Azabicyclo chemistry. 7 . Ultra‐high field (600 MHz) NMR spectroscopy in solving conformational problems: cis‐Octahydroindoles

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.