Robust synthesis of enantiopure cyclohexenyl analogues of 2/3-deoxyribose sugars as carbocyclic nucleoside precursors

Article abstract of DOI:10.1016/j.tet.2011.06.003

An expedient synthesis of 2-deoxy (10) and 3-deoxy (11) cyclohexenyl analogues of 2-deoxy and 3-deoxy-d-ribose sugar from commercially available starting materials is reported. Highly efficient enzymatic resolution of the key compound 10 is described usin

Full text of DOI:10.1016/j.tet.2011.06.003

Tetrahedron 67 (2011) 5744e5749
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Robust synthesis of enantiopure cyclohexenyl analogues of 2/3-deoxyribose
sugars as carbocyclic nucleoside precursors
*
Manojkumar Varada, Venubabu Kotikam, Vaijayanti A. Kumar
Organic Chemistry Division, National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An expedient synthesis of 2-deoxy (10) and 3-deoxy (11) cyclohexenyl analogues of 2-deoxy and
3-deoxy-D-ribose sugar from commercially available starting materials is reported. Highly efficient
enzymatic resolution of the key compound 10 is described using lipase under hydrolytic conditions. The
robust methodology applied here will be useful to synthesize cyclohexenyl nucleosides, which possess
potent antiviral activity and are capable of gene silencing via RNAi or antisense applications.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 24 December 2010
Received in revised form 1 June 2011
Accepted 3 June 2011
Available online 13 June 2011
Keywords:
DA reaction
Carbocyclic mimics of 2-deoxy-D-ribose and
3-deoxy- -ribose
D
Enzymatic resolution
CeNA
1. Introduction
Additionally, the conformational flexibility allowed CeNA oligo-
mers to have tremendous application in antisense therapeutics
Investigations on synthetic oligonucleotides that can mimic the
properties of oligoribonucleotides have been much focused on the
synthesis of conformationally restricted N-type nucleoside ana-
logues.1 The LNA (Locked Nucleic Acids) are the best examples that
show very high DNA/RNA duplex stability due to its locked N-type
sugar conformation.2 In six-membered ring analogues, the cyclo-
hexene nucleosides are structurally considered as one of the best
mimics of natural furanose nucleosides (Fig. 1),3 as the conforma-
tional equilibrium and the values of the thermodynamic parame-
because of the consequential ability to induce RNase-H activity7 as
compared to any other modified nucleic acid oligomers except for
the FANAs.8 They have also been shown to be useful in siRNA ap-
plications.9 The main limitation that has stymied the scope of this
highly valuable discovery is the lack of a robust and scalable syn-
thetic strategy that would ensure access to the useful 20-de-
oxyribose sugar analogue 14 as an intermediate in enantiomerically
pure form. There are two synthetic methodologies that have been
applied so far. In the first case, R-(ꢀ)-carvone10 was used as starting
ters are very similar between a cyclohexenyl nucleoside (
D
G 1.8 kJ/
material in a multistep synthesis to obtain both
D and L isomers but
mol between 2H3 or S-type and 3H2 or N-type and equilibrium
occurs via the eastern hemisphere with a barrier of 10.9 kJ/mol) and
in natural ribose nucleosides (difference between N-type and
S-type is 2 kJ/mol, and equilibrium occurs via the eastern hemi-
the overall yield of this synthesis was low (2e3%), which is not
HO
5
HO
1
O
4
4
3
1
sphere with a barrier of 4e20 kJ/mol).4 Also, the
pe
s* interaction
OH
OH
5
mimics the anomeric effect in furanose nucleoside.5 At the nucle-
oside level the CeNA (Cyclohexenyl Nucleic Acids) exists in both
N-type and S-type sugar conformations due to highly flexible
cyclohexene ring with low energy barrier between the two forms.
The cyclohexenyl nucleosides have exhibited potent antiviral ac-
tivity and the CeNA oligomers have been shown to mimic the
function of RNA with increased enzymatic and chemical stability.6
3
2
2
OH
14
OH
HO
Cyclohexene nucleosides
'
'
1
4
B
'
5
B = adenine, thymine,
guanine, cytosine
'
2
'
3
OH
* Corresponding author. Fax: þ91 2025902629; e-mail address: va.kumar@
ncl.res.in (V.A. Kumar).
