STEREOSPECIFIC ENAMMONIUM IMINIUM REARRANGEMENTS IN A BENZO[A]QUINOLIZIDINE SYSTEM

Reductive deoxygenation of amino alcohols 5a.HBr or 5b.HBr with borane-THF in trifluoroacetic acid produced a 68:32 mixture of amines 8a and 8b. This is a significant departure from the 8:92 ratio of amines 2a:2b obtained in the reduction of amino alcohols 1a.HBr or 1b.HBr. The diminished trans selectivity with 5 arises from a reduced bias for a cis ring fusion in the N-protonated 6,6 system relative to the 5,6 system. By proton NMR, we observed dehydration of 5a.HBr in CF3CO2D to a 75:25 mixture of enammonium salts trans-6:cis-6, each of which rearranged stereospecifically to give a 75:25 mixture of iminium salts cis-7:trans-7. Rate data for this rearrangement were acquired and computationally analyzed. The dehydration of free base 5b in CF3CO2D was also studied. In this case, we were able to characterize the rate of disappearance of 5b, as well as the rate of the stereospecific enammonium-iminium rearrangement. We also address slow, "post-rearrangement" epimerization at ring position 7, H/D exchange at ring position 6, and mechanistic aspects of the overall process.