Structural versatility of peptides from Cα,α‐disubstituted glycines: Preferred conformation of the Cα,α‐diphenylglycine residue

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Abstract
The preferred conformation of the Cα,α‐diphenylglycine residue was determined in simple derivatives and dipeptides. The dipeptides were synthesized by the 5(4H)‐oxazolone (from the N‐para‐bromobenzoylated amino acid) method. This activated intermediate and a reaction by‐product, para‐bromobenzoylbenzhydrylamine, were characterized inter alia by x‐ray diffraction. Conformational energy calculations on the Cα,α‐diphenylglycine mono‐peptide, Ac‐Døg‐NHMe, indicate that this Cα,α‐symmetrically disubstituted residue is conformationally restricted and that its minimum energy conformation falls in the fully extended (C5) region. The results of the theoretical analysis are in agreement with the solution and crystal‐state structural tendency of mClAc‐Døg‐OH, Z‐Døg‐OtBu, pBrBz‐Døg‐Gly‐OMe and its tert‐butyl ester analogue, determined by ir absorption, lH‐nmr, and x‐ray diffraction, and also described in this work. The implications for the use of the Døg residue in designing conformationally constrained analogues of bioactive peptides are briefly discussed.

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