Reductive deoxygenation of 5a, 5b, 7a, or 7b with borane-THF in trifluoroacetic acid yielded a product mixture (6a/6b or 4a/4b) highly biased (90-95%) in favor of the trans diastereomer (6b or 4b). The mechanism of this process was probed by NMR spectroscopy and the use of deuterium-labeled reagents and substrates. Thus, we eliminated several possible mechanisms and determined that the reaction proceeds via (1) dehydration of the substrate to an enammonium salt (lla/llb or 17a/17b) comprised mainly of the cis-fused diastereomer (lib or 17b), (2) prototropic rearrangement to an iminium salt (12a/12b or 18a/18b) highly enriched in the (6,10b)-trans isomer (12b or 18b), and (3) hydride transfer to the iminium carbon at the 5-position. In further support, a mixture of iminium species 12a and 12b (35:65) was isolated and characterized as a perchlorate salt. Also, the stereochemistry of the enammonium salts was proven by studies with methiodides 22a and 22b, the latter of which was characterized by single-crystal X-ray analysis. Three critical factors appear to govern the stereochemical outcome of the reductive deoxygenation: (1) the much greater thermodynamic stability of the cis-fused enammonium salt, (2) a highly stereocontrolled (at least 99% for the major cis-fused diastereomer), suprafacial proton transfer from N4 to C6, and (3) reduction of the iminium salts before they equilibrate. Protonation of enamine 14 with CF3CO2D occurred directly on N4 and the resultant enammonium salts (11a/11b), present in an 8:92 ratio, rearranged in the usual manner. In the enammonium-iminium rearrangement, we observed a considerable degree of intramolecular proton transfer, while some exchange with the medium was occurring. The reaction rates for rearrangement of enammonium salts from various acid-addition salts of 7a/7b in CF3CO2D were measured and found to increase by a factor of 75 in going from HClO4 to CF3CO2H (pKa ranging from-10 to 0). The body of evidence suggests that the enammonium-iminium rearrangement takes place largely within a tight solvent cage, in which the proton or deuteron on nitrogen is conducted suprafacially from N4 to C6. This internal control is responsible for the >98% stereoselectivity of the rearrangement. The synthesis of 4b (McN-5652-Z) from 4-(methylthio) benzaldehyde and N-vinylpyrrolidin-2-one and an enantiospecific synthesis of (6S,10bR)-(+)-4b (McN-5652-Z) are described in detail. A single-crystal X-ray analysis of 5a·HBr showed the trans-fused ring geometry and a hydrogen-bond bridge involving the bromide atom, HO, and HN4 (O-H⋯Br⋯H-N). On dissolution in CDC13, 5a·HBr rapidly converted to a mixture of cis and trans-fused species. © 1990, American Chemical Society. All rights reserved.