Asymmetric synthesis of methyl bacteriopheophorbide- d and analogues by stereoselective reduction of the 3-acetyl to the 3-(1-hydroxyethyl) group

Abstract

Asymmetric borane-reduction of 3-acetyl-13 1-oxo-tetrapyrroles (=bacteriochlorin, chlorin and porphyrin) in the presence of a chiral source gave selectively chiral 3-(1-hydroxyethyl)-13 1-oxo-tetrapyrroles. Oxazaborolidines were effective as chiral auxiliaries. Reduction with ( S)-oxazaborolidines led to the (3 1 S)-alcohol as the major product, whose stereoselectivity was the opposite of that in the same asymmetric reduction of usual prochiral ketones ArCOR. The asymmetric reduction of the 3-acetyl group in methyl bacteriopyropheophorbide- a and the 7,8-oxidation afforded 3 1-epimeric methyl (3 1 S)-bacteriopheophorbide- d (89% de).