Conformational studies on substituted ε-caprolactams by X-ray crystallography and NMR spectroscopy

Gruber, Tobias and Thompson, Amber L. and Odell, Barbara and Bombicz, Petra and Schofield, Christopher J. (2014) Conformational studies on substituted ε-caprolactams by X-ray crystallography and NMR spectroscopy. New J. Chem., 38 (12). pp. 5905-5917. ISSN 1144-0546

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Abstract

The synthesis and conformational analysis of epsilon-caprolactams containing a -COOMe group at the C-6 position is described. The influence of different C-2, C-6 and N substituents on ring conformation was studied using X-ray crystallography and NMR spectroscopy. The results provide evidence that all the analysed caprolactams adopt a chair type conformation with a planar lactam. In the 6-substituted caprolactam, the -COOMe residue prefers to reside in an equatorial position, but can be induced to occupy an axial orientation by the introduction of a bulky tert-butyloxycarbonyl (BOC) group on the lactam nitrogen or by C-2/C-3 ring desaturation. The BOC protected caprolactam was found to undergo exchange between two chair forms as detected by solution NMR, one with the C-6 ester equatorial (30%) and the other with it in the axial position (70%); the tatter was observed by X-ray crystallography. For the C-2 dithiocarbamate substituted C-6 methyl ester seven-membered rings, a single chair form is observed for cis-isomers with both substituents equatorial. The analogous trans-isomers, however, exist as two chair forms in a 1:1 equilibrium ratio of C-1,N(4) and C-4(1,N) conformers, where either substituent can occupy axial or equatorial positions.

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