Quinolizidines. XXVIII. Racemic and Chiral Syntheses of Ochromianine, an Indoloquinolizidine Alkaloid from Neisosperma miana

抄録

A full account is given of the first racemic and chiral syntheses of 11-methoxydihydrocorynantheol [(-)-1], a candidate structure for the Neisosperma alkaloid ochromianine. Coupling of (±)-trans-6-ethoxy-3-ethyl-2, 3, 4, 5-tetrahydro-4-pyridineacetic acid ethyl ester [(±)-6] with 2-chloro-1-(6-methoxy-1H-indol-3-yl)ethanone (4) in the presence of KBr produced the lactam ketone (±)-7, which was then converted into the lactam (±)-9 through the oxazolium salt (±)-8. Bischler-Napieralski cyclization of (±)-9 followed by catalytic hydrogenation gave the tetracyclic ester (±)-11. On reduction with LiAlH₄, (±)-11 yielded the racemic target (±)-1. A parallel synthetic route starting from (+)-6 and 4 afforded the chiral target (-)-1 via (+)-7, 8, (+)-9, and (-)-11. Identity of synthetic (-)-1 with ochromianine unequivocally established the structure and absolute stereochemistry of this alkaloid.