Diastereoselective synthesis of 2,5-disubstituted 3-hydroxypyrrolidine and 2,6-disubstituted 3-hydroxypiperidine derivatives by radical cyclisation; synthesis of (+)-bulgecinine and (–)-desoxoprosopinine

Abstract

Cyclisation of the O-stannyl ketyl, generated from the aldehyde 17 by reaction with tributyltin hydride in the presence of AIBN, gives the 5-benzyloxymethyl-7-hydroxypyrrolooxazolidin-2-ones as a diastereoisomeric mixture of 7α-ol 18 and 7β-ol 19 (2 : 1), with high diastereoselectivity with respect to the 5,7a positions. (+)-Bulgecinine 8 is enantioselectively synthesised by stereospecific reduction of the ketone 20, derived from compounds 18 and 19. In a similar way, cyclisation of compound 40 gives a 2 : 1 mixture of compounds 41 and 42. Conversion of compound 41 into (–)-desoxoprosopinine 9 is successfully achieved.