Synthesis of optically active pyrrolizidine bases

Abstract

Regiospecific 1,3-dipolar cycloaddition of ethyl propiolate to the NO-diformyl derivative (1) of natural (–)-4-hydroxy-L-proline followed by stereospecific hydrogenation gave ethyl (+)-6α-hydroxy-β-pyrrolizidine-1α-carboxylate (4). Confirmation of the absolute configuration of this ester was obtained by its conversion into the naturally occurring 8β-pyrrolizidine bases (+)-isoretronecanol (6), (+)-laburnine (7), and (+)-supinidine (8). The synthesis of the corresponding 8α-bases (–)-isoretronecanol (12), (–)-trachelanthamidine (13), and (–)-supinidine (14) was achieved after epimerisation of ethyl (+)-(2R)-2-hydroxy-2,3-dihydro-1H-pyrrolizine-7-carboxylate (3). The optical purity of all six naturally occurring bases which were synthesized was >80%. Two new optically active pyrrolizidine bases (+)-6α-hydroxy-1α-hydroxymethyl-8β-pyrrolizidine (15) and (–)-6α-acetoxy-1-acetoxymethyl-5,6,7,8-tetrahydro-3H-pyrrolizine (16) were also prepared.