Conformational Analysis of (1R~*,4S~*,7S~*)-7-[(R~*)-1,2-Butadienyl]-1-methyl-bicyclo[2.2.1]hept-2-ene and (1R~*,4S~*,7S~*)-7-[(S~*)-1,2-Butadienyl]-1-methyl-bicyclo[2.2.1]hept-2-ene. Molecular Modeling as a Complement to NMR for Stereochemical Analysis

摘要

Earlier we reported on the synthesis and thermal rearrangement of (±)-(1R~*,4S~*,7S~*)-7-[(R~*)-1,2-butadi-enyl]-1-methylbicyclo[2.2.1]hept-2-ene (1a) and (±)-(1R~*,4S~*,7S~*)-7-[(S~*)-1,2-butadienyl]-1-methylbicyclo-[2.2.1]hept-2-ene (1b). Both 1a and 1b were shown to undergo Cope rearrangements to (±)-cis-1-ethylidene-3a,4,5,7a-tetrahydro-6-methylindene (2) and (±)-cis-1-ethylidene-3a,4,5,7a-tetrahydro-3a-methylindene (3) with greater than 90% stereoselectivity. Assignment of absolute stereochemistry to these rearrangements relies upon proper assignment of stereochemistry to the diastereomeric allenylnorbornenes 1a and 1b as well as trienes 2 and 3.