Stereoselective synthesis of 1,2:4,5-di-O-isopropylidene-3-C-(5-phenyl-1,2,4-oxadiazol-3-yl)-β-D- psicopyranose and its X-ray crystallographic analysis

Article abstract of DOI:10.1016/S0008-6215(02)00401-9

In a novel procedure, when 3-O-benzoyl-3-C-(N-hydroxycarbamimidoyl)-1,2:4,5-di-O-isopropylidene-β- D-psicopyranose (1) is treated with acetic anhydride, chloroacetyl chloride, propanic anhydride and benzoyl chloride, the 3-O-benzoyl group undergoes an int

Full text of DOI:10.1016/S0008-6215(02)00401-9

Carbohydrate Research 338 (2003) 257–261
www.elsevier.com/locate/carres
Stereoselective synthesis of
1,2:4,5-di-O-isopropylidene-3-C-(5-phenyl-1,2,4-oxadiazol-3-yl)-
b- -psicopyranose and its X-ray crystallographic analysis
D
Jianxin Yu,a,b,* Suna Zhang,a Zhongjun Li,b Wenjie Lu,c Rong Zhou,a Yuting Liu,a
Mengshen Caib
aDepartment of Chemistry, Xinjiang Uni6ersity, Wulumqi 830046, China
bNational Research Laboratory of Natural and Biomimetic Drugs, School of Pharmaceutical Sciences, Peking Uni6ersity, Beijing 100083, China
cDepartment of Chemistry, Wuyi Uni6ersity, Jiangmen 529020, China
Received 2 August 2002; accepted 15 October 2002
Abstract
In a novel procedure, when 3-O-benzoyl-3-C-(N-hydroxycarbamimidoyl)-1,2:4,5-di-O-isopropylidene-b-D-psicopyranose (1) is
treated with acetic anhydride, chloroacetyl chloride, propanic anhydride and benzoyl chloride, the 3-O-benzoyl group undergoes
an intramolecular replacement reaction with neighbouring group participation and transfer resulting in a more stable conjugated
system by the formation of a 1,2,4-oxadiazol ring. A possible mechanism is reported. The structure has been determined by
spectroscopic data and X-ray crystallographic analysis. © 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction
2. Results and discussion
The methodologies of branched-chain sugar synthesis
have attracted considerable attention1 because of their
biological importance. Recently, a number of oxadiazo-
lines have been reported to possess anti-HIV activity.2
These findings prompted us to synthesise oxadiazole
moieties and incorporate them with carbohydrate frag-
ments.3 Here, we would like to report a novel proce-
dure in which the 3-O-benzoyl group undergoes an
intramolecular replacement reaction with neighbouring
group participation and transfer in the formation of a
1,2,4-oxadiazol ringwhen 3-O-benzoyl-3-C-(N-hydroxy-
For our purpose, we chose the most frequently encoun-
tered route for the synthesis of substituted 1,2,4-oxadia-
zoles involving the acylation and subsequent cyclization
of amidoximes.4–6 Thus, the key intermediate would be
of 1,2:4,5-di-O-isopropylidene-3-C-amidoximino-3-O-
benzoyl-b- -psicopyranose (1), which was obtained in
D
the yield of 48%, by controlling the pH of the reaction
mixture between pH 7 and 8, through refluxing of
3-O-benzoyl-3-C-cyano-1,2:4,5-di-O-isopropylidene-b-
D-psicopyranose with hydroxylamine in anhydrous
methanol.7 The amidoxime 1 was then treated with
different acylation agents for ring closure giving the
substituted 1,2,4-oxadiazole derivatives. First of all, it
was treated with acetic anhydride at 80 °C under nitro-
gen atmosphere for 18 h, presumably giving 1,2:4,5-di-
O-isopropylidene-3-C-(5-methyl-1,2,4-oxadiazol-3-yl)-
carbamimidoyl)-1,2:4,5-di-O-isopropylidene-b-D-psico-
pyranose (1) is treated with acetic anhydride,
chloroacetyl chloride, propanic anhydride or benzoyl
chloride. A possible mechanism for its formation, as
well as its structural analysis, is described.