Fig. 1. Cyclohexene mimics of 2-deoxyribose and cyclohexene nucleosides.
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.06.003

M. Varada et al. / Tetrahedron 67 (2011) 5744e5749
5745
practical for oligonucleotide work (Scheme 1, A). Another improved
2. Results and discussion
method involved a synthetic approach11 to the sugar precursor in
the racemic form starting from ethyl (2E)-3-acetoxyprop-2-enoate
as dienophile12 and Danishefsky’s diene13 in a DielseAlder reaction
to construct the six-membered ring skeleton (Scheme 1, B).
DielseAlder reaction between commercially available 5,5-
dimethoxy-1,2,3,4-tetrachlorocyclopentadiene 1 and vinyl acetate
2 at 120 ꢁC gave exclusively the endo adduct (ꢂ)-3a.15 The ester
group in (ꢂ)-3a was hydrolyzed under acidic conditions to get the
free alcohol (ꢂ)-3b. Compound (ꢂ)-3b was then subjected to re-
ductive dehalogenation under Birch conditions to get (ꢂ)-4a. The
purpose of the acetate hydrolysis prior to dehalogenation was to
improve the yields of the dehalogenation reaction (Scheme 2).16 The
bicyclic system and the formation of endo adduct set the desired
trans-relationship of the substituents corresponding to the position
3 and 4 in ribose sugar. The ketal hydrolysis of compound (ꢂ)-4a to
get the free ketone (ꢂ)-5 proved to be difficult as we found that the
formation of compound (ꢂ)-6 was the major product under variety
of ketal hydrolysis conditions. We employed different protecting
groups for the secondary hydroxyl group in (ꢂ)-4a, such as TBDMS,
benzyl or acetate15 but each time compound (ꢂ)-6 was formed as
a major product in acidic hydrolysis. The silyl ether, benzyl ether and
the acetate protecting group probably underwent hydrolytic cleav-
age, concomitant with the ketal hydrolysis and as a consequence,
(ꢂ)-6 is obtained as the major fragmentation product. A report was
found in the literature where a similar retro aldol type of rear-
rangement was observed and a similar major reaction product was
obtained.17 Then we used benzoyl protection as suggested by Sgarbi
and Clive18 and found that the aqueous acetic acid reflux conditions
gave a clean ketal hydrolyzed product (ꢂ)-5 in 81% yield. The ben-
zoyl protection was found to be stable under these conditions and
the rearranged product (ꢂ)-6 was not observed. As expected, the
BaeyereVilliger’s oxidation of ketone (ꢂ)-5 gave two regioisomeric
inseparable mixture of lactones (ꢂ)-7 and (ꢂ)-8 in 7:3 proportion,
respectively, in high yield (Scheme 3). It was confirmed by 2D COSY
NMR spectroscopy (Supplementary data).
A
R3O
OR4
R1O
4
OH
R1O
3
R2O
O
R2O
R1 = R2= TBDMS
R3 = H, R4= Bn
R-(-)-carvone
R1 = R2= TBDMS
B
HO
4
OH
OH
OMe
O
OMe
3
EtOOC
HO
OEt
+
OTMS
AcO
OTMS
OAc
E-3-acetoxyprop
-2-enoate
EtOOC
Scheme 1. Reported synthetic methods.
We have been recently involved in developing synthetic ap-
proaches towards conformationally restricted analogues of 20-50
isoDNA for their applications in antisense research.14 During
a careful search of the literature for the synthesis of isomeric car-
bocyclic mimic of 3-deoxyribose and the analogues nucleoside for
its incorporation in 20-50 isoDNA, we came across a very interesting
report15 about the synthesis of carbasugars and confused carba-
sugars from commercially available starting materials. The required
2-deoxy and 3-deoxy cyclohexenyl analogues of 2-deoxy and
3-deoxyribose sugars in racemic form were found to be the in-
termediate compounds in this synthesis. We report in this article
the optimized synthetic protocols to achieve the synthesis of
cyclohexene containing mimic of 2-deoxy-(
cyclohexene containing mimic of 3-deoxy-( )-ribose and an im-
portant cyclohexene analogue of 2-deoxy- -ribose sugar with
inverted configuration at C4 centre. We also report the enzymatic
resolution of the DielseAlder adducts as well as that of the ben-
zylidene protected intermediate to obtain enantiomerically pure
carbocyclic sugars.
