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.