Structure, stereochemistry, and conformation of diastereoisomeric cis- and trans-3-ethyl-1,2,3,4,4a,9a-hexahydrocarbazol-4-ones by means of 13C and two-dimensional 1H nuclear magnetic resonance spectroscopy. An example of diastereoselection in a photocyclisation reaction

Abstract

3-Ethyl hexahydrocarbazol-4-ones (3a) and (3b) are stereospecifically obtained in a 4a,9a-trans-stereochemistry by a photocyclisation reaction in which appreciable diastereoselection (40%) due to the ethyl chain is observed. They are quantitatively converted into cis-compounds by treatment with acid or by angular alkylation via a benzylic carbanion. Complete analysis of the ¹H (1D, 2D) NMR spectra allows the conformation of the c ring in each compound to be established. Moreover, the collected ¹H and ¹³C NMR data constitute references for further identification of the cis- and trans-series and further determination of the position of the 3-ethyl chain. Equilibrium studies between the different isomers show the higher stability of the cis-derivatives which possess the stereochemistry of the natural products of the Aspidosperma indole alkaloids.