Synthesis of polyhydroxylated pyrrolizidine alkaloids of the alexine family by tandem ring-closing metathesis-transannular cyclization. (+)-australine

Dienes 23 and 54, prepared from epoxy alcohol 9 via oxazolidinones 15 and 51, respectively, underwent ring-closing metathesis with Grubbs's catalyst to give azacyclooctenes 26 and 55. Treatment of each azacyclooctene with m-chloroperoxybenzoic acid afforded beta -epoxide 28 from 26 and alpha -epoxide 61 from 60. Basic:hydrolysis of each of these oxazolidinones was accompanied by transannular attack at the epoxide by nitrogen, resulting in 2-(O-benzyl)-7-deoxyalexine (29) and, 1,2-di-(O-benzyl)australine (62). The latter was converted:to the alkaloid australine (3) upon hydrogenolysis. Attempts to effect ring-closing metathesis of dienes 37, 38, and 46 were unsuccessful, suggesting that, while one allylic-oxygen substituent can be tolerated by Grubbs's catalyst, two cannot.