The mixture of lactones was reduced with LiAlH4 to result (ꢂ)-9
(the epimerized product of (ꢂ)-10) and inseparable mixture of
trihydroxy substituted cyclohexene derivatives (ꢂ)-10 as 20-deox-
yribosugar analogue and (ꢂ)-11 as 30-deoxyribosugar analogue.
Epimeric triol (ꢂ)-9 could be separated by column chromatog-
raphy and was characterized by NMR and mass spectroscopy. The
mixture of regioisomers (10 and 11) obtained was subjected to
chemoselective 1,3-diol protection and only compound (ꢂ)10 was
protected to furnish (ꢂ)12 quantitatively. The unreacted triol
(ꢂ)-11 could be then easily separated chromatographically from
(ꢂ)-12. After purification, the structure of triol (ꢂ)-11 was
D,L)-ribose sugar,
D,L
L
MeO
OMe
MeO
OMe
Cl
O
MeO
OMe
Cl
aq AcOH
120O
C
Na/Liq NH3
O
Cl
Cl
reflux,
60-80 %
+
THF-EtOH,
Cl
4h, 84%
Cl
OH
4a
20 min, 78%
OR
OAc
Cl
Cl
6
MeOH
5%H2SO4
5 h, 95%
3a R = Ac
3b R = H
2
1
BzCl,
py, 93%
O
MeO
OMe
OBz
aq AcOH
reflux, 81%
OBz
5
4b
Scheme 2. Synthesis of desired dehalogenated endo adduct 5.

5746
M. Varada et al. / Tetrahedron 67 (2011) 5744e5749
HO
HO
OH
O
OH
O
O
O
OH
O
mCPBA, dry
LAH, dry
(55%)
10
o
HO
DCM, Na2CO3
THF, -15 C
80%
+
+
+
4
6h, 92%
3
OBz
OBz
OH
OBz
OH
9
5
8
7
(30%)
(70%)
OH
OR
(15%)
(30%)
11
RO
PhCH(OCH3)2
dry dioxane
OH
O
+
10
(
11)
+
O
PTSA
OR
Ph
12
11, R=H
11a, R=Ac
Scheme 3. Synthesis and separation of the triols 9, 11 and protected 12.
confirmed by converting it to the known triacetate (ꢂ)-11a.15 All
the three isomers were easily purified in very good yield. The
separation protocol employed here avoids the difficult chromato-
graphic separation15 of the two regioisomers 7 and 8, which had to
be further individually processed to get 9, 10 and 11. With this we
completed the racemic synthesis of both the 3-deoxy-cyclohexenyl
sugar analogues (ꢂ)-11 and benzylidene protected 2-deoxy-cyclo-
hexenyl sugar analogues (ꢂ)-12 in high yields. Compound (ꢂ)-12
was then oxidized using CrO3/pyridine in less than 3 h to get the
enone (ꢂ)-13. The reported MnO2 oxidation reported earlier11 takes
very long time and depends mainly on the quality of MnO2 used in
the reaction. The use of CrO3 reduces the reaction time and is highly
reproducible. Compound (ꢂ)-13 was then subjected to Luche re-
duction to get epimeric alcohol (ꢂ)-14 using NaBH4 in the presence
of CeCl3$7H2O (Scheme 4).11 Compound (ꢂ)-14 can then be con-
verted to the cyclohexenyl nucleosides units using reported
procedures.19
by using CCL for esterification using vinyl acetate as acyl donor in
diethyl ether solvent. The acylation was enantioselective, giving the
acetate (48% yield and >98% ee) in 3 h (Scheme 5). The enantio-
meric identity of the resolved acetate 3a and ent-3b was estab-
lished after comparing with the known compound and the
enantiomeric purity was established by chiral HPLC analysis and 1H
NMR using chiral shift reagent (Supplementary data). Following the
same reactions as in Schemes 2e4, compounds 9, 11, 12 and 14, can
now be synthesized in enantiomerically pure form from enantio-
merically pure DielseAlder adducts 3a/b.
