SYNTHESIS OF VINCA ALKALOIDS AND RELATED-COMPOUNDS .63. A NEW SYNTHETIC PATHWAY FOR PREPARING ALKALOIDS AND RELATED-COMPOUNDS WITH THE ASPIDOSPERMA SKELETON - TOTAL SYNTHESES OF (+/-)-VINCADIFFORMINE, (+/--TABERSONINE, AND (+/-)-3-OXOTABERSONINE

Compound 3, which has an indole skeleton containing a masked acryl ester function, was synthesized from the hydrochloride of 2-(ethoxycarbonyl)tryptamine (2). The cycloaddition of 3 with methyl 4-formylhexanoate (21) or with 5-(benzoyloxy)-2-ethylpentanal (33) yielded the starting materials for target compounds (+/-)-vincadifformine (4), (+/-)-3-oxotabersonine (42), and (+/-)-tabersonine (43). The synthesis of vincadifformine (4) was achieved in two different ways: via 3-oxovincadifformine (7) and via tetracyclic benzoate esters 37 and 38. The double bond required in tabersonine (43) and 3-oxotabersonine (42) was introduced by treatment of 3-thioxovincadifformine (39) with p-toluenesulfinyl chloride.