M. Mokotoff, S. Hill
1988
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0
Influential Citations
8
Citations
Journal
Journal of Heterocyclic Chemistry
Abstract
Studies using ultra-high field (600 MHz) nmr spectroscopy has allowed us to assign the conformational preference of cis-octahydroindoles, including the position of their nitrogen lone-pair electrons. Lithium aluminum hydride reduction of cis-octahydroindol-2-one (4) gave cis-octahydroindole (5a) which showed three distinct nmr resonances, the C7a proton at δ 3.04 (apparent q, J = 5.3 Hz) and the two C2 protons at δ 3.10 and 2.94. The latter two resonances were absent in the C2 deuterated compound 5b. Deuteromethylation of 5a, via a urethane intermediate, gave 1-deuteromethyl-cis-octahydroindole (8a) which showed resonances at δ 3.12 for the α-faced C2 proton, the β-faced C2 proton at δ 2.17, and the C7a proton at δ 2.15. The resonances at δ 3.12 and 2.17 are absent in the corresponding C2 deuterated compound 8b, and the C7a remains as a doublet of triplets (d, J = 5.5 Hz; t, J = 3.8 Hz). Interpretation of the above data led us to assign 8a as the cis compound with nitrogen axial, and the lone pair electrons in a pseudo-axial position and antiperiplanar to the C7a and β-faced C2 protons. The amine 5a is also cis and has the nitrogen axial, but the lone pair electrons are pseudo-equatorial. Inspection of the Bohlmann band region in the ir of 5a and 8a supports the above assignments. These investigations thus suggest, in cis-octahydroindoles, the following steric order: methyl > lone pair > H.