The title compound has been studied by molecular mechanics calculations. There is no single, well defined global minimum energy conformation and the low energy conformations are unrelated to the conformation of the analogous alkene, trans,trans,trans-cyclodeca-l,5,9-triene. Flectrostatic interactions between the amide groups are shown to be an important conformational determinant in the cyclic tripeptide. Not unexpectedly attempts to characterize a single conformation of cyclo-[P-ala13 by X-ray crystal structure analysis and l H n.m.r. were unsuccessful.CYCLOTRI-p-ALANYL (CTBA) is probably the smallest cyclic peptide which can accommodate all trans-amide groups without introducing unacceptable Baeyer or Pitzer strain into the ring system. Cyclodi-p-alanyl, which we have previously studied by means of X-ray crystal structure analysis,l is forced to have cis-amide g m u ~i n _ o r d e r _ t o _effect ringclosure and shows a ring conformation similar to that of cis,&-cyclo-octa-l,5diene.2*3 This conformational homology between cyclic peptides and the corresponding cyclic alkene has been noted and commented on in a number of case^.^*^ These observations can be utilized to good effect in the present instance by bearing in mind the results of both the X-ray crystal structure analysis and the n.m.r.-molecular mechanics study of trans,trans,trarcs-cyclodeca-l,5,9triene (CDDT).The present work describes the synthesis of CTBA and the analysis of its molecular conformation by means of molecular mechanics calculations, X-ray crystal structure analysis, and n.m.r.
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