An efficient route to thioglycosides with the 2,3-anhydro-D-ribo stereochemistry

Article abstract of DOI:10.1016/j.carres.2004.10.009

An improved route for the synthesis of p-tolyl 2,3-anhydro-5-O-benzoyl-1- thio-β-d-ribofuranoside and its α anomer, which are important intermediates in the synthesis of α- and β-d-arabinofuranosides, has been developed. The products are obtained in six steps from d-xylose in 39% overall yield.

Full text of DOI:10.1016/j.carres.2004.10.009

Carbohydrate
RESEARCH
Carbohydrate Research 339 (2004) 2895–2899
Note
An efficient route to thioglycosides with the 2,3-anhydro-D-ribo
stereochemistry
Jayant N. Tilekar and Todd L. Lowary*
Department of Chemistry and Alberta Ingenuity Centre for Carbohydrate Science, Gunning–Lemieux Chemistry Centre,
University of Alberta, Edmonton, AB, Canada T6G 2G2
Received 30 September 2004; accepted 15 October 2004
Available online 21 November 2004
Abstract—An improved route for the synthesis of p-tolyl 2,3-anhydro-5-O-benzoyl-1-thio-b-D-ribofuranoside and its a anomer,
which are important intermediates in the synthesis of a- and b-D-arabinofuranosides, has been developed. The products are obtained
in six steps from D-xylose in 39% overall yield.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Keywords: 2,3-Anhydro sugars; Thioglycosides; Thiofuranosides; Synthesis
We have previously demonstrated the utility of 2,3-
anhydrosugar thioglycosides 1 and 2 and the corre-
sponding glycosyl sulfoxides in the synthesis of a- and
b-arabinofuranosides1,2 and 20,30-anhydronucleosides.3
The routine use of this methodology is dependent upon
convenient methods for the synthesis of these glycosyl
donors, and in our earlier studies,1,2 we had developed
an efficient route for the preparation of 1. In contrast,
the method we used previously for the preparation of
2 was rather long and cumbersome. When considering
alternate approaches for the synthesis of 2, the other
published routes4–7 to this ring system were unattractive
given the large number of steps required and/or the
incompatibility of the chemistry with a thioglycoside
moiety.
Mindful of the need to develop a better route for the
preparation of 2, we were attracted to a recent paper by
Takatsuki et al.8 that described the preparation of 20,30-
anhydro-D-ribofuranosyl-adenine (4) from 30,50-O-sulfin-
yl-D-xylofuranosyl-adenine (5). In this Note, we report
the use of the Takatsuki methodology for the prepara-
tion of 2 and the corresponding a-anomer 3, which is
also a substrate for our glycosylation methodology.2
The route (Scheme 1) began with the tetraacetate 6,
which was synthesized in 91% yield from D-xylose using
the boric acid-mediated process developed by Furneaux
et al.9 The thioglycoside was then installed upon reac-
tion of 6 with p-thiocresol and boron trifluoride ether-
ate, which afforded a 5:1 b:a mixture of p-thiocresyl
thioglycosides 7 and 8 in 89% combined yield. The ano-
mers could be separated at this stage, and each was
carried forward separately. Thus, treatment of 7 with so-
dium methoxide yielded 9 in 92% yield. Reaction of this
triol with thionyl chloride in acetonitrile and pyridine
gave an 83% yield of the 3,5-O-sulfinyl-b-D-xylofurano-
syl thioglycoside 11, which was then converted to the
epoxide 13 upon treatment with sodium bicarbonate
BzO
STol
BzO
BzO
O
O
O
O
O
STol
O
STol
3
1
2
NH2
NH2
N
N
N
N
N
N
O
N
O
HO
N
O
O
O
S
O
OH
4
5
*
Corresponding author. Tel.: +1 760 492 1861; fax: +1 760 492 7705;
e-mail: tlowary@ualberta.ca
0008-6215/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.carres.2004.10.009

2896
J. N. Tilekar, T. L. Lowary / Carbohydrate Research 339 (2004) 2895–2899
RO
STol
RO
STol
STol
O
HO
O
O
e
e
O
d
O
c
c
S
2
3
O
OR
O
OH
11
7, R = Ac
b
AcO
9, R = H
AcO
13
a
O
+
OAc
RO
RO
HO
O
OAc
O
O
O
O
d
6
S
O
STol
OR
STol
STol
OH
O
8, R = Ac
b
12
14
10, R = H
Scheme 1. Reagents and conditions: (a) p-TolSH, BF3ÆOEt2, CH2Cl2, 0 ! 25ꢁC, 89% 5:1 b:a; (b) NaOCH3, CH3OH, rt, 92% for 9, 89% for 10; (c)
SOCl2, CH3CN, pyridine, 5ꢁC, 83% for 11, 87% for 12; (d) NaHCO3, Et3N, DMF, 90ꢁC, 60% for 13, 70% for 14; (e) BzCl, pyridine, 0ꢁC ! rt, 93%
for 2, 88% for 3.
