Syntheses of Morphin-like Structures. III. Stereochemical Control of Addition of Borane to 9-Methylenebenzomorphan

Abstract

The addition of borane to 9-methylenebenzomorphan at high temperature has been examined in the hope of clarifying stereochemical course of the hydroboration which gave selectively the 9β-hydroxymethyl derivative in the previous study. Reaction of pyridine-borane with I in benzene at 70° gave the benzomorphan-borane (II) in high yield. Heating II in anisole followed by the oxidation gave the 9α-hydroxymethyl derivative (IV) along with the 9βisomer and a neutral product (V). The formation of 9α isomer was rationalized in terms of the dissociation of the amine-borane prior to the addition of borane to the double bond. α-Orientation of the 9-hydroxymethyl group of IV was established by converting IV to the known dl-2, 5, 9α-trimethylbenzomorphan (VIII) via the toluene-p-sulfonate (VII).