Enantioselective synthesis of C1-C4 and C5-C14 fragments of cytospolide D

A convergent approach for the synthesis of the two key fragments (C1-C4 and C5-C14) of cytospolide D is described. Key transf ormations include MacMillan’s crossed aldol, Sharpless asymmetric dihydroxylation (AD) and Mitsunobu inversion reactions.


Introduction
3] A number of cytospolides showed cytotoxic effects to various human carcinoma cell lines.The C-2 methyl group inversion in cytospolide E 5 from 2R of 4 to 2S of 5 was found to lead to a surprise increase in cytotoxic activity against the A-549 tumor cell lines. 2The structure of all the cytospolides were determined by extensive studies of chemical, spectroscopic and single crystal X-ray analysis. 3Cytospolide D 4 has been a synthetic target of considerable interest due to its bioactivities and unique structure with an array of functionalities.][6][7][8] As part of our ongoing program towards the syntheses of bioactive natural products, [9][10][11][12][13][14] we became interested in developing a simple and flexible route to the key fragments of cytospolide D 4. Herein, we are reporting a new and efficient enantioselective synthesis of key fragments C1-C4 and C5-C14 for the cytospolide D 4 employing the MacMillan's crossed aldol, Sharpless AD and Mitsunobu inversion reaction as key steps.

Results and Discussion
Our retrosynthetic analysis for the synthesis of both the fragments of cytospolide D 4 is displayed in Scheme 1.We envisaged that the cytospolide D 4 in its E-form could be synthesized from the key precursor bis-olefin derivative 10 via Grubbs RCM reaction followed by deprotection of protecting groups.The bis-olefin derivative 10 could be accessed from the olefinic alcohol fragment 11 (C5-C14) and acid fragments 12 (C1-C4) by intermolecular Mitsunobu esterification.The alcohol derivative 11 in turn could be synthesized from easily accessible trans-olefinic ester 14 via Sharpless AD followed by standard organic transformation.The olefinic acid fragment 12 could be assembled from alcohol derivative 13 by employing PMB protection of primary alcohol, silyl ether cleavage, regioselective primary hydroxyl o-tosylation, base treatment to get terminal epoxide, one carbon homologation, TBS protection and PMB deprotection followed by oxidation.The 1,3-diol derivative 13 in turn could be obtained from the aldehyde intermediate of monosilylated ethylene glycol derivative 15 via asymmetric MacMillan's crossed aldol reaction followed by in-situ aldehyde reduction.Scheme 1. Retrosynthetic analysis of cytospolide D 4.

Synthesis of the C5-C14 Fragment (11).
The synthesis of the key fragment olefinic alcohol 11 is depicted in Scheme 2. The synthesis started from readily available α, β-unsaturated ester 14 15 by treatment with osmium tetroxide and potassium ferricyanide as co-oxidant in the presence of (DHQ)2PHAL (hydroquinine 1,4-phthalazinediyl diether) under Sharpless asymmetric conditions [16][17][18] to afford the diol derivative 16 in 92% yield with >98% ee {[α]D 25 -14.37 (c 1.00, EtOH), Lit 19-20 -14.40 (c 1.00, EtOH) 19 }.The diol derivative 16 on LiAlH4 reduction afforded triol intermediate, which on regioselective primary alcohol o-tosylation by TsCl and Et3N in the presence of catalytic amount of Bu2SnO [21][22] (dibutyltin(IV) oxide) followed by base treatment afforded the epoxy alcohol derivative 17 as the sole product in 80% yield.The free secondary hydroxyl group of 17 was subjected to imidazole-promoted protection with TBSCl which furnished the TBS protected epoxide 18 in 90% yield.The protected epoxide derivative 18 was then subjected to copper-catalyzed (CuI) regioselective ring opening with allylMgBr to give the alkenol derivative 19 in excellent yield.The free hydroxyl group of alkenol derivative 19 on pmethoxybenzyl chloride (PMBCl) protection in the presence of NaH and selective desilylation of the TBS ether with TBAF (tetrabutylammonium fluoride) furnished the alkenol derivative 11 in excellent yield.

