Manipulation of substituents at nitrogen in tropanes, homotropanes, and dehydro- derivatives

Article abstract of DOI:10.1016/S0040-4020(98)00643-7

Trop-6-ene (8-methyl-8-azabicyclo[3.2.1]oct-6-ene) and substituted N- benzylnortrop-6-enes are synthesised. N-Debenzylation of tropanes fails using a wide variety of non-hydrogenolytic conditions despite effective application in model piperidine derivatives; debenzylation of homotropanes is more successful, giving norhomotropane and norhomotrop-7-ene. Bridged N-benzyl compounds react more slowly than N-methyl analogues with chloroformates; the resulting quaternary intermediates are more prone than the N-methyl analogues to suffer competitive reversion to the starting amine, especially in the tropanes, consistent with slower S(N)2 attack at the benzylic carbon during decomposition; the use of an N-p-methoxybenzyl group does allow partial debenzylation to nortrop-6-ene, suggesting greater S(N)1 character in the decomposition step in this case. 'Equatorial' attack is preferred in methylation of N-benzyl-nortrop-6-ene and -homotrop-7-ene with CH₃I; subsequent reductive debenzylation of the quaternary salt achieves overall replacement of N-benzyl by N-methyl in both cases. VT NMR studies show the presence of a single 'equatorial' invertomer (CI syn- to the 2C-bridge) in N- chloro-norhomotropanes and -7-enes and by analogy, in the N-alkyl compounds.