D. Duquet, R. Callens, M. Anteunis
Sep 1, 2010
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Journal
Bulletin des Sociétés Chimiques Belges
Abstract
The solution conformation of both Z and E rotamers of trans and cis isomers of N-trifluoroacetyl-2-methoxy-4-t.butylpiperidine and N-trifluoroacetyl-2-methoxy-4-methylpiperidine is determined from a complete analysis and assignment of proton and carbon nuclear magnetic resonance data. The trans isomers have chair conformations and a Z/E amide bond rotation barrier of ca. 76 kJ.mol−1. The cis 4-t.butyl analogue is a flexible D2 twist with the two-fold symmetry axis through Cα,Cδ for the Z rotamer, but rather through Cβ,Ce for the E rotamer, with a lowered interconversion barrier of ca. 63 kJ.mol−1. The cis 4-methyl compound is a greatly distorted chair with a Z/E activation barrier of ca. 65 kJ.mol−1. These results are relevant in relation to the potential importance of twist-boat cyclic imino acids as β-bend inducing entities in synthetic peptides.