A synthetic entry to ervatamine alkaloids - Synthesis of (+/-)-6-oxo-16-episilicine and (+/-)-6-oxosilicine

Catalytic hydrogenation of several 3-acyl-4-[2-(indolyl)carbonylmethyl]-5-( methoxycarbonyl)-1,4-dihydropyridines 4 gives chemoselectively the correspo nding 1,2,3,4-tetrahydropyridyl esters 5, which have been elaborated into t he tetracyclic 2,3-diacylindole system 6 of oxosilicine alkaloids. Barton d ecarboxylation of the N-benzyl derivative 6e, followed by debenzylation and subsequent stereoselective reduction of the 5,16-double bond gives (+/-)-6 -oxo-16-episilicine. This compound is converted into (+/-)-6-oxosilicine by base-catalyzed epimerization.