Macrocyclic pyrrolizidine alkaloids. Synthesis and stereochemistry of (+)-dicrotaline (13β-hydroxy-13α-methyl-1,2-didehydrocrotalanine) and (+)-13-epi-dicrotaline

Abstract

Treatment of (+)-retronecine (1) with the trimethylsilyl ether of 3-hydroxy-3-methylglutaric anhydride gave a mixture of the 7- and 9-monoesters of (+)-retronecine (1). Lactonisation of this mixture was achieved using the corresponding pyridine-2-thiol esters to give (+)-dicrotaline (13β-hydroxy-13α-methyl-1,2-didehydrocrotalanine)(2) and (+)-13-epi-dicrotaline (3). The absolute configuration at C-13 in both compounds was established by a sequence of selective reactions on each epimer to yield optically active mevalonolactone.