Synthesis of the C1-C4 Fragment (12)
The synthesis of olefinic acid fragment 12 (C1-C4) is illustrated in Scheme 3. The 1,3-Diol 13 was synthesized from readily available monosilylated ethylene glycol derivative 15 via MacMillan's crossed aldol 23 reaction with propanal catalyzed by D-proline and subsequent NaBH4 reduction in 86% yield with >99% ee and anti:syn 98:2 ratio. 24Treatment of 1,3-diol 13 with K2CO3 and PMBCl in acetone under reflux conditions 25 successfully furnished the regioselective primary alcohol PMB protected derivative 21 in 83% yield.The synthesis of the epoxide derivative 22 from alcohol 21 was carried out by a process including silyl deprotection and selective primary alcohol tosylation by TsCl and Et3N in the presence of catalytic amount of Bu2SnO [21][22] followed by base treatment in 89% yield (over three steps).7] With allyl alcohol derivative 23 in hand, we then subjected it to imidazolepromoted protection with TBSCl and selective deprotection of the PMB ether with DDQ to afford the primary alcohol derivative 25 in excellent yield.Finally, TEMPO {(2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl}/BIAB {(diacetoxyiodo)benzene} mediated oxidation of the primary alcohol 25 furnished the key fragment 12 in 87% yield.The fragments alkenol derivative 11 and acid fragment 12 could be used for the coupling reactions to accomplish the synthesis of key precursor bis-olefin derivative 10 which could be used for the total synthesis of cytospolide D 4 as shown in Scheme 4. The ester derivative 10 could be prepared by coupling of alcohol fragment 11 and acid fragment 12 under Mitsunobu conditions.Alternatively, to access the key precursor bisolefin 10, the Yamaguchi esterification [28][29][30] could be attempted.Therefore, alkenol derivative 11 was subjected to Mitsunobu inversion in the presence of p-nitrobenzoic acid, Ph3P, and DIAD followed by K2CO3 treatment in MeOH to afford required alkenol derivative 11a in 82% yield (Scheme 2).The Yamaguchi esterification of acid fragment 12 and alkenol derivative 11a could also lead to the required bis-olefin derivative 10. 31 The bis-olefin derivative 10 could then be converted into the target compound cytospolide D 4 by the Grubbs RCM and deprotection of both protecting group following the standard organic transformations.Ramana et al. reported the synthesis of Cytospolide E 5 with similar fargments with different stereochemistry. 32We can expect to get E geometry with the current stereochemistry and would be investigated.

Conclusions
We have developed a new and efficient enantioselective approach to the synthesis of the two key fragments (C1-C4 and C5-C14) of cytospolide D employing MacMillan's crossed aldol, Sharpless AD and Mitsunobu inversion reactions as key steps.The synthetic approach described has significant potential for stereochemical variations in all the positions and further extension to other stereoisomers and analogues.Currently, the efforts are in progress and the results will be disclosed in due course.

Experimental Section
General.All reactions were carried out under argon or nitrogen in oven dried glassware using standard glass syringes and septa.The solvents and chemicals were purchased from Merck and Sigma Aldrich Company.Solvents and reagents were purified and dried by standard methods prior to use.Progress of the reactions was monitored by TLC using precoated aluminium plates of Merck kiesel gel 60 F254.Column chromatography was performed on silica gel (60-120 and 100-200 mesh) using a mixture of n-hexane/ethyl acetate and Dichloromethane/MeOH. IR spectra were recorded on Agilent resolution Pro 600 FT-IR spectrometer, fitted with a beam condensing ATR accessory. 1H and 13 C NMR spectra were recorded in CDCl3 (unless otherwise mentioned) on JEOL ECS operating at 400 and 100 MHz, respectively.Chemical shifts are reported in δ (ppm), referenced to TMS.HRMS were recorded on Agilent 6530 Accurate-Mass Q-TOF using Electron Spray Ionization.Optical rotations were measured on automatic polarimeter AA-65.

