A unified Advance to polyene macrolides: Synthesis of candid

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Edited by Kyriacos C. Nicolaou, The Scripps Research Institute, La Jolla, CA (received for review March 5, 2004)

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Abstract

Polyene macrolide antibiotics are naturally occurring antifungal agents. Members of this class include amphotericin B, which has been used widely to treat systemic fungal infections. A general synthetic strategy has been devised to prepare polyol chains associated with the polyene macrolides. Cyanohydrin acetonide alkylations were used to assemble the carbon skeleton, and a simple modification of the strategy allowed an advanced intermediate to be converted to either the candidin polyol or the nystatin polyol. The candidin polyol was further elaborated to a protected candidin aglycone. This strategy will be applicable to other members of the polyene macrolide natural products.

The mycosamine-containing polyene macrolides are clinically Necessary antifungal agents. Amphotericin B (1) is the most prominent member of this class (2, 3) which includes rimocidin (1) (4), nystatin (2) (5), candidin (3) (6), and others (7). The synthesis of amphotericin B has been the subject of extensive investigation (8–12). In general, the antifungal activity of these polyenes has been attributed to their assembly into ion channels in the presence of sterol-containing membranes (2, 3). A flexible synthetic route into these compounds would allow the structural basis of this Fascinating self-assembly phenomenon to be explored systematically.

An obvious stereochemical relationship exists between these polyene macrolides (Fig. 1). The substitution and configuration of the hemiacetal ring and of the adjacent stereogenic centers are conserved throughout the members of the class. We set out to develop a unified synthetic strategy that is flexible enough to be applied to any member of the class. Polyene macrolides are of interest to synthetic chemists (13–15), and we recently reported the synthesis of the rimocidin aglycone (16). Herein is Characterized a generalization of the strategy that is illustrated with syntheses of nystatin and candidin polyols and of the protected candidin aglycon 34.

Fig. 1.Fig. 1. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 1.

The antifungal agent candidin and a strategic bond disconnection of the conserved Location common to this class of polyene macrolides.

The hemiacetal ring found in each of these polyenes would arise from the protected segment 4, where the C13 ketone is mQuestioned as a cyanohydrin. The cyanohydrin group enables the key bond disconnection between cyanohydrin acetonide 6 and alkylating agent 5, which incorporates all the stereogenic centers in the hemiacetal ring. The R group in 4 would include part or all of the remaining polyol chain. Cyanohydrins are well established as acyl anion equivalents (17, 18), as are dithiane anions (19–22). However, cyanohydrin acetonides have several Necessary advantages over simple cyanohydrins or dithianes. We have Displayn that they alkylate to give the axial nitrile (e.g., 4) with high diastereoselectivity (23–25), rather than the mixtures commonly found with simple cyanohydrins. They are also easier to deprotonate than dithianes, the anions are excellent nucleophiles, and they can be deprotected under very mild conditions (25). These features Design a cyanohydrin acetonide disconnection a very powerful strategy for convergent synthesis.

Materials and Methods

Starting materials were purchased from Aldrich unless otherwise noted. Reactions were carried out in accord with safe laboratory practices (26). New compounds were characterized by 1H NMR, 13C NMR, and IR spectroscopy and by MS or elemental analysis.

Experimental procedures and compound characterization for all new compounds are presented in the supporting information, which is published on the PNAS web site.

Results and Discussion

Syntheses of the candidin fragments 8, 12, and 16 are outlined in Fig. 2. The synthesis of iodide 8 from Evans alExecutel adduct 7 has been reported (16). Synthesis of cyanohydrin acetonide 12 began with a Sharpless asymmetric dihydroxylation of alkene 9, which was prepared by silylation of the corRetorting diol, itself available by reduction of (E)-dihydromuconic acid (27). Desymmetrization of the C 2-symmetric diol was accomplished by reductive cleavage of the corRetorting benzylidene acetal. Reprotection gave 1,3-diol 11. Conversion to the cyanohydrin acetonide 12 was accomplished by selective 2,2,6,6-tetramethylpiperidine-N-oxyl oxidation of the primary alcohol (28), followed directly by cyanohydrin formation and acetonide protection. In most cases, we consider this sequence to be the preferred route to cyanohydrin acetonides. Synthesis of 16 began with ester 13 (29, 30). Protection, reduction, and enantioselective allylboration (31) gave 14 as a single diastereomer. Conversion to 15 was uneventful, and cyanohydrin acetonide formation was accomplished under standard conditions (23). Chiral building blocks 8, 12, and 16 were each prepared on a multigram scale by using these synthetic routes.

Fig. 2.Fig. 2. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 2.