3-O-benzoyl-b- -psicopyranose (2). The FABMS and
D
ESIMS all gave m/z 427 [M+Na]+, 443 [M+K]+ and
405 [M+1]+ which indicated its molecular formula to
be C20H24N2O7 (3) and not C22H26N2O8 which is the
substituted 1,2,4-oxadiazole derivative 2 we desired.
The characteristics from the H–H COSY, DEPT and
* Corresponding author. Present address: Advanced Radio-
logical Sciences, MC 9058, Department of Radiology, The
University of Texas Southwestern Medical Center at Dallas,
5323 Harry Hines Blvd., Dallas, TX 75390-9058, USA. Tel.:
+1-214-648-8872; fax: +1-214-648-2991
E-mail address: jian-xin.yu@utsouthwestern.edu (J. Yu).
0008-6215/03/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0008-6215(02)00401-9

258
J. Yu et al. / Carbohydrate Research 338 (2003) 257–261
Table 2
TOCSY spectra by comparison with 3-C-cyano-1,2:4,5-
Fractional atomic coordinates (×104) and equivalent thermal
parameters of 3
di-O-isopropylidene-b-
D
-psicopyranose, 3-O-acetyl-3-
-psicopyran-
3-O-benzoyl-3-C-cyano-1,2:4,5-di-O-isopropyl-
-psicopyranose, 3-O-benzoyl-3-C-cyano-
-fructopyranose and 3 -
C-cyano-1,2:4,5-di-O-isopropylidene-b-
D
ose,
Atom
x
y
z
U(eq)×10
idene-b-
D
1,2:4,5-di-O-isopropylidene-b-
D
O-1
O-2
O-3
O-4
O-5
O-6
O-7
N-1
N-2
C-1
C-2
C-3
C-4
C-5
C-6
C-7
C-8
C-9
C-10
C-11
C-12
C-13
C-14
C-15
C-16
C-17
C-18
C-19
C-20
1189(2)
2332(1)
4526(1)
5169(1)
6471(1)
4311(2)
1023(2)
2061(2)
1986(2)
2480(2)
3212(2)
3693(2)
4373(2)
5328(2)
4974(2)
1234(2)
1461(2)
34(2)
2036(1)
1450(1)
4018(1)
3804(1)
2452(2)
1385(1)
5103(2)
4232(2)
4946(2)
2403(2)
2123(2)
3302(2)
2835(2)
1758(2)
830(2)
287(1)
1262(1)
802(1)
2248(1)
1620(1)
638(1)
1974(1)
2059(1)
926(1)
52(1)
43(1)
43(1)
50(1)
52(1)
46(1)
61(1)
59(1)
40(1)
47(1)
39(1)
35(1)
39(1)
46(1)
52(1)
45(1)
58(1)
68(1)
38(1)
40(1)
41(1)
49(1)
57(1)
60(1)
60(1)
51(1)
57(1)
91(1)
88(1)
O - benzoyl - 3 - C - (N - hydroxycarbamimidoyl) - 1,2:4,5-
di-O-isopropylidene-b-
-psicopyranose (1)7 showed
that the 1,2:4,5-di-O-isopropylidene- -pyranose frag-
D
D
ment still remained in 3. Thus the remainder is
C8H6N2O2, in which two parts of it contained as 5-
phenyl-1,2,4-oxadiazol-3-yl [C8H5N2O] on the basis of
NMR spectral data with a hydroxyl group ꢀOH at
n
max: 3397.0 (s, OH) cm−1, dH: 3.33 (1H, s, HO-3,
136(1)
828(1)
exchangeable with D2O).4–6 We tried to determine the
configuration of C-3 by a NOESY spectrum, but the
problem was that there were no correlations between
HO-3 or benzene hydrogens and H-2, 4, 5 and 6 of
sugar ring. We fortunately obtained the crystals of 3
from 1:1 acetone–cyclohexane that were suitable for
X-ray crystallographic analysis.
