Self‐association of N‐protected α‐amino acids. Optically active and racemic N‐tert‐butyloxycarbonyl‐alanine

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AbstractThe ir absorption and x‐ray diffraction analysis of N‐tert‐butyloxycarbonyl‐DL‐alanine (t‐Boc‐DL‐Ala‐OH) in the solid state has revealed a new mode of self‐association for a N‐urethanyl‐α‐amino acid, i.e., ribbons of hydrogen‐bonded cyclic dimers formed through the —COOH groups. In contrast to the recemate, a water molecule, incorporated into the crystal of the chiral t‐Boc‐D‐Ala‐OH, alters in part that hydrogen‐bonding scheme. In the two independent molecules of the unit cell of the optically active alanine derivative, as in that of the racemic derivative: (i) the conformation of the —CONH group is trans, also a new observation for a N‐urethanyl‐α‐amino acid, and (ii) the overall conformation is quasi‐extended. These findings exclude the occurrence of an oxy‐C7 peptide conformation. In solvents of high polarity, strongly solvated species predominate, as shown by ir absorption spectroscopy. In deuterochloroform nonassociated and associated species occur simultaneously. No differences were observed between the optically active and racemic derivatives. The type of self‐association near saturation seems to differ, at least in part, from that found in the anhydrous racemic compound in the crystal state.