MeO
OMe
Cl
MeO
Cl
Cl
OMe
Cl
Cl
Cl
50% w/w CCL, MeCN
pH 7.2, phosphate buffer
Cl
45o C, 4days, 45%, >98%ee
Cl
OH
OAc
ent-3b
rac-3a
O
OH
OH
NaBH4
CrO3.Py, Ac2O
dry DCM, 1.5 h
MeO
Cl
Cl
MeO
OMe
Cl
OMe
Cl
CeCl3.7H2O
MeOH, 80%
O
O
O
92%
Cl
Cl
2, 20% w/w CCL, Et O
2
O
Ph
O
Ph
O
12
Ph
3 h, 48% >98% ee
Cl
14
13
Cl
OAc
OH
Scheme 4. Epimerization at C-1.
ent-3a
rac-3b
Scheme 5. Enzymatic resolution of 3a/3b.
After achieving the synthesis of racemic 9, 11, 12 and 14, we
undertook the enantiopure synthesis of these sugar analogues.
Enzymatic resolution of compound (ꢂ)-3b is known in the litera-
ture using Candida cylindracea lipase (CCL) and vinyl acetate as an
acyl donor.20
In our laboratory this reaction was very sluggish, took almost 7
days to complete and could not be scaled up. As the acylation was
sluggish but enantiospecific, we thought of doing enzymatic hy-
drolysis of (ꢂ)-3a using the same lipase enzyme. Use of acetonitrile
as co-solvent along with phosphate buffer (pH 7.2) gave very good
enantioselective hydrolysis in 4 days (Scheme 5, yield 45%, ee 98%).
Continuing the hydrolysis of the remaining enantiomerically
enriched acetate for 24 h, the other isomer as unreacted acetate 3a
was obtained in good yield and high ee (yield 42%, ee 98%). Alter-
natively, the improvement of the resolution time could be achieved
As the enzymatic resolution at such an early stage to get the
different sugar analogues, such as 14 and enantiopure compound
12, ent-12 would be time consuming, we embarked to resolve the
two enantiomers of (ꢂ)-12 using enzymatic acylation. Compound
(ꢂ)-12 was subjected to acylation using vinyl acetate as donor and
C. cylindracea lipase (CCL) to get 15 in excellent yield and high
enantiomeric purity (Scheme 6). The high enantiomeric purity was
established by 1H NMR using chiral shift reagent (Supplementary
data). The enantiomeric identity of the resolved acetate 15 and ent-
12 was established after converting the acetate 15 (hydrolysis of the
acetate followed by oxidation and reduction as described in
Scheme 4) to the known compound, i.e., enantiomerically pure
D
-isomer 14 and comparing the reported19 HPLC retention time on
chiral HPLC column (Supplementary data). The enriched ent-12 was

M. Varada et al. / Tetrahedron 67 (2011) 5744e5749
5747
further treated under same enzymatic conditions and ent-12 could
(7.82 mL, 90.9 mmol) and allowed to stir at 120 ꢁC for 5 h. The
excess vinyl acetate was removed in vacuo and the residue was
purified by silica gel chromatography (pet. ether/EtOAc¼97:3) to
afford exclusively the endo adduct (ꢂ)-3a (13.36 g) in 84% yield.
be isolated by silica gel chromatography. Synthesis of pure 20deoxy-
D
-sugar analogue ent-12 was not possible by the earlier reported
route.11
1H NMR (200 MHz, CDCl3):
d
5.51 (dd, 1H, J¼7.7, 2.3 Hz, CHOAc),
3.60 (s, 3H, OMe), 3.56 (s, 3H, OMe), 2.83 (dd, 1H, J¼12.6, 7.8 Hz,
CH2), 2.07 (s, 3H,OAc), 1.76 (dd, 1H, J¼12.8, 2.6 Hz, CH2); 13C NMR
OH
OAc
OH
(50 MHz, CDCl3):
43.8, 20.6.
d 170.2, 131.0, 127.8, 111.8, 76.4, 73.9, 52.6, 51.7,
2, 20%w/w
CCL, Et2O, 4 h
Ph
+
O
O
O
4.1.2. (ꢂ) (1S,2S,4R)-1,4,5,6-Tetrachloro-7,7-dimethoxybicyclo[2.2.1]
hept-5-en-2-ol (3b). To a solution of (ꢂ)-3a (20 g, 57.14 mmol) in
methanol (150 mL) was added 5% aqueous H2SO4 (40 mL) and
stirred at 65 ꢁC for 5 h. Methanol was removed on rotavapor in
vacuo. The residue was diluted with EtOAC and water, saturated
aqueous NaHCO3 and brine wash were given. Organic layer was
dried over Na2SO4, filtered, concentrated in vacuo and purified by
silica gel chromatography (pet. ether/EtOAc¼92:8) to result 3b
(16.8 g) in 95% yield as a white solid, along with the recovered 3a
(400 mg, 2% yield).
