Conformational analysis of substituted hexahydropyrrolo [2,3-b]indoles and related systems. An unusual example of hindered rotation about sulfonamide SN bonds. An X-ray crystallographic and NMR study

Abstract An indepth comparison of the solution (CDCl 3 , 1 H-NMR) and solid state (X-ray) conformations of the hexahydropyrrolo[2,3- b ]indoles 4 , 5 , and 6 is made. Close parallels with the literature conformations of the aflatoxin furo[2,3- b ]benzofuran skeleton and with the conformation of the naturally occuring hexahydropyrrolindole physostigmine are noted. In the solid state the sulfonamide N in 4 – 6 is non-planar with various degrees of rotation about the SN bond. In solution, variable temperature 1 H-NMR evidence indicates hindered rotation about the NS bond of the sulfonamide group in 4 – 6 , possibly coupled to inversion of the pyramidal sulfonamide N atom. Reaction of 5 with LDA followed by quenching with methyl iodide resulted in alkylation with clean inversion of configuration.