Synthesis of rac-(1R,4aR,9aR)-2-methyl-1,3,4,9a-tetrahydro-2H-1,4a-propanobenzofuro[2,3-c]pyridin-6-ol. An unusual double rearrangement leading to the ortho- and para–f oxide-bridged phenylmorphan isomers

Abstract

In an attempt to obtain the para-f isomer, rac-(1R,4aR,9aR)-2-methyl-1,3,4,9a-tetrahydro-2H-1,4a-propanobenzofuro[2,3-c]pyridin-6-ol, via mesylation of an intermediate 9α-hydroxyphenylmorphan, we obtained, instead, a rearranged chloro compound with a 5-membered nitrogen ring, 7-chloro-3a-(2,5-dimethoxyphenyl)-1-methyl-octahydroindole. This indole underwent a second rearrangement to give us the desired para-f isomer. The structures of the intermediate indole and the final product were unequivocally established by X-ray crystallography. A resynthesis of the known rac-(1R,4aR,9aR)-2-methyl-1,3,4,9a-tetrahydro-2H-1,4a-propanobenzofuro[2,3-c]pyridin-8-ol, the ortho-f isomer, was achieved using the reaction conditions for the para-f isomer, as well as under Mitsunobu reaction conditions where, unusually, the oxide-bridge ring in the 5-phenylmorphan was closed to obtain the desired product. The synthesis of the para-f isomer adds an additional compound to those oxide-bridged phenylmorphans that were initially visualized and synthesized; the establishment of the structure and configuration of 8 of the theoretically possible 12 racemates has now been achieved. The X-ray crystallographic structure analysis of the para-f isomer provides essential data that will be needed to establish the configuration of a ligand necessary to interact with an opioid receptor.