1253(1)
1943(1)
1809(1)
1233(1)
828(1)
1083(2)
218(2)
503(1)
1181(3)
4178(2)
5475(2)
6396(2)
7115(2)
7985(2)
8144(2)
7432(3)
6553(2)
3502(2)
3129(3)
4612(3)
1273(2)
1426(1)
1287(1)
1037(1)
1509(1)
1253(1)
528(2)
55(1)
308(1)
2099(1)
2789(2)
1739(2)
2573(2)
1059(2)
85(2)
Table 1
650(2)
Crystal data and structure refinement for 3
1539(2)
1693(2)
974(2)
Empirical formula
Formula weight
Temperature
Wavelength
Crystal system
Space group
C20H24N2O7
404.41
291(2) K
89(2)
6508(2)
7175(3)
7151(3)
,
0.71073 A
Orthorhombic
P212121
,
Unit cell dimensions
a=10.309(1) A, a=90°
,
b=10.474(1) A, b=90°
,
c=18.646(3) A, g=90°
2013.3(4) A , 4
Crystallographic data, and details on data collection
and structure refinement are summarized in Table 1.
The coordinates of the non-hydrogen atoms are listed
in Table 2. The bond lengths and bond angles are listed
in Tables 3 and 4, respectively. The ORTEP plot for
compound 3 is shown in Fig. 1 together with the
numbering scheme, and the packing arrangement of the
molecules is shown in Fig. 2. The pyranoid ring retains
3
,
Volume, Z
Dcalc.
Absorption coefficient
F(000)
1.334 Mg/m3
0.102 mm−1
856
Crystal size
q Range for data
collection
Limiting indices
Reflections collected
Independent reflections
Absorption correction
Refinement method
0.56×0.50×0.42 mm
2.18 to 26.99°
05h513, 05k513, 15l523
2766
2625 (Rint=0.0180)
None
a
4C1 chair conformation, 5-phenyl-1,2,4-oxadiazole
and 3-OH groups are bonded to C-3 in equatorial and
axial positions, respectively. The X-ray diffraction anal-
ysis also indicates that the crystal has molecular stack-
ing along a one-dimensional chain. Fig. 2 exhibits the
interaction among the molecules in the columnar stack-
ing. The molecules assemble through 3-OH···O-1 inter-
Full-matrix least-squares on
F2
Data/restraints/parameters 2625/0/268
Goodness-of-fit on F2
0.914
Final R indices [I\2s(I)] R1=0.0334, WR2=0.0662
,
molecular hydrogen bonds (2.88 A; 157.1°).
R indices
Absolute structure
parameter
Extinction coefficient
Largest diff. peak and
hole
R1=0.0526, WR2=0.0711
2.0 (11)
In order to confirm the unexpected reaction and
study the mechanism more deeply, we explored the
reactions of 1 with chloroacetyl chloride, propanoic
anhydride or benzoyl chloride instead of acetic anhy-
dride, at 120–130 °C under nitrogen atmosphere for 22
h. Only one product, the 5-phenyloxadiazole 3, but not
0.0283 (13)
0.147 and 0.137 e A
−3
,

J. Yu et al. / Carbohydrate Research 338 (2003) 257–261
259
4–6, was obtained in yields 81–88% from each of the
above three reactions. The products were homogeneous
as evidenced by TLC and had the same sharp melting
point as well as correct elemental analyses, IR, MS and
1H, and 13C NMR spectral data (Scheme 1).
Table 3
,
The bond lengths (A) for 3
O(1)ꢀC(1)
O(1)ꢀC(7)
O(2)ꢀC(2)
O(2)ꢀC(7)
O(3)ꢀC(3)
O(4)ꢀC(4)
O(4)ꢀC(18)
O(5)ꢀC(18)
O(5)ꢀC(5)
O(6)ꢀC(2)
O(6)ꢀC(6)
O(7)ꢀC(11)
O(7)ꢀN(1)
N(1)ꢀC(10)
N(2)ꢀC(11)
N(2)ꢀC(10)
C(1)ꢀC(2)
1.414(3)
1.420(2)
1.405(2)
1.443(2)
1.417(2)
1.424(2)
1.444(3)
1.417(3)
1.430(3)
1.416(2)
1.427(3)
1.339(2)
1.415(2)
1.293(3)
1.293(3)
1.372(3)
1.525(3)
C(2)ꢀC(3)
C(3)ꢀC(10)
C(3)ꢀC(4)
C(4)ꢀC(5)
C(5)ꢀC(6)
C(11)ꢀC(12)
C(12)ꢀC(17)
C(12)ꢀC(13)
C(13)ꢀC(14)
C(14)ꢀC(15)
C(15)ꢀC(16)
C(16)ꢀC(17)
C(7)ꢀC(9)
1.549(3)
1.510(3)
1.544(3)
1.518(3)
1.493(3)
1.468(3)
1.381(3)
1.385(3)
1.378(3)
1.372(3)
1.372(3)
1.379(3)
1.492(3)
1.510(3)
1.497(4)
1.511(4)
Fig. 1. The ORTEP plot for compound 3.
