An improved formal total synthesis of (-)-anisomycin

( R )-1-Benzyloxycarbonyl-2-(4-methoxyphenyl)methyl-3-pyrroline 14 was synthesized from ( S )- N , O -dibenzyl malimide 5 via a highly regio and trans stereoselective reductive alkylation of 5 . This constitutes an improved formal asymmetric synthesis of natural (-)-anisomycin.


Introduction
Polysubstituted pyrrolidines represent a class of bioactive natural products as exemplified by the antifungal antibiotics (-)-anisomycin 1, (+)-preussin 2 and (-)-codonopsinine 3. Since its first isolation from Streptomyces griseolus and S. roseochromogenes 1 in the early fifties of last century, anisomycin has become an important tool in molecular biology. 2 It also has been used for the treatment of trichomonas vaginitis 3 and amebic dysentery, 4 and as an agricultural fungicide. 5More recently, it was reported that anisomycin showed high in vitro antitumor activity, 6 and could be used in a synergistic fashion with a cyclin-dependent protein kinase inhibitor to kill carcinoma cells. 7Consequently, many approaches have been developed for the asymmetric synthesis of anisomycin. 8In continuation of our efforts on the development of amalic acid-based methodology to bioactive N-containing compounds, 9 we have communicated an asymmetric approach to (-)-anisomycin. 10In that approach, the key reductive alkylation of 4 led to 9 in modest regioselectivity.We would like to report herein an improved formal asymmetric synthesis to (-)-anisomycin, which featured the reductive alkylation of (S)-N,O-dibenzyl malimide 5 in high regio-and stereo-selectivity.

Results and Discussion
The synthesis began with the reductive alkylation of (S)-N,O-dibenzyl malimide 5 (Scheme 1), which was prepared starting from (S)-malic acid as described previously.Thus, the addition of p-methoxybenzylmagnesium chloride to 5 led smoothly to the desired α-hydroxylactam 6 as a diastereomeric mixture and in high regioselectivity, which was deduced in the followed step.The stereochemistry of the isomeric 6 was not assigned.This diastereomeric mixture, although separable by flash chromatography, was used in the next step as it was, since the followed Lewis acid mediated ionic hydrogenation 11 was considered to proceed by the intermediacy of the N-acyliminium 7. 9,12 Indeed, in the presence of 3.0 equivalents of boron trifluoride etherate, hydroxylactams 6 were reduced with excess of triethylsilane (CH 2 Cl 2 , -78°C~ r.t.) to yield predominantly trans-8 in 94.8% yield.The coupling constant of ~0 Hz for H-4/H-5 in 8 indicates a trans orientation 9 of these two protons, was later confirmed by converting 8 to known (R)-(-)-14 8e,f,i .Catalytic hydrogenation (10% Pd/C, H 2 ,1atm) of 8 afforded 9 in quantitative yield. 4

Experimental Section
General Procedures.Melting points were determined on a Yanaco MP-500 micro melting point apparatus.Infrared spectra were measured with a Shimadza IR-408 spectrometer or a Nicolet Avatar 360 FT-IR spectrometer using film KBr pellet techniques. 1 H-NMR spectra were recorded in CDCl 3 on a Varian unity +500 spectrometer with tetramethylsilane as an internal standard.Chemical shifts are expressed in δ (ppm) units downfield from TMS. Mass spectra were recorded by Finnigan Mat-LCQ (ESI direct injection).Optical rotations were measured with Perkin-Elmer 341 automatic polarimeter.THF and diethyl ether used in the reactions were dried by distillation over metallic sodium and benzophenone; dichloromethane were distilled over P 2 O 5 .Silica gel (Zhifu, 300~400 mesh) was used for column chromatography, eluting (unless otherwise stated) with ethyl acetate/petroleum ether (PE) (60-90°C) mixtures.
To an anhydrous CH 2 Cl 2 (7 mL) solution of the crude pyrrolidine 11 and a catalytic amount of DMAP, was successively added Et 3 N (0.4 mL, 2.9 mmol) and CbzCl (0.6 mL, 1.25 mmol) dropwise at 0 o C and under an atmosphere of N 2 .The mixture was stirred at room temperature for 4 hours.After concentrated at reduced pressure, the residue was purified by flash chromatography on silica gel (eluent: ethyl acetate: petroleum ether = 1 : 1) affording

benzyloxycarbonylation (CbzCl, NEt 3 , CH 2 Cl 2 , 0 °C → room temperature) gave N-protected pyrrolidine 12 in an overall yield of 77 %. Xanthation of alcohol 12 then provided 13 in 72% yield. Upon thermolysis of 13 at 190°C under reduced pressure, the
(R)-14]} was obtained in 87.3% yield, of which the spectroscopic data fully matched those reported in the literature 6m .Since (+)-14 has been converted into unnatural enantiomer (+)-anisomycin 1 8h,l,m , the present work constitutes a new formal asymmetric synthesis of the natural enantiomer of this antibiotic.