HOMOPROLINE HOMOLOGATION BY ENOLATE CLAISEN REARRANGEMENT OR DIRECT ALLYLATION - SYNTHESES OF (-)-TRACHELANTHAMIDINE, (-)-ISORETRONECANOL AND (+ -)-TURNEFORCIDINE/

The pyrrolizidine precursors 13 and 14 are obtained both by enolate Cl aisen rearrangement of the homoproline allyl ester 12 and by direct al lylation of N-protected homoproline ethyl ester 15. In both cases, the reactions show poor levels of stereoselectivity. Reduction gives the corresponding alcohols 2Oa and 21a which are separated and subsequentl y elaborated to (-)-trachelanthamidine 24 and (-)-isoretronecanol 25 r espectively via reductive alkene cleavage, mesylation and spontaneous cyclisation following N-deprotection. The chiral integrity of the orig inal proline-derived asymmetric centre is preserved throughout. A simi lar enolate allylation gives, with high stereoselectivity, the homolog ue 34 of the Geissman-Waiss lactone 32, which is similarly transformed into (+/-)-turneforcidine 31.