C(7)ꢀC(8)
C(18)ꢀC(20)
C(18)ꢀC(19)
Table 4
The bond angles (°) for 3
C(2)ꢀO(6)ꢀC(6) 114.21(16) O(1)ꢀC(1)ꢀC(2)
C(1)ꢀO(1)ꢀC(7) 107.59(16) O(2)ꢀC(2)ꢀO(6)
C(2)ꢀO(2)ꢀC(7) 108.57(15) O(2)ꢀC(2)ꢀC(1)
C(4)ꢀO(4)ꢀC(18) 108.53(16) O(6)ꢀC(2)ꢀC(1)
C(18)ꢀO(5)ꢀC(5) 105.15(17) O(2)ꢀC(2)ꢀC(3)
C(11)ꢀO(7)ꢀN(1) 105.81(16) O(6)ꢀC(2)ꢀC(3)
C(10)ꢀN(1)ꢀO(7) 103.64(17) C(1)ꢀC(2)ꢀC(3)
104.15(18)
112.79(16)
105.30(16)
106.76(17)
108.22(16)
107.91(15)
116.00(17)
C(11)ꢀN(2)ꢀC(10) 102.94(17) O(3)ꢀC(3)ꢀC(10) 105.60(15)
O(3)ꢀC(3)ꢀC(4) 112.80(16) N(1)ꢀC(10)ꢀC(3) 122.26(19)
C(10)ꢀC(3)ꢀC(4) 111.21(16) N(2)ꢀC(10)ꢀC(3) 123.29(18)
O(3)ꢀC(3)ꢀC(2) 108.18(15) N(2)ꢀC(11)ꢀO(7) 113.19(19)
C(10)ꢀC(3)ꢀC(2) 110.40(16) N(2)ꢀC(11)ꢀC(12) 128.40(2)
Fig. 2. The arrangement of the molecules in the unit cell.
C(4)ꢀC(3)ꢀC(2)
108.58(15) O(7)ꢀC(11)ꢀC(12) 118.42(19)
Taking these facts into account, we believe that the
3-O-benzoyl group undergoes intramolecular replace-
ment reaction through neighbouring group participa-
tion, and a more stable conjugation system is formed in
compound 3 during the cyclization of 1 with acetic
anhydride, ClCH2COCl, propanic anhydride or benzoyl
chloride. As shown in Scheme 2, the novel procedure
proceeds as follows: acetylation of amidoxime 1 takes
place first at the hydroxyl group to form 3-C-(N-ace-
toxycarbamimidoyl)-3-O-benzoyl-1,2:4,5-di-O-isoprop-
O(4)ꢀC(4)ꢀC(5) 102.82(16) C(17)ꢀC(12)ꢀC(13) 119.40(2)
O(4)ꢀC(4)ꢀC(3) 111.63(16) C(17)ꢀC(12)ꢀC(11) 118.70(2)
C(5)ꢀC(4)ꢀC(3)
113.11(17) C(13)ꢀC(12)ꢀC(11) 121.90(2)
O(5)ꢀC(5)ꢀC(6) 110.81(19) C(14)ꢀC(13)ꢀC(12) 120.20(2)
O(5)ꢀC(5)ꢀC(4) 101.38(16) C(15)ꢀC(14)ꢀC(13) 120.00(2)
C(6)ꢀC(5)ꢀC(4)
116.31(18) C(14)ꢀC(15)ꢀC(16) 120.30(2)
O(6)ꢀC(6)ꢀC(5) 114.31(18) C(15)ꢀC(16)ꢀC(17) 120.10(2)
N(1)ꢀC(10)ꢀN(2) 114.42(19) C(16)ꢀC(17)ꢀC(12) 120.10(2)
O(1)ꢀC(7)ꢀO(2) 103.69(16) O(5)ꢀC(18)ꢀO(4) 105.43(17)
O(1)ꢀC(7)ꢀC(9) 108.60(2) O(5)ꢀC(18)ꢀC(20) 109.40(2)
O(2)ꢀC(7)ꢀC(9) 108.70(17) O(4)ꢀC(18)ꢀC(20) 109.90(2)
O(1)ꢀC(7)ꢀC(8) 110.73(18) O(5)ꢀC(18)ꢀC(19) 110.40(2)
O(2)ꢀC(7)ꢀC(8) 110.13(19) O(4)ꢀC(18)ꢀC(19) 109.10(2)
ylidene-b-
-psicopyranose (A).6 The lone electron pair
D
on NH2 with high density attacks the carbon of the
CꢁO group in the 3-O-benzoyl group to produce the
intramolecular replacement reaction through neigh-
C(9)ꢀC(7)ꢀC(8)
114.40(2) C(20)ꢀC(18)ꢀC(19) 112.40(2)

260
J. Yu et al. / Carbohydrate Research 338 (2003) 257–261
bouring-group participation and transfer. The resulting
intermediate B undergoes ring-closure reactions with
the action of anhydrides or benzoyl chloride under
refluxing to remove the hydrogen from NH2 and give
rise to the compound 3.