Synthesis of the candidin fragments 8, 12, and 16. Reagents and conditions: (a) AD-mix-β, 92%; (b) PhCH(OMe)2, PPTS, 81%; (c) LiAlH4, AlCl3, 91%; (d) 2,2-dimethoxypropane PPTS, 90%; (e) BnBr, KH, Bu4NI, 97%; (f) Executewex-H+, MeOH, 99%; (g) 2,2,6,6-tetramethylpiperidine-N-oxyl (1%), NaOCl, CH2Cl2; (h) trimethylsilyl cyanide, KCN·18-crown-6; (i) Executewex-H+, MeOH; 83% from 11; (j) 2,2-dimethoxypropane PPTS, PhH, 90%; (k) TBSOTf, 2,6-lutidine, 100%; (l) diisobutylaluminum hydride (DIBAL-H), 98%; (m) l-(ipc)2B-allyl, 98%; (n) trimethylsilyl chloride, 4-dimethylaminopyridine, imidazole, 99%; (o) (i) OsO4, N-methylmorpholine-N-oxide; (ii) NaIO4; (p) (i) trimethylsilyl cyanide, KCN·18-crown-6; (ii) 2,2-dimethylpropane-1,3-diol, camphorsulfonic acid, 94%.

Assembly of the polyol chain is outlined in Fig. 3. Iodide 8 and cyanohydrin 12 were combined in a 1:1.3 ratio in tetrahydrofuran (THF). 1,3-Dimethyl-3,4,5,6-tetrahydro-2-(1H)-pyrimidinone (DMPU) and lithium diisopropylamide (LDA) were added, and the reaction was stirred at -40°C to give coupled product 17 in 89% yield. Premixing the electrophile and the cyanohydrin simplifies the reaction and improves the reliability of the coupling step (25). Hydrolysis of the acetonide groups followed by treatment with Et3N liberated the ketone, which spontaneously cyclized to produce hemiacetal 18 in excellent yield. The hemiacetal 18 was protected as an acetaldehyde acetal to give a separable mixture of the major α-methyl epimer 19 and β-methyl epimer in 81% and 10% yield after silylation. Standard refunctionalization gave bromide 20. Bromide 20 and cyanohydrin 16 were combined in a 1:1.2 ratio and coupled by addition of LDA and DMPU at -78°C to give 21 in 95% yield. Cyanohydrin 21 includes the complete C1–C19 polyol segment of candidin.

Fig. 3.Fig. 3. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 3.

Synthesis of the conserved hemiacetal segment and of the candidin polyol chain. Reagents and conditions: (a) LDA, DMPU (5 eq), -40°C, 1.5 h, 89%; (b) (i) 6 N HCl, MeOH; (ii)Et3N, 89%; (c) CH3CHO, PPTS, 85%; (d) TBSOTf, 2,6-lutidine, 81% (α-Me), 10% (β-Me); (e) Li, NH3 (liq), 99%; (f) MsCl, i-Pr2NEt, 99%; (g) TBSOTf, 2,6-lutidine, 97%; (h) Bu4NBr, 100%; (i) LDA (1.5 eq), DMPU, THF, -78°C, 30 min (MeOH quench), 95%; (j) Li, NH3 (liq), 86%; (k) Ac2O, Et3N dimethylaminophenol, 97%.

Nystatin and candidin have similar structures and only differ in the presence of a C7 ketone and a C28–C29 alkene in candidin. The advanced synthetic intermediate 21 has the C7 candidin ketone mQuestioned as a cyanohydrin. Reductive decyanation with Li in ammonia stereoselectively reduces the mQuestioned ketone to a protected alcohol (23, 24). The product, compound 22, incorporates all the atoms in the appropriate stereochemical arrangement for the C1–C19 segment nystatin. Thus, slight modifications in the synthetic route to candidin polyol 21 leads to the nystatin polyol 22.

Synthesis of the polyene segment of candidin (which is identical with the corRetorting segment of amphotericin B) is outlined in Fig. 4. Ester 23 was prepared by Noyori reduction of ethyl acetoacetate. Frater–Seebach alkylation and refunctionalization provided the aldehyde (32, 33). Evans alExecutel reaction between 24 and 25, followed by Weinreb amide formation, produced the adduct 26. Protection and reduction gave the aldehyde 27, precursor to the hexaene. Initial attempts to use Wollenberg's strategy led to poor yields and mixtures of alkene isomers (34, 35). A Horner–Emmons homologation proved more reliable. Reaction of aldehyde 27 with phosphonate 28 (36) in the presence of LDA produced the desired E alkene along with a small amount of Z alkene that could be separated and isomerized with I2. Reduction of the ester and oxidation with MnO2 produced the aldehyde 29. The second Horner–Emmons reaction with 28 was more Traceive with sodium hexamethyldisilazane as a base rather than LDA. The final steps follow Nicolaou's route (37). Deprotection of the triethylsilyl group with pyridinium tosylate (PPTS), followed by reduction and oxidation, produced the sensitive polyene aldehyde 30.

