39. A New Enantioselective Total Synthesis of Natural Vincamine via an Intramolecular Mannich Reaction of an Silyl Enol Ether
    Oppolzer, W.; Hauth, H.; Pfäffli, P.; Wenger, R.
    Helv. Chim. Acta 1977, 60, 1801-1810

Natural Vincamine (1) has been synthesized in an enantioselective manner starting from the ethylpentenal 7. In the key step a mixture of the diastereoisomeric racemates, 14 and 15, was directly obtained from the silyl enol ether 11 and the dihydro-β-carboline 12 by the way of an intramolecular Mannich reaction of the intermediate 13(Scheme 4). The undesired stereoisomers, 14 and 15b, were recycled to 15ausing the related reversible Mannich reaction 18 ⇄ 14 + 15, followed by crystallization of the salt from 15a and (+)malic acid. 15a was converted to natural vincamine (1) in several steps including the known transformation 20→1.

DOI : 10.1002/hlca.19770600533