O
Ph
O
Ph
O
enriched ent-12
rac-12
15
(44% 98%ee) (56 % 87%ee)
OH
OH
1H NMR (200 MHz, CDCl3):
d 4.66 (m, 1H, CHOH), 3.58 (s, 3H,
O
O
OMe), 3.55 (s, 3H, OMe), 2.67 (dd, 1H, J¼12.4, 8.0 Hz, CH2), 2.12 (d,
1H, J¼4.8 Hz), 1.79 (dd, 1H, J¼12.3, 2.4 Hz, CH2); 13C NMR (200 MHz,
Ph
O
Ph
14 98 % ee
O
CDCl3):
d 130.8, 127.3, 112.0, 79.8, 76.4, 74.2, 52.5, 51.6, 44.2.
12
(D)
Scheme 6. Synthesis of 14 via enzymatic resolution and epimerization of 12.
4.1.3. (ꢂ)(1S,2S,4S)-7,7-Dimethoxybicyclo[2.2.1]hept-5-en-2-ol
(4a). To a suspension of liquid ammonia (700 mL) and sodium
(4.78 g, 207.8 mmol) at ꢀ78 ꢁC, solution of (ꢂ)-3b (8 g, 25.97 mmol)
in 10/1 mixture of THF (100 mL)/EtOH (10 mL) was added dropwise.
After completion of addition, stirring was continued for 15 min,
reaction mixture was quenched with saturated aqueous NH4Cl
solution and kept at rt overnight to allow liquid ammonia to
evaporate. THF/EtOH was removed on rotavapor in vacuo, residue
was diluted with DCM. Water wash and brine wash were given to
the organic layer, dried over Na2SO4, filtered, concentrated in vacuo
and purified by silica gel chromatography (pet. ether/EtOAc¼85:15)
to result (ꢂ)-4a (3.4 g) in 78% yield as a pale yellow thick liquid.
3. Conclusions
In conclusion, this paper outlines a robust method to synthesize
important cyclohexenyl-2-deoxyribose sugar and 3-deoxyribose
sugar analogues in excellent yields from commercially available
starting materials. The synthesis of both the sugars in enantiopure
form can be achieved from the resolved DA adducts. The enzymatic
resolution of cyclohexene analogue of 2-deoxyribose sugar was
achieved in high yields and enantiomeric purity. The easy access to
these sugars as outlined in this paper would allow further exploi-
tation of the cyclohexenyl as well as cyclohexane nucleic acid an-
alogues. These synthons will have applications not only in the
synthesis of nucleoside/oligonucleotide analogues but also in car-
bohydrate chemistry where modified carbasugars as sugar mimics
have potential applications.21
1H NMR (200 MHz, CDCl3):
d 6.49 (m, 1H, Alkene CH), 6.10 (m,
1H, Alkene CH), 4.85 (s, 1H, OH), 4.56 (m, 1H, CHOH), 3.26 (merged,
1H, CH), 3.18 (s, 3H, OMe), 3.15 (s, 3H, OMe), 2.87 (m, 1H, CH), 2.42
(m, 1H, CH2), 0.85 (dd, 1H, J¼12.3, 2.2 Hz, CH2); 13C NMR (50 MHz,
CDCl3):
d 137.9, 128.3, 119.2, 76.7, 70.2, 51.6, 50.7, 49.6, 45.6, 36.2.
4.1.4. (ꢂ) (1S,2S,4S)-7,7-Dimethoxybicyclo[2.2.1]hept-5-en-2-yl
benzoate (4b). To a solution of (ꢂ)-4a (9 g, 17.6 mmol) in pyridine
(45 mL) was added benzoyl chloride (8.1 mL, 58.23 mmol) and the
reaction mixture was stirred at rt for 4 h. Pyridine was removed in
vacuo and the residue was diluted with EtOAc. Water wash and brine
wash were given to the organic layer, dried over Na2SO4, concen-
trated in vacuo. The residue was purified by silica gel chromatogra-
phy (pet. ether/EtOAc¼89:11) to afford (ꢂ)-4b (13.5 g) in 93% yield.
