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

Two crystal structures of urethane‐protected derivatives of aspartic acid dimethyl ester are presented, namely dimethyl (2S)‐2‐(tert‐butoxycarbonyl)amino]butanedioate, C₁₁H₁₉NO₆, and dimethyl (2S)‐2‐{bis(tert‐butoxycarbonyl]amino}butanedioate, C₁₆H₂₇NO₈. 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.