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Issue 12, 2014
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Conformational studies on substituted ε-caprolactams by X-ray crystallography and NMR spectroscopy

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Abstract

The synthesis and conformational analysis of ε-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 latter 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 1,NC4 and 4C1,N conformers, where either substituent can occupy axial or equatorial positions.

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

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Publication details

The article was received on 08 Aug 2014, accepted on 24 Sep 2014 and first published on 25 Sep 2014


Article type: Paper
DOI: 10.1039/C4NJ01339E
New J. Chem., 2014,38, 5905-5917
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    Conformational studies on substituted ε-caprolactams by X-ray crystallography and NMR spectroscopy

    T. Gruber, A. L. Thompson, B. Odell, P. Bombicz and C. J. Schofield, New J. Chem., 2014, 38, 5905
    DOI: 10.1039/C4NJ01339E

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