radiation in a u/2u scan mode. The unit cell parameters
were determined from least-squares refinement based
on the setting angles of 25 reflections. The stability of
conditions was controlled by three control measure-
ments every hundred reflections.
3.1.1.
1,2:4,5-di-O-isopropylidene-b-
lution of 3-O-benzoyl-3-C-cyano-1,2:4,5-di-O-isopro-
pylidene-b- -psicopyranose (1.20 g, 3.08 mmol) and
3-O-Benzoyl-3-C-(N-hydroxycarbamimidoyl)-
D-psicopyranose (1). A so-
3. Experimental
D
3.1. General procedures
hydroxylamine (6.5 mmol) in anhyd MeOH (15 mL)
(pH 7–8) was refluxed with stirring for 8 h. After the
reaction was complete (TLC 1:2 cyclohexane–EtOAc),
the solvent was removed by distillation, the residue was
purified by silica gel column chromatography with 2:3
cyclohexane–EtOAc to afford 1 (0.75 g, 58%) as white
crystals: mp 109–112 °C, Rf 0.62 (1:2 cyclohexane–
EtOAc), [a]2D2 −102.3° (c 1.0, CH2Cl2). nmax: 3394.0 (vs,
OH, NH2), 1687.0 (m, CO) cm−1; dH: 9.85 (1H, bs,
OH, exchangeable with D2O), 8.11–7.45 (5H, m,
PhꢀH), 5.23 (2H, s, NH2, exchangeable with D2O), 4.81
(1H, d, J1,1% 7.5 Hz, H-1), 3.82 (1H, d, H-1%), 5.61 (1H,
d, J4,5 7.8 Hz, H-4), 4.43 (1H, ddd, J5,6 2.7 Hz, J5,6% 6.0
Hz, H-5), 4.18 (1H, dd, J6,6% 10.8 Hz, H-6), 3.86 (1H,
dd, H-6%), 1.55, 1.53, 1.50, 1.41 (12H, 4s, 4×CH3) ppm;
dC: 170.71 (CꢁO), 151.86 (CꢁN), 133.59, 130.19, 128.42
(PhꢀC), 111.86, 103.30 (2×CMe2), 73.92 (C-1), 110.97
(C-2), 73.45 (C-3), 75.73 (C-4), 65.04 (C-5), 63.04 (C-6),
26.63, 26.19, 25.96, 25.01 (4×CH3) ppm; ESIMS (%):
m/z 445 [M+Na]+(100). Anal. Calcd for C20H26N2O8:
C, 56.80; H, 6.16; N, 6.64. Found: C, 56.76; H, 6.18; N,
6.70.
Melting points were determined using an X4 micromelt-
ing point apparatus and are uncorrected. Optical rota-
tions were determined with a Perkin–Elmer Model
241MC polarimeter. IR spectra were recorded with a
Biorad FT-40 spectrophotometer (KBr pellets). All
NMR spectra were recorded on a Varian MERCURY
1
400 spectrometer (400 MHz for H, 100 MHz for 13C)
with CDCl3 as solvent, Me4Si as an internal standard.