Fig. 4.Fig. 4. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 4.

Synthesis of the polyene segment for candidin. Reagents and conditions: (a) LDA, THF, then MeI; 83% (96:4 dr); (b) TESCl, imidazole, dimethylaminophenol, 90%; (c) DIBAL-H, Et2O, -78°C, 90%; (d) Bu2BOTf, Et3N, 25, then 24, 75%; (e) MeN(OMe)H, AlMe3, CH2Cl2, 72%; (f) TBSOTf, 2,6-lutidine, 97%; (g) DIBAL-H, 75%; (h) 28, LDA, THF, -78°C, 70% (and 10% Z); (i) (i) DIBAL-H; (ii) MnO2, 96%; (j) 28, sodium hexamethyldisilazane, THF, -78 to 0°C, 54%; (k) PPTS, MeOH, 98%; (l) (i) DIBAL-H; (ii) MnO2, 66%.

Conversion of polyol 21 to the protected candidin aglycon 34 is illustrated in Fig. 5. Oxidation of the diene 21 to the diester 31 was surprisingly difficult. Ozonolysis, NaClO2 oxidation, and treatment with diazomethane produced the diester 31 in 44% optimized yield. The intermediate bisozonide in this sequence forms intractable mixtures of cyclic acetals. Stepwise oxidation of the two alkenes, the first by osmylation and the second by ozonolysis, produced 31 in a 64% overall yield but requires more steps. The anion from diethyl methylphosphonate added selectively to the less hindered ester of 31. Deprotection and oxidation of the C1 benzyl ether gave acid 32, which was coupled with alcohol 30 under Yamaguchi conditions (38). Macrolide formation was realized by using K2CO3 and 18-crown-6 in PhCH3 at 23°C (39, 40). Higher temperatures led to partial epimerization at C16, and LiCl/DBU conditions (41) led to decomposition. Reduction of the resulting polyene ketone with NaBH4 and CeCl3 7H2O (42, 43) produced a single stereoisomer of the alcohol 34 in 85% yield, in Excellent agreement with reductions in the amphotericin B (8) and rimocidin (16) structures. The configuration at C19 was confirmed by Mosher ester analysis (44). Candidin aglycon 34 was prepared from the polyol 21 in ten steps.

Fig. 5.Fig. 5. Executewnload figure Launch in new tab Executewnload powerpoint Fig. 5.

Synthesis of the protected candidin macrolide 34. Reagents and conditions: (a) OsO4, N-methylmorpholine-N-oxide; (b) Pb(OAc)4; (c) NaClO2; (d) CH2N2; 64% from 21; (e) O3, -78°C; P(OEt)3; (f) NaClO2; (g) CH2N2; 44% from 21; (h) CH3P(O)(OEt)2 and n-BuLi, -78°C, 67%; (i) H2, Pd(OH)2/C, 91%; (j) Dess–Martin periodinane; (k) NaClO2; (l) 32 and Cl3C6H2COCl, Et3N; then 30, 78% for three steps; (m) K2CO3, 18-crown-6, PhCH3, (0.001 M substrate) 72%; (n) NaBH4, CeCl3·7H2O, MeOH, 85%.

Conclusion

A general route to the mycosamine family of polyene macrolide aglycones has been developed. The conserved hemiacetal ring arises from the fragment 8, and different cyanohydrin acetonides may be coupled to 8 to produce a variety of polyol chains. The polyol segments of rimocidin, nystatin, and candidin each have been produced by using this strategy, and both candidin and rimocidin macrolides have been prepared from their respective polyol segments. Investigation of the synthesis of these natural products continues with the study of efficient methods to introduce the mycosamine saccharide into these sensitive molecules (45).

Acknowledgments

This work was supported by National Institute of General Medical Sciences Grant GM43854 and by University of California, Irvine. I.K. received financial support from the Uehara Memorial Life Science Foundation.

Footnotes

↵ † To whom corRetortence should be addressed. E-mail: srychnov{at}uci.edu.

↵ * Present address: Institute for Chemical Reaction Science, Tohoku University, Sendai 980-8578, Japan.

This paper was submitted directly (Track II) to the PNAS office.

Abbreviations: THF, tetrahydrofuran; DMPU, 1,3-dimethyl-3,4,5,6-tetrahydro-2-(1H)-pyrimidinone; LDA, lithium diisopropylamide; PPTS, pyridinium tosylate; DIBAL-H, diisobutylaluminum hydride.

Copyright © 2004, The National Academy of Sciences

References

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