STEREOCONTROLLED ELABORATION OF QUATERNARY CARBON CENTERS THROUGH THE ASYMMETRIC MICHAEL REACTION USING CHIRAL IMINES - ENANTIOSELECTIVE SYNTHESIS OF (+)-ASIDOSPERMIDINE

An enantioselective synthesis of (+)-aspidospermidine (1b) has been developed. The key strategic element was the stereocontrolled elaboration of quaternary carbon centers through the asymmetric Michael reaction, using chiral imines under neutral conditions. Thus, addition of imine 21, prepared from 2-ethylcyclohexanone and (R)-1-phenylethylamine, to methyl acrylate, led to cyclohexanone (S)-20 with 90% stereoselectivity (Scheme 3). The latter compound was then converted in six steps into dione 19 (Chart 6). Synthesis of [ABC]-type tricyclic carbazolone 18 was next accomplished, starting from this dione, by using the indole synthesis protocol developed by Suzuki. Critical to the success of this approach was the evolution, after extensive experimentation, of an efficient sequence for assembling D ring to carbazolone 18, having controlled during the events the ''natural'', cis CD ring junction. Thus, treatment of alcohol 57 with trifluoroacetic acid led to tetracycle 58 obtained as a single isomer with 94% yield (Chart 10). The intramolecular capture of a putative intermediary iminium ion, as illustrated in 52, by the carbamate nitrogen atom of 57 has been evoked to rationalize the observed stereoselectivity. The strategy we have adopted for the construction of the fifth E ring of 1b was in fact the methodology proposed by Magnus, based on an intramolecular Pummerer rearrangement (17 --> 59). Thus, synthesis of (+)-aspidospermidine (1b) has been achieved by a linear sequence of 22 chemical operations, starting with 2-ethylcyclohexanone, with an overall yield of 2.7%.