Mass spectra were obtained on either ZAB-HS or HP
1100-MSD mass spectrometers. Column chromatogra-
phy was performed on silica gel (200–300 mesh), and
silica gel GF254 for TLC was purchased from the Qing-
dao Chemical Company (China). Detection was ef-
fected by spraying the plates with 5% ethanolic H2SO4
(followed by heating at 110 °C for 10 min) or by direct
UV irradiation of the plate. All the crystallographic
measurements were carried out on a KUMA KM-4
diffractometer with graphite-monochromated MoKa
3.1.2. 1,2:4,5-Di-O-isopropylidene-3-C-(5-phenyl-1,2,4-
oxadiazol-3-yl)-b-D-psicopyranose (3). A solution of 1
(0.20 g, 0.47 mmol) in redistilled Ac2O (8 mL) was
heated at 80 °C under a nitrogen atmosphere for 18 h
until TLC (1:2 acetone–cyclohexane) showed the reac-
tion complete. Anhyd EtOH (10 mL) was added with
stirring for 30 min. Then the solvent was distilled off,
and the residue was further coevaporated with toluene.
The resultant brown syrup was purified on a column of
silica gel using 1:3 acetone–cyclohexane as eluent to
give white crystals 3 (0.17 g, 88%): mp 116–117 °C, Rf
0.26 (1:2 acetone–cyclohexane), [a]2D2 −148.5° (c 1.0,
Scheme 1. Reaction conditions: Ac2O, ClCH2COCl,
(CH3CH2CO)2O or benzoyl chloride, respectively, N2, 80 or
120–130 °C.
Scheme 2. A possible mechanism for the intramolecular replacement reaction to form 3.

J. Yu et al. / Carbohydrate Research 338 (2003) 257–261
261
CH2Cl2). nmax: 3397.0 (s, OH), 1603.0 (s, CꢁN), 1555.0
(s, CꢀN) cm−1; dH: 8.16 (2H, dd, J2,3=J5,6=6.0 Hz,
tures, have been deposited with the Cambridge Crystal-
lographic Data Centre. These data may be obtained, on
request, from the CCDC, 12 Union Road, Cambridge
CB2 1EZ, UK. Tel. +44 1223 336408; fax: +44 1223
336033; e-mail: deposit@ccdc.cam.ac.uk; web: http://
www.ccdc.cam.uk/conts/retrieving/html.
J
2,4=J4,6=1.8 Hz, ArH-2,6), 7.57 (1H, dd, J3,4
=
J4,5=4.8 Hz, ArH-4), 4.60 (1H, d, J1,1% 9.5 Hz, H-1),
4.03 (1H, d, H-1%), 5.23 (1H, d, J4,5 5.5 Hz, H-4), 4.41
(1H, m, H-5), 4.37 (1H, dd, J5,6 3.0 Hz, J6,6% 12.5 Hz,
H-6), 4.30 (1H, dd, H-6%), 3.33 (1H, s, HO-3), 1.63,
1.48, 1.43, 1.11 (12H, 4s, 4×CH3) ppm; dC: 175.84
(C-3%), 170.89 (C-5%), 132.83, 129.05, 128.23, 124.01
(PhꢀC), 112.97, 105.36 (2×CMe2), 72.18 (C-1), 109.58
(C-2), 71.89 (C-3), 73.83 (C-4), 71.01 (C-5), 60.03 (C-6),
25.04, 25.79, 25.64, 25.21 (4×CH3) ppm; FABMS (%):
m/z 405 [M+1]+(100). Anal. Calcd for C20H24N2O7:
C, 59.40; H, 5.94; N, 6.93. Found: C, 59.38; H, 5.96; N,
6.91.
Acknowledgements
The authors are very grateful for the financial sup-
port of the National Natural Science Foundation of
China (No. 29862006) and the National Laboratory of
Natural and Biomimetic Drugs of Peking University.
Compound 1 (0.20 g, 0.47 mmol) was heated with
ClCH2COCl, propanoic anhydride and benzoyl chlo-
ride (10 mL), respectively, at 120–130 °C under nitro-
gen atmosphere for 22 h until TLC (1:2 acetone–
cyclohexane) showed the reaction complete. The mix-
ture was then poured into ice-water, extracted with
CH2Cl2 (3×20 mL), washed, dried (Na2SO4), concen-
trated and purified by column chromatography on sil-
ica gel to give compound 3 (81–83%) as white crystals,
which was consistent with the product obtained in the
above procedure in all physical and spectral data.
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Full crystallographic details, excluding structure fea-