Studies on the syntheses of heterocyclic compounds. DCXLI. A convenient synthesis of hexadehydroyohimbine and a total synthesis of yohimbine.

Abstract
A total synthesis of (.+-.)-yohimbine was achieved from 1,2,3,4,5,6,7,12b-octahydroindolo[2,3-a]quinolizin-2-one via 15,16-dehydroyohimbinone and yohimbinone. Hexadehydroyohimbine was synthesized from 2-bromo-5-methoxybenzaldehyde (VIII). .alpha.-Cyanophenylpropionic acid, prepared from VIII through .alpha.-cyano-2-bromo-5-methoxycinnamic acid, was cyclized to 4-bromo-2-carbamoyl-7-methoxyindan-1-one, which was converted into 6-bromo-3-methoxy-2-methoxycarbonylphenylpropionic acid [BMMP] via the corresponding dicarboxylic acid and diester. Debromination of BMMP, followed by cyclization, gave the indanone, which was transformed into the indan-1,2-dione [ID] through the .alpha.-hydroxyimino ketone. Pictet-Spengler reaction of ID with tryptamine afforded the spirobenzyl-.beta.-carboline, whose photolysis yielded the decadehydroyohimbane and the decadehydroyohimban-21-one. Reduction of both products gave O-methylhexadehydroyohimbine.