to alcohol, followed by the DIBAL-H mediated reduction of the lactone to the hemiacetal, and a final Wittig olefination furnished cloprostenol (1) in 44% over three steps (Fig. 5). The applicability of our developed route was further demonstrated by the synthesis of another four PGs: bimatoprost, PGF_2α, fluprostenol, and travoprost (Fig. 5). When enones 12b–12d with different ω-side-chains were subjected to ChKRED20-catalyzed reduction, the desired allylic alcohols 13b–13d were isolated in 80–90% yields with 87 : 13 to 99 : 1 dr. Analogous to the above synthesis of cloprostenol (1), bimatoprost (2), PGF_2α (3), and fluprostenol (4) were prepared from 13b, 13c, and 13d in a three-step sequence with 31%, 63%, and 51% yields, respectively. Finally, the transformation from fluprostenol (4) to travoprost (5) was accomplished in 68% yield by using 2-iodopropane and Cs₂CO₃ in the mixed solvent of DCM and DMF.^4b Our chemoenzymatic synthesis of cloprostenol (1), bimatoprost (2), PGF_2α (3), fluprostenol (4), and travoprost (5) was completed in 11–12 steps from bicyclic ketone 6a with 3.8–8.4% overall yields.

Table 3 ChKRED20-catalyzed reduction of 12a^a

Entry	DMSO (v/v, %)	12a (mM)	Conv.^b (%)	dr (C-15, α : β)^b
a Reaction conditions (0.5 mL): 12a (5 mM or 10 mM), glucose (2 equiv. relative to 12a), NADP^+ (0.2 mM), ChKRED20 (1 mg mL^−1), 0.1 mL of 15% (w/v) cell-free extract (CFE) of GDH, and DMSO (v/v, 3–20%) in KPi buffer (100 mM, pH 7.0). Reaction mixtures were incubated at 30 °C with 200 rpm shaking for 17 h. b Determined by SFC analysis. c The reaction volume was 40 mL and the reaction time was 33 h. d Isolated yield.
1	3	10	43	97.8 : 2.2
2	5	10	58	97.4 : 2.6
3	10	10	71	96.7 : 3.3
4	20	10	83	93.9 : 6.1
5	10	5	90	97.9 : 2.1
6^c	10	5	96 (91^d)	97.9 : 2.1

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Conclusions

In summary, a unified, biocatalytic retrosynthesis-guided route has been developed for the synthesis of cloprostenol (1), bimatoprost (2), PGF_2α (3), fluprostenol (4), and travoprost (5) from the readily available dichloro-containing bicyclic ketone 6a in 11–12 steps with 3.8–8.4% overall yields, featuring a BVMO-catalyzed stereoselective oxidation of 6a (99% ee), a time-economical three-step transformation of 7a into diol 10 using flow chemistry, a copper(II)-catalyzed regioselective p-phenylbenzoylation of the secondary alcohol of diol 10 (9.3 : 1 rr), and a KRED-catalyzed diastereoselective reduction of enones 12 (87 : 13 to 99 : 1 dr). Compared to chemocatalytic reactions, these two key enzymatic transformations were performed under milder conditions and meanwhile exhibited superior stereoselectivity. Our study not only provides an alternative route to the highly stereoselective synthesis of prostaglandins, but also showcases the usefulness and great potential of biocatalysis in construction of complex molecules.

Data availability

The experimental data is included in the ESI.

Author contributions

Z. H. and F. C. conceived and directed the project and wrote the paper with assistance from K. Z. K. Z., M. J., B. Y., G. Z., W. L., and P. T. performed the experiments and analyzed the data. All authors discussed the results and commented on the paper.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the National Natural Science Foundation of China (no. 22071033 and 21801047) and Shanghai Sailing Program (18YF1402100) is greatly appreciated.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1975866. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1sc03237b

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