Synthetic studies on lythraceae alkaloids via (3+2)cycloaddition. I. Total synthesis of some simple phenylquinolizidine alkaloids and ester alkaloids.

Abstract

A general synthetic route to Lythraceae alkaloids based on a regio- and stereoselective [3+2] cycloaddition reaction has been developed. The [3+2] cycloaddition of the nitrone 5 with the olefins 14 and 26 gave the adducts 15 and 27 which, upon treatment with methanesulfonyl chloride followed by reductive cleavage of the N-O bonds and acetylation, gave the trans-quinolizidines 18 and 30 and the cis-isomers 20 and 32 in reasonable yields. The cis-isomers were converted into the simple phenylquinolizidine alkaloid, (±)-2, and the ester alkaloid, (±)-10-epi-desmethoxyabresoline (4). On the other hand, (±)-1, (±)-lasubine II (23), and (±)-abresoline (3) were synthesized efficiently from the trans-isomers, after inversion of the configuration at the C-2 position by means of the Mitsunobu reaction.