and triethylamine in DMF at 90ꢁC for 60h. The epox-
ide-forming reaction proceeded in 60% yield, with the
byproducts being unreacted starting material and triol
9. Formation of the epoxide was readily apparent from
the 13C NMR spectrum of 13, which showed two
methine carbons at 58.67 and 57.74 ppm, as would be
expected for these 2,3-anhydrosugars.2,10 Once epoxide
13 was in hand, it was converted to 2 in 93% yield using
concentrated under vacuum, and all column chromato-
graphy was performed on Silica Gel 60 (40–60lm).
The ratio between Silica Gel and crude product ranged
from 100 to 50:1 (w/w). Optical rotations were measured
at 22 2ꢁC and are in units of degreesmL/gdm. 1H
NMR spectra were recorded at 500MHz, and chemical
shifts are referenced to either Me4Si (d 0.0, CDCl3),
HOD (d 4.78, D2O) or CD3OH (d 4.78, CD3OD). 13C
NMR spectra were recorded at 100MHz, and 13C chem-
ical shifts are referenced to internal CDCl3 (d 77.23,
CDCl3), CD3OD (d 48.9, CD3OD), or external dioxane
1
conventional benzoylation conditions. The H and 13C
NMR spectra of 2 obtained via the route shown in
Scheme 1 were identical to those previously reported
for this compound.2 The overall yield for the conversion
of 7 into 2 was 43%. Using identical transformations,
the a-thioglycoside 8 was converted to epoxide 3 in
48% overall yield. It was also possible to convert the
mixture of 7 and 8 into the epoxy thioglycosides 2 and
3. When this mixture was carried through the sequence,
the two products were obtained in a combined 43%
overall yield.
1
(d 67.40, D2O). H NMR chemical shifts were assigned
though 1H–1H COSY experiments, and 13C NMR
chemical shifts were assigned by comparison with values
in the literature11 and/or by inspection. Electrospray-
ionization mass spectra (ESIMS) were recorded on sam-
ples suspended in mixtures of THF and CH3OH with
added NaCl.
In summary, an improved route for the synthesis of
thioglycosides 2 and 3 has been developed. The products
are obtained in six steps from D-xylose in 39% overall
yield. Our previous route to these glycosylating agents,
which also started with D-xylose, provided the two com-
pounds in nine steps and 27% overall yield. Thus, this
new route is an improvement both in terms of number
of steps and overall yield.
1.2. p-Tolyl 2,3,5-tri-O-acetyl-1-thio-b-D-xylofuranoside
(7) and p-tolyl 2,3,5-tri-O-acetyl-1-thio-a-D-xylofurano-
side (8)
Tetraacetate 69 (1.0g, 3.14mmol) was dissolved in dry
CH2Cl2 (20mL) and cooled to 0ꢁC before p-thiocresol
(0.46g, 3.77mmol) was added. The mixture was stirred
for 20min under argon, and BF3ÆOEt2 (1.99g,
15.7mmol) was added via syringe. After 1.5h, the mix-
ture was neutralized with Et3N (2.2mL, 15.7mmol),
diluted with CH2Cl2, and then washed with water and
brine. The organic layer was dried (Na2SO4) and evapo-
rated, and the crude product was purified by chromato-
graphy (10:1 hexane–EtOAc) to give 7 (0.88g, 74%) as
an oil: Rf 0.38 (3:1 hexane–EtOAc); [a]D À97.4 (c 0.5,
CHCl3); 1H NMR (500MHz, CDCl3): dH 7.43 (dd,
2H, J 7.9, 2.2Hz, aryl), 7.13 (dd, 2H, J 8.0, 0.5Hz, aryl),
5.29 (dd, 1H, J2,3 2.2Hz, J3,4 5.0Hz, H-3), 5.25 (dd, 1H,
J1,2 3.3Hz, J2,3 2.2Hz, H-2), 5.18 (d, 1H, J1,2 3.3Hz, H-
1), 4.44 (ddd, 1H, J3,4 5.0Hz, J4,5a 10.2Hz, J4,5b 6.4Hz,
H-4), 4.30 (dd, 1H, J4,5a 5.2Hz, J5a,5b 11.7Hz, H-5a),
1. Experimental
1.1. General methods
Reactions were carried out in oven-dried glassware. Sol-
vents were distilled from appropriate drying agents be-
fore use. Unless stated otherwise, all reactions were
carried out under a positive pressure of argon and were
monitored by TLC on Silica Gel 60 F254 (0.25mm, E.
