Geometric changes around an N atom due to a urethane-type bis(tert-butoxycarbonyl) substituent

Abstract

Two crystal structures of urethane-protected derivatives of aspartic acid dimethyl ester are presented, namely dimethyl (2S)-2-[(tert-butoxycarbonyl)amino]butanedioate, C(11)H(19)NO(6), and dimethyl (2S)-2-{bis[(tert-butoxycarbonyl]amino}butanedioate, C(16)H(27)NO(8). The geometry at the N atom is discussed and compared with similar structures. The analysis of singly and doubly N-substituted derivatives reveals an elongation of all bonds involving the N atom and conformational changes of the amino acid side chain due to steric interactions with two bulky substituents on the amino group.