4. Experimental section
4.1. General
All the non-aqueous reactions were carried out under the inert
atmosphere of Nitrogen/Argon and the chemicals used were of
laboratory or analytical grade. All solvents used were dried and
distilled according to standard protocols. TLCs were carried out on
1H NMR (200 MHz, CDCl3):
d 7.99e7.94 (m, 2H, Aromatic),
pre-coated silica gel GF
sheets (Merck 5554). Column chro-
7.55e7.27 (m, 3H, Aromatic), 6.43 (m, 1H, Alkene), 6.11 (m, 1H, Al-
kene), 5.61 (m, 1H, CHOBz), 3.41 (m, 1H, CH), 3.26 (s, 3H, OMe), 3.19
(s, 3H, OMe), 2.97 (m, 1H, CH), 2.58e2.48 (m, 1H, CH2), 1.18 (dd, 1H,
254
matographic separations were performed using silica gel
60e120 mesh (Merck) or 200e400 mesh (Merck) and using the
solvent systems EtOAc/pet. ether and MeOH/DCM. IR spectra were
recorded on an infrared Fourier Transform spectrophotometer us-
ing chloroform or neat. 1H and 13C spectra were obtained using
Bruker AC-200, AC-400 and AC-500 NMR spectrometers. The
J¼12.5, 2.4 Hz, CH2); 13C NMR (50 MHz, CDCl3):
d 166.4, 136.0, 133.6,
132.8, 130.1, 129.5, 128.3, 119.0, 73.8, 51.9, 49.9, 48.7, 45.2, 33.4;
HRMS: mass calculated for C16H18O4Na (MþNa)þ 297.1102, ob-
served 297.1113; IR (CHCl3): nmax 2950, 1720, 1602 cmꢀ1
chemical shifts are reported in delta (d) values and referred to in-
ternal standard TMS for 1H. Enzyme C. cylindracea lipase was pur-
chased from Ltd. Aldrich Inc.
4.1.5. (ꢂ) (1S,2S,4S)-7-Oxobicyclo[2.2.1]hept-5-en-2-yl benzoate
(5). Solution of (ꢂ)-4b (4.5 g,16.4 mmol) in 180 mL of 6/1 mixture of
acetic acid/water was refluxed for 4 h. Solvent was removed in vacuo,
residue diluted with the EtOAc and water wash, saturated aqueous
NaHCO3 wash and finally brine wash were given. The organic layer
was dried over Na2SO4, concentrated in vacuo. 1H NMR (200 MHz,
4.1.1. (ꢂ)(1S,2S,4R)-1,4,5,6-Tetrachloro-7,7-dimethoxybicyclo[2.2.1]
hept-5-en-2-yl acetate (3a). To 5,5-dimethoxy-1,2,3,4-tetrachloro-
cyclopentadiene (12 mL, 45.45 mmol) was added vinyl acetate

5748
M. Varada et al. / Tetrahedron 67 (2011) 5744e5749
CDCl3):
d
8.06e7.94 (m, 2H, Aromatic), 7.58e7.39 (m, 3H, Aromatic),
65.2, 40.4, 37.2; HRMS: mass calculated for C14H17O3 (MþH)þ
233.1177, observed (MþH)þ 233.1173.
6.79 (m, 1H, Alkene CH), 6.50 (m, 1H, Alkene CH), 5.68e5.60 (m, 1H,
CHOBz), 3.57 (m, 1H, CH), 3.09 (m, 1H, CH), 2.66e2.53 (m, 1H, CH2),
1.49 (dd, 1H, J¼13.5, 3.0 Hz, CH2); 13C NMR (50 MHz, CDCl3):
d
201.0,
4.1.9. (ꢂ) (4aR,8aS)-2-Phenyl-8,8a-dihydro-4H-benzo[d][1,3]dioxin-
7(4aH)-one (13). To a solution of CrO3 (90 mg, 0.9 mmol), Ac2O
(0.085 mL, 0.9 mmol), pyridine (0.14 mL, 1.81 mmol) in dry DCM
(15 mL), was added 5 mL solution of 12 (200 mg, 0.9 mmol) in DCM
stirred for 1.5 h at rt. Reaction mixture was filtered on Celite and
purified by silica gel chromatography (pet. ether/EtOAc¼91:9) to
afford 13 (185 mg) in 92% yield.