Merck). Spots were detected under UV light or by char-
ring with 10% H2SO4, in EtOH. All solutions were

J. N. Tilekar, T. L. Lowary / Carbohydrate Research 339 (2004) 2895–2899
2897
4.26 (dd, 1H, J4,5b 6.4Hz, J5a,5b 11.7Hz, H-5b), 2.33 (s,
3H, aryl-CH3), 2.09 (s, 3H, C(O)CH3), 2.07 (s, 3H,
C(O)CH3), 2.04 (s, 3H, C(O)CH3); 13C NMR
(100MHz, CDCl3): dC 170.51 (C@O), 169.59 (C@O),
169.17 (C@O), 138.25 (aryl), 133.29 (2, aryl), 129.74
(2, aryl), 129.36 (aryl), 90.19 (C-1), 80.40 (C-4), 78.37
(C-2), 75.26 (C-3), 62.05 (C-5), 21.12 (aryl-CH3), 20.79
(C(O)CH3), 20.73 (C(O)CH3), 20.57 (C(O)CH3);
ESIMS: m/z calcd for [C18H22O7S]Na+: 405.0979.
Found: m/z 405.0980. Further elution with 10:1 hex-
ane–EtOAc gave 8 (0.18g, 15%) as an oil: Rf 0.39 (3:1,
hexane–EtOAc); [a]D +148 (c 0.5, CHCl3); 1H NMR
(500MHz, CDCl3): dH 7.39 (dd, 2H, J 8.0, 1.7Hz, aryl),
7.12 (dd, 2H, J 8.0, 0.4Hz, aryl), 5.78 (d, 1H, J1,2 5.4Hz,
H-1), 5.44–5.40 (m, 2H, H-3, H-2), 4.65 (ddd, 1H, J3,4
5.4Hz, J4,5a 5.4Hz, J4,5b 5.4Hz, H-4), 4.25 (dd, 1H,
J4,5a 5.4Hz, J5a,5b 11.8Hz, H-5a), 4.18 (dd, 1H, J4,5b
5.4Hz, J5a,5b 11.8Hz, H-5b), 2.33 (s, 3H, aryl-CH3),
2.09 (s, 3H, C(O)CH3), 2.08 (s, 3H, C(O)CH3), 2.03 (s,
3H, C(O)CH3); 13C NMR (100MHz, CDCl3): dC
170.45 (C@O), 169.66 (C@O), 169.50 (C@O), 138.02
(aryl), 132.83 (2, aryl), 129.80 (2, aryl), 129.62 (aryl),
89.77 (C-1), 77.49 (C-4), 75.61 (C-2), 75.01 (C-3),
61.23 (C-5), 21.10 (aryl-CH3), 20.78 (C(O)CH3), 20.62
(C(O)CH3), 20.61 (C(O)CH3); ESIMS: m/z calcd for
[C18H22O7S]Na+: 405.0979. Found: m/z 405.0981.
