organic compounds
0.222 (8). In molecule A, the C—C bonds within the pyranose
ring have bond lengths ranging between 1.510 (3) and
˚
1.523 (3) A, in accordance with those in a survey of 27
pyranoid rings reported by Arnott & Scott (1972). The
˚
exocyclic C1—O1 bond has a length of 1.398 (3) Awhich is the
shortest of the C—O bonds in the pyranose ring. The C1—O5
˚
and C5—O5 bond lengths are 1.437 (2) and 1.422 (3) A,
acetyl groups directly connected to the sugar ring [i.e.
C(ring)—O—C( O)] are close to coplanar with the H atom
bound to the same ring C atom (Haines & Hughes, 2007). The
O5—C5—C6—O6 [ꢂ71.0 (2)ꢁ] and O50—C50—C60—O60
[ꢂ63.2 (2)ꢁ] torsion angles indicate a gg arrangement of the
exocyclic C6—O6 (C60—O60) bond (i.e. H5 anti to O6 and H50
anti to O60).
respectively, suggesting that the C1—O1 bond adopts an
equatorial orientation according to Arnott & Scott (1972).
The bond angles at the C atoms of the ring vary between
107.28 (15) and 114.27 (17)ꢁ, with the angles at C1 and C2
larger than the expected values of 109.2 and 110.5ꢁ, respec-
tively, and the angle at C5 smaller than the expected value of
110.0ꢁ. Although the O5—C1—O1 angle [106.47 (18)ꢁ]
appears normal for equatorial C1—O1, the C5—O5—C1
angle [113.45 (17)ꢁ] is somewhat larger than the expected
value of 112.0ꢁ. These data indicate that C1—O1 is distorted
from an idealized equatorial orientation and is bisectional.
This could be explained by the presence of the ethoxyethyl-
idene group at the 1,2-positions. The pyranose ring of mol-
ecule B exhibits similar structural features to those of
molecule A.
There are some structural differences on the O1 (O10) and
O2 (O20) sides of the five-membered dioxolane ring. In mol-
˚
ecule A, the C1—O1 bond [1.398 (3) A] is the shortest of the
four C—O bonds involving O1 and O2, and the other three
˚
bonds are almost identical in length [average 1.435 (2) A]. The
O1—C7—C8 angle [113.10 (18)ꢁ] is larger than O2—C7—C8
[110.08 (18)ꢁ] and the O1—C7—O9 angle [108.90 (17)ꢁ] is
smaller than O2—C7—O9 [112.19 (17)ꢁ]. The C7—O9 bond
˚
[1.380 (3) A] is the shortest of the three ether bonds at C7,
with the C7—O1 and C7—O2 bonds effectively identical
˚
[average 1.435 (2) A]. The same structural features are found
in molecule B. This might give an indication of why hydrolysis
of (I) yields almost equivalent amounts of the two major
isomeric products (Toyokuni et al., 2004).
Atom C7 (C70) is the only new chiral centre formed during
the synthesis of (I) (Toyokuni et al., 2004). It has an R
configuration, with the ethoxy group exo to the manno-
pyranose ring. The 1H NMR spectrum (CDCl3) gives a single
peak at 1.75 p.p.m. for the C8 methyl group, consistent with
The endocyclic torsion angles in the pyranose rings vary
from 40.2 (3) to 65.3 (2)ꢁ (absolute values), indicating that the
4
six-membered rings are distorted from an idealized C1 chair
conformation. The conformation in (I) is different from the
skew conformation (3S5) reported for 3,4,6-tri-O-acetyl-l,2-O-
[1-(exo-ethoxy)ethylidene]-ꢂ-d-glucopyranose (Heitmann et
al., 1974). Further insight into the distortions is obtained from
the Cremer–Pople puckering parameters (Cremer ꢁ& Pople,
1
the conclusion of earlier H NMR studies of glucopyranose
1,2-(alkyl orthoacetates) (Lemieux & Morgan, 1965), that the
diastereomer for which the C8 methyl H atoms resonate at a
lower field has the configuration in which the alkoxy group is
exo to the pyranose ring.
The bicyclic ring structure of (I) has five pendant groups,
each of which terminates in a methyl group. In the packing of
the crystal structure, the pendant groups of one molecule lie
between pendant groups from other molecules (Fig. 2). The
partially occupied water molecules bridge between molecules
A and B, forming hydrogen bonds (Table 1). There is no direct
hydrogen bonding between molecules A and B. Thermo-
gravimetric analysis was carried out for (I), but it is difficult to
detect any water loss prior to decomposition of the compound,
˚
1975): foꢁr molecule A, Q = 0.553 (2) A, ꢀ = 16.2 (2) and ’ =
ꢁ
˚
290.4 (8) ; for molecule B, Q = 0.529 (2) A, ꢀ = 15.3 (3) and
’ = 268.2 (9)ꢁ. The extent of the ring distortion, embodied in
the value of ꢀ, is slightly greater for molecule A than for
molecule B. The direction of the ring distortion, embodied in
the value of ’, shows a distortion towards a BC2,C5 confor-
C1
mation for molecule A and
Yates, 1979).
S
for molecule B (Jeffrey &
C5
In the five-membered ring of molecule A, the C2—O2—
C7—O1 torsion angle [5.2 (2)ꢁ] indicates an approximately
planar structure for these four atoms (r.m.s. deviation from the
˚
mean plane = 0.023 A). Atom C1 deviates from the mean
˚
plane by 0.535 (3) A. In molecule B, the corresponding values
ꢁ
0
˚
are 3.3 (2) , r.m.s. deviation = 0.015 A and deviation of C1 =
˚
0.525 (3) A. Thus, the conformation of the dioxolane ring is an
˚
envelope, with Cremer–Pople parameters of Q = 0.349 (2) A
ꢁ
˚
and ’ = 43.2 (3) for molecule A, and Q = 0.339 (2) A and ’ =
40.7 (4)ꢁ for molecule B.
The bond lengths and angles in the acetoxy groups and
acetoxymethyl substituent all appear normal, although the
bond angles at C6 [107.47 (16)ꢁ] and C60 [107.26 (18)ꢁ] are,
respectively, 4.3 and 4.5ꢁ smaller than the average bond angle
(111.8ꢁ) found in 23 other pyranose derivatives (Arnott &
Scott, 1972). The three substituents all adopt the same equa-
torial orientation as in the parent compound, ꢁ-d-mannose,
Figure 2
The molecular packing of (I), viewed along the a axis. Dashed lines
indicate hydrogen bonds.
4
due to the C1 conformation of the mannopyranose ring.
Similar to what is observed for many acetylated sugars, the
ꢃ
Acta Cryst. (2012). C68, o338–o340
Liu et al. C16H24O10ꢀ0.11H2O o339
Liu, Ya-Ling