166.1, 134.4, 133.3, 133.2, 129.7, 129.5, 128.9, 128.4, 69.0, 51.8, 47.4,
32.5, IR (CHCl3): nmax 3064, 2947, 1790, 1720, 1600 cmꢀ1
.
4.1.6. (ꢂ) (1S,4S,8S,)-3-Oxo-2-oxabicyclo[2.2.2]oct-5-en-8-yl benzo-
ate (7). To a solution of (ꢂ)-5 (5 g, 21.1 mmol) in 100 mL of dry
DCM, was added Na2CO3 (2.23 g, 21.1 mmol), stirred and cooled to
0
ꢁC. m-CPBA (5.2 g, 21.1 mmol) was added to this suspension and
1H NMR (200 MHz, CDCl3):
d 7.55 (m, 2H, Aromatic), 7.43e7.38
stirred it for 6 h at rt. The reaction mixture was quenched with 10%
aqueous solution of Na2S2O5 (30 mL). Organic layer was separated
and aqueous layer was extracted with DCM. The combined organic
layer was washed with saturated aqueous NaHCO3 followed by
brine, and dried over anhydrous Na2SO4. The organic layer was
concentrated in vacuo followed by silica gel chromatography (pet.
ether/EtOAc¼90:10) to give a mixture of 7 and 8 (92%) in 70:30
ratio along with the recovered 5 (4%).
(m, 3H, Aromatic), 6.63 (dd, 1H, J¼9.8, 1.8 Hz, Alkene CH), 6.18 (m,
1H, Alkene CH), 5.64 (s, 1H, CHPh), 4.49 (dd, 1H, J¼10.9, 4.5 Hz,
CH2O), 4.07 (m, 1H, CHeO), 3.81 (t, 1H, J¼11.3 Hz, CH2O), 3.02e2.86
(m, 2H, CH2,CH), 2.68 (dd, 1H, J¼16.4, 12.8 Hz, CH2); 13C NMR
(50 MHz, CDCl3): d 196.8,145.0,137.5,132.1,129.2,128.4,126.1,101.7,
76.5, 69.3, 44.5, 40.0; IR (CHCl3) : nmax 1720, 1610 cmꢀ1
.
4.1.10. (ꢂ) (4aR,7R,8aS)-2-Phenyl-4a,7,8,8a-tetrahydro-4H-benzo[d]
[1,3]dioxin-7-ol (14). To a solution of 13 (165 mg, 0.717 mmol) in
10 mL dry MeOH, was added CeCl3$7H2O (400 mg, 1.07 mmol)
stirred for 1 h at rt. NaBH4 (33 mg, 0.86 mmol) was added in por-
tions, stirred for 2 h at rt. Reaction was quenched with crushed ice
and stirred for 30 min. Reaction mixture concentrated in vacuo,
residue was dissolved in EtOAc and washed with water, brine.
EtOAc layer was dried over Na2SO4 concentrated in vacuo and pu-
rified by silica gel chromatography (pet. ether/EtOAc¼90:10) to
afford 14 (135 mg) in 82% yield.
1H NMR (200 MHz, CDCl3):
d 7.98 (m, 2H, Aromatic), 7.56e7.41
(m, 3H, Aromatic), 6.78 (m, 1H, minor), 6.72 (m, 1H, major) 6.54 (m,
1H), 5.60 (m, 1H, minor), 5.48 (m, 1H, major), 5.42 (m, 1H, minor),
5.32 (m, 1H, major), 4.05 (m, 1H, major), 3.57 (m, 1H, minor), 2.85
(m, 1H, major), 2.60 (m, 1H, minor), 1.75 (d, J¼6.0 Hz, 1H, major),
1.59 (m, 1H, minor); 13C NMR (50 MHz, CDCl3):
d 172.2, 170.7, 165.5,
134.3, 133.4, 133.3, 132.5, 129.5, 129.3, 129.0, 128.9, 128.4, 128.2