CH3OH (8mL). After stirring for 3h, the reaction
mixture was neutralized by the addition of dry ice and
then concentrated. The crude product was purified by
chromatography (1:1, hexanes–EtOAc) to yield 10
(0.89g, 89%) as a white solid: Rf 0.11 (1:1, hexane–
EtOAc); [a]D +247 (c 0.5, CH3OH); mp 144–
146ꢁC; 1H NMR (500MHz, CD3OD): dH 7.39 (dd,
2H, J 6.5, 1.7Hz, aryl), 7.08 (dd, 2H, J 8.0, 0.5Hz, aryl),
5.63 (d, 1H, J1,2 4.3Hz, H-1), 4.30–4.27 (m, 1H, H-4)
4.26 (dd, 1H, J1,2 4.3Hz, J2,3 2.1Hz, H-2), 4.15 (dd,
1H, J2,3 2.1Hz, J3,4 3.8Hz, H-3), 3.79 (dd, 1H, J4,5a
5.1Hz, J5a,5b 11.7Hz, H-5a), 3.68 (dd, 1H, J4,5b 6.0Hz,
J5a,5b 11.7Hz, H-5b), 2.28 (s, 3H, aryl-CH3); 13C
NMR (100MHz, CD3OD): dC 137.87 (aryl), 133.71
(aryl), 132.41 (2, aryl), 130.57 (2 aryl), 93.19 (C-1),
81.77 (C-4), 80.31 (C-2), 78.07 (C-3), 61.61 (C-5),
21.10 (aryl-CH3). Anal. Calcd for C12H16O4S: C,
56.23; H, 6.29. Found: C, 56.13; H, 6.49; ESIMS: m/z
calcd for [C12H16O4S]Na+: 279.0662. Found: m/z
279.0664.
1.5. p-Tolyl 3,5-O-sulfinyl-1-thio-b-D-xylofuranoside (11)
To an ice-cooled solution (5ꢁC) of 9 (1.3g, 5mmol) in
CH3CN (25mL) was added pyridine (2.05mL,
25mmol) and thionyl chloride (1.85mL, 25mmol), and
the mixture was stirred at 5ꢁC. After 3h water (5mL)
was added, and the mixture was stirred for 10min. The
solution was extracted with EtOAc (3 · 25mL), and
the organic layer was washed with water and brine,
before being dried (Na2SO4), filtered, and concentrated.
The crude product was purified by chromatography
(1:6 hexanes–EtOAc) to yield 11 (1.27g, 83%) as a white
solid: Rf 0.41 (1:1 hexane–EtOAc); [a]D À106.1 (c 0.5,
1.3. p-Tolyl 1-thio-b-D-xylofuranoside (9)
A solution of 7 (100mg, 0.26mmol) in CH2Cl2 (10mL)
and CH3OH (10mL) at rt was treated with 1M
NaOCH3 in CH3OH (2mL). After stirring for 3h, the
reaction mixture was neutralized by the addition of
dry ice and then concentrated. The crude product was
purified by chromatography (1:1 hexanes–EtOAc) to
yield 9 (62mg, 92%) as a white solid: Rf 0.11 (1:1,
hexane–EtOAc); [a]D À196 (c 0.5, CH3OH); mp 108–
109ꢁC; 1H NMR (500MHz, CD3OD): dH 7.47 (dd,
2H, J 6.5, 1.7Hz, aryl), 7.26 (d, 2H, J 8.0Hz, aryl),
5.15 (d, 1H, J1,2 4.4Hz, H-1), 4.27 (ddd, 1H, J3,4
3.4Hz, J4,5a 4.3Hz, J4,5b 6.6Hz, H-4), 4.23–
4.20 (m, 1H, H-3), 4.12 (dd, 1H, J1,2 4.2Hz, J2,3
3.6Hz, H-2), 3.80 (dd, 1H, J4,5a 4.3Hz, J5a,5b 12.1Hz,
H-5a), 3.68 (dd, 1H, J4,5b 6.6Hz, J5a,5b 12.1Hz, H-5b),
2.33 (s, 3H, aryl-CH3); 13C NMR (100MHz, D2O): dC
139.90 (aryl), 133.50 (2 aryl), 130.89 (2 aryl), 129.67
(aryl), 91.34 (C-1), 82.74 (C-4), 81.28 (C-2), 76.36 (C-
3), 61.32 (C-5), 21.05 (aryl-CH3). Anal. Calcd for
C12H16O4S: C, 56.23; H, 6.29. Found: C, 56.28; H,
6.13; ESIMS: m/z calcd for [C12H16O4S] Na+:
279.0662. Found: m/z 279.0661.
1
CHCl3); mp 127–129ꢁC; H NMR (500MHz, CDCl3):
dH 7.45 (dd, 2H, J 6.4, 1.8Hz, aryl), 7.14 (dd, 2H, J
8.0, 0.4Hz, aryl), 5.19 (d, 1H, J1,2 2.1Hz, H-1), 4.89
(dd, 1H, J4,5a 2.2Hz, J5a,5b 12.8Hz, H-5a), 4.77 (d, 1H,
J3,4 2.7Hz, H-3), 4.40 (dd, 1H, J2,OH 4.2Hz, J1,2
2.1Hz, H-2), 4.18–4.12 (m, 2H, H-4, H-5b), 2.57 (d,
1H, JOH,2 4.2Hz, exchanges with D2O), 2.34 (s, 3H,
aryl-CH3); 13C NMR (100MHz, CDCl3): dC 138.04
(aryl), 132.58 (2, aryl), 130.12 (aryl), 129.78 (2, aryl),
93.14 (C-1), 81.35 (C-4), 73.20 (C-2), 71.46 (C-3),
55.79 (C-5), 21.12 (aryl-CH3). Anal. Calcd for
C12H14O5S2: C, 47.67; H, 4.67. Found: C, 47.61; H,
4.39; ESIMS: m/z calcd for [C12H14O5S2]Na+:
325.0175. Found: m/z 325.0174.
1.6. p-Tolyl 3,5-O-sulfinyl-1-thio-a-D-xylofuranoside (12)
To an ice-cooled solution (5ꢁC) of 10 (0.28g, 10.9mmol)
in CH3CN (15mL) was added pyridine (0.45mL,
54.6mmol), and thionyl chloride (0.40mL, 54.6mmol)
and reaction mixture was stirred at 5ꢁC. After 3h water
(3mL) was added, and the mixture was stirred for
1.4. p-Tolyl 1-thio-a-D-xylofuranoside (10)
A solution of 8 (1.5g, 3.9mmol) in CH2Cl2 (25mL) and
CH3OH (25mL) at rt was treated with 1M NaOCH3 in

2898
J. N. Tilekar, T. L. Lowary / Carbohydrate Research 339 (2004) 2895–2899
10min. Workup of the reaction as described for the
preparation of 11 provided a crude product that was
purified by chromatography (1:8 hexanes–EtOAc) to
yield 12 (0.287g, 87%) as a white solid: Rf 0.43 (1:1 hex-
ane–EtOAc); [a]D +194 (c 0.5, CHCl3); mp 149–
3H, H-3, H-5a, H-5b), 2.34 (s, 3H aryl-CH3), 1.68 (dd,
1H, JOH,5a 5.9Hz, JOH,5b 5.9Hz, exchanges with D2O);
13C NMR (100MHz, CDCl3): dC 137.81 (aryl), 132.50
(2, aryl), 130.33 (2, aryl), 129.79 (aryl), 87.65 (C-1),
80.44 (C-4), 62.89 (C-5), 59.45 (C-2), 57.95 (C-3),
21.10 (aryl-CH3); ESIMS: m/z calcd for [C12H14O3S]-
Na+: 261.0478. Found m/z 261.0550.
1
150ꢁC; H NMR (500MHz, CDCl3): dH 7.43 (d, 2H, J
8.1Hz, aryl), 7.15 (d, 2H, J 7.9Hz, aryl), 5.75 (d, 1H,
J1,2 4.0Hz, H-1), 4.96 (d, 1H, J3,4 1.6Hz, H-3), 4.92
(dd, 1H, J4,5a 2.1Hz, J5a,5b 13.0Hz, H-5a), 4.44 (dd,
1H, J1,2 3.9Hz, J2,OH 3.7Hz, H-2), 4.33 (m, 1H, H-4),
4.14 (d, 1H, J5a,5b 13.0Hz, H-5b), 2.94 (d, 1H, JOH,2
3.6Hz, exchanges with D2O), 2.34 (s, 3H, aryl-CH3);
13C NMR (100MHz, CDCl3): dC 138.20 (aryl), 132.41
(2, aryl), 130.06 (2, aryl), 129.24 (aryl), 93.59 (C-1),
76.60 (C-4), 71.87 (C-2), 71.21 (C-3), 55.97 (C-5),
21.11 (aryl-CH3). Anal. Calcd for C12H14O5S2: C,
47.67; H, 4.67. Found: C, 47.49; H, 4.48; ESIMS: m/z
calcd for [C12H14O5S2]Na+: 325.0175. Found: m/z
325.0177.
