organic compounds
C12—N1—C3 angle (ANG2). As atoms C7 and C12 should
create the same chemical environment, this is the effect of
stereochemistry and resulting steric correlations with the Cꢀ—
C(O)OMe group of aspartic acid. Only a single H atom (H3)
points towards atom C7 (of the Boc group), while the rest of
the amino acid chain points towards atom C12 from the other
Boc group and therefore decreases the value of ANG1, as the
result of steric repulsion between two bulky Boc groups and
the side chain. It is of interest that the side-chain ester group
has dramatically changed its conformation, as shown in Fig. 5.
It seems that this effect was forced by the introduction of the
second Boc group and provides the best spatial location of the
substituents on the N atom: two Boc groups and the ester side
chain of aspartic acid.
The elongation of bonds in (II) is similar to two other Boc
and bis-Boc structures (see Scheme 2), as shown in Table 5.
The N-atom out-of-plane value is much greater in (II) than in
XUDQOD or MUTTUQ. In (II), DIST3 is longer than in the
two bis-Boc aromatic structures, while the other two distances
(DIST1 and DIST2) are similar. The differences in the angle
values are greater in the two aromatic CSD structures than in
(II), as the Boc substituents are rotated out of the plane of the
aromatic ring, whereas in (II) the side chain of the aspartic
acid limits the available space, even with the conformational
change of the ester side chain (C2—C3—C4—C5 torsion
angle).
Figure 4
Definitions of the geometric parameters presented in Table 5.
Nonetheless, each chain interacts with five others (related by a
b-axis translation and symmetries of two 21 axes), utilizing
ester, tert-butyl and urethane groups to give a three-dimen-
sional network of hydrogen bonds, as shown in Fig. 3(b).
The limited data available on mono- and bis-Boc substi-
tuted compounds prompted us to take a closer look at the
N-atom geometry. We found almost no information on the
values of C—N bond lengths in such urethane-type structures
in International Tables for Crystallography, Vol. C (Prince,
2004). The only available structural information is the H—N
3
˚
distance in X2—N—H of 1.01 A (Prince, 2004) and the Csp —
Nsp3 distance in C*—NH—C O (acyclic amides) of 1.45 A
˚
(Prince, 2004), which is the best available representation of a
urethane group. We found 874 urethane-type organic struc-
tures in the CSD [tert-butoxycarbonyl-, benzyloxycarbonyl-,
ethoxycarbonyl-, methoxycarbonyl-, (9H-fluoren-9-ylmeth-
oxy)carbonyl-, and others] and only 11 bis-urethane struc-
tures. The mean values of selected geometric parameters,
defined in Fig. 4, are presented in Table 5 for comparison with
(I) and (II), and with acetyl-substituted compounds (non-
urethane derivatives).
All bonds at the N atom in (II) are longer than those in (I)
due to the introduction of a second bulky tert-butoxycarbonyl
substituent. Elongation of these bonds should be expected
from resonance theory, as the introduction of a second
substituent provides a decrease in the double-bond character
of the N—C bonds, while the elongation of the N1—C3 bond
˚
(DIST3) from 1.4438 (16) to 1.4669 (13) A seems to be caused
by steric effects. It is worth noting that the C O and C—O
bonds in the Boc substituent in (II) are shorter [C O =
˚
1.2067 (14) and 1.2057 (13) A, and C—O = 1.3241 (13) and
˚
1.3312 (13) A] than the respective bonds in (I) [C O =
˚
˚
1.2169 (14) A and C—O = 1.3455 (15) A]. The same is true for
the other two sets of Boc and bis-Boc structures (Macleod et
al., 2003; Ikonen et al., 2009).
The hybridization of atom N1 in both structures is sp2, with
angles tending towards 120ꢁ. The urethane group in (I) is
˚
almost planar and atom N1 is out-of-plane by 0.031 (2) A. In
The interatomic distances in (I) and (II) are within the
range of other tert-butoxycarbonyl- and bis(tert-butoxy-
carbonyl) structures, and those of general urethane and bis-
urethane structures. They are also close to those in acyl-
substituted species. Resonance theory allows the participation
of the second noncarbonyl O atom in resonance structures.
Urethane groups are known for their racemization-protecting
properties during peptide synthesis, as the resonance of the
urethane group lowers the rate of cyclic transient product
(II), atom N1 is out of the plane of the neighbouring atoms
3
˚
(C3, C7 and C12) by 0.184 (2) A, suggesting a partial sp
character. The shift of electron density around atoms C7 and
C12 towards the O atoms in (II) may be a consequence of this
change, probably related to the limited delocalization of
electrons in a bis-protected amino acid derivative. It is also
possible that the steric repulsion between two bulky Boc
substituents also affects the N1 out-of-plane distance. In (II),
the C7—N1—C3 angle (ANG1) is much smaller than the
ꢃ
84 Wojewska et al.
C11H19NO6 and C16H27NO8
Acta Cryst. (2013). C69, 82–86