(S)-1-((S)-Oxiran-2-yl)hexan-1-ol (17).
To a solution of LiAlH4 (1.56 g, 41.12 mmol) in dry THF (20 mL) at 0 o C was added a solution of 16 (4.20 g, 20.56 mmol) in dry THF (40 mL) dropwise.The resulting mixture was stirred at 0 o C for 2 h.After this 10% aq.NaOH (20 mL) solution was added slowly to quench the reaction.The mixture was extracted with EtOH (2 x 30 mL), the combined organic layers were dried over Na2SO4 and concentrated in vacuo to get the triol intermediate which is used for the next step without further purification.To a solution of above triol in dry DCM (40 mL), was added Et3N (2.86 mL, 20.56 mmol), TsCl (3.91 g, 20.56 mmol), and Bu2SnO (1.02 g, 4.11 mmol) sequentially.The resulting solution was stirred for 30 min and quenched with H2O (20 mL).The aqueous layer extracted with DCM (3 x 30 mL).The combined organic phase was washed with brine, dried over anhydrous Na2SO4 and concentrated in vacuo.The crude mono tosylated derivative was formed and was used as such in next reaction without further purification.
To a stirred solution of above aldehyde and D-proline (192 mg, 1.66 mmol) in dioxane (17 mL) at 4 o C was added dropwise a precooled (4 o C) solution of propionaldehyde (5.95mL, 83.20 mmol) in dioxane (17 mL) over 24 hours via syringe pump.The mixture was continuously stirred for an additional 24 hours at the same temperature.After completion, the reaction was diluted with ethyl acetate washed with brine and organic layer was dried over anhydrous Na2SO4 and concentrated in vacuo.The crude aldehyde obtained was directly used for the next reaction.To a stirred suspension of above aldehyde in dry MeOH (40 mL) was added NaBH4 (1.26 g, 33.28 mmol) in small lots at 0 o C.After stirring the mixture at room temperature for 30 min, reaction mixture was quenched with slow addition of saturated NH4Cl solution.The organic layer was extracted with EtOAc (3 x 40 mL), dried over anhydrous Na2SO4, and evaporated under vacuum.Silica gel column chromatography (hexane/EtOAc 4:1 v/v) of the residue afforded 1,3-diol 13 (5.10

3(S)-2((S)-1-(4-Methoxybenzyloxy)propan-2-yl)oxirane (22).
To a stirred solution of 21 (5.0 g, 10.4 mmol) in dry THF (50 ml), TBAF (1.0 M in THF, =15.66 mL, 15.66 mmol) was added, and resulting solution was stirred for 1.5 hours at room temperature.Saturated solution of NH4Cl was added to quench the reaction.The aqueous phase was extracted with EtOAc (3 x 50 mL).The combined organic phase was washed with brine, dried over anhydrous Na2SO4 and concentrated in vacuo to get the diol.The crude residue was taken for the next reaction without further purification.To a solution of above diol at 0 o C in dry DCM (20 mL), was added Et3N (1.73 mL, 12.48 mmol), TsCl (2.37 g, 12.48 mmol) and Bu2SnO (517 mg, 2.08 mmol) sequentially.The resulting solution was stirred for 30 min at room temperature and quenched with water (20 mL).The aqueous layer extracted with DCM (3 x 20 mL).The combined organic phase was washed with brine, dried over anhydrous Na2SO4 and concentrated in vacuo.The crude monotosylated derivative formed was utilized for the next reaction without further purification.To a solution of monotosylated derivative formed in the above reaction in ether (20 ml) was added KOH (1.75 g, 31.2 mmol).The resulting solution was stirred for 2 hours at room temperature and after which the turbid solution was quenched with H2O (20 mL).The aqueous layer was extracted with ether (3 x 20 mL), organic layer separated, washed with brine, dried over anhydrous Na2SO4 and concentrated under reduced pressure.