1.9. p-Tolyl 2,3-anhydro-5-O-benzoyl-1-thio-b-D-ribofur-
anoside (2)
Compound 13 (0.12g 0.50mmol) was dissolved in dry
pyridine (8mL) and cooled to 0ꢁC. After 10min benzoyl
chloride (0.087mL, 0.75mmol) was added dropwise
over 10min. The reaction mixture was allowed to stir
for 2h while warming to rt, and then it was diluted with
CH2Cl2. The organic layer was washed with 1N HCl
(5mL), satd aq NaHCO3 (5mL) and then water. The
organic layer was dried with Na2SO4, filtered, and
concentrated. The product was purified by chromato-
graphy (12:1 hexanes–EtOAc) to yield 2 (0.16g, 93%).
The NMR data obtained for the product were identical
to those previously reported.2
1.7. p-Tolyl 2,3-anhydro-1-thio-b-D-ribofuranoside (13)
To a solution of thioglycoside 11 (80mg, 0.26mmol) in
DMF (5mL) was added Et3N (0.19mL, 1.3mmol) and
NaHCO3 (111mg, 1.3mmol), and the reaction mixture
was heated at 90ꢁC. After 60h the solution was
concentrated and the crude product thus obtained was
purified by chromatography (1:6 hexanes–EtOAc) to
yield 13 (35mg, 60%) as an oil: Rf 0.41 (1:1 hexane–
EtOAc); [a]D À276 (c 0.5, CHCl3); 1H NMR
(500MHz, CDCl3): dH 7.40 (d, 2H, J 8.1Hz, aryl),
7.14 (d, 2H, J 7.9Hz, aryl), 5.54 (s, 1H, H-1), 4.31
(dd, 1H, J4,5a 4.0Hz, J4,5b 4.0Hz, H-4), 3.96 (d, 1H,
J2,3 2.7Hz, H-3), 3.89 (d, 1H, J2,3 2.7Hz, H-2), 3.85–
3.75 (m, 2H, H-5a, H-5b), 2.62 (d, 1H, JOH,5 4.2Hz, ex-
changes with D2O), 2.34 (s, 3H, aryl-CH3); 13C NMR
(100MHz, CDCl3): dC 138.36 (aryl), 132.43 (2, aryl),
130.09 (2, aryl), 128.81 (aryl), 87.62 (C-1), 81.11 (C-4),
62.61 (C-5), 58.67 (C-2), 57.74 (C-3), 21.11 (aryl-CH3);
ESIMS: m/z calcd for [C12H14O3S]Na+: 261.0478.
Found: 261.0552.
1.10. p-Tolyl 2,3-anhydro-5-O-benzoyl-1-thio-a-D-ribo-
furanoside (3)
Compound 14 (0.14g 0.58mmol) was dissolved in dry
pyridine (10mL) and cooled to 0ꢁC. After 10min benz-
oyl chloride (0.11mL, 0.88mmol) was added dropwise
over 10min. The reaction mixture was allowed to stir
for 2h while warming to rt and then diluted with
CH2Cl2. Workup as described for the preparation of
2, gave a crude product that was purified by chromato-
graphy (10:1 hexanes–EtOAc) to yield 3 (0.176g, 88%).
The NMR data obtained for the product were identical
to those previously reported.2
Acknowledgements
We thank the Natural Science Research Council of Can-
ada, the Alberta Ingenuity Centre for Carbohydrate Sci-
ence, and The University of Alberta for support of this
work.
1.8. p-Tolyl 2,3-anhydro-1-thio-a-D-ribofuranoside (14)
To a solution of thioglycoside 12 (65mg, 0.20mmol) in
DMF (5mL) was added Et3N (0.15mL, 1.0mmol) and
NaHCO3 (90mg, 1.0mmol), and the reaction mixture
was heated at 90ꢁC. After 48h the reaction mixture
was concentrated, and the crude product thus obtained
was purified by chromatography (1:4 hexanes–EtOAc)
to yield 14 (35mg, 70%) as a oil: Rf 0.37 (1:1 hexane–
EtOAc); [a]D +50.6 (c 0.5, CHCl3); 1H NMR
(500MHz, CDCl3): dH 7.45 (d, 2H, J 8.2Hz, aryl),
7.14 (d, 2H, J 7.9Hz, aryl), 5.42 (d, 1H, J1,2 1.0Hz, H-
1), 4.42 (dd, 1H, J4,5a 4.2Hz, J4,5b 4.2Hz, H-4), 4.02
(dd, 1H, J1,2 1.0Hz, J2,3 2.8Hz, H-2), 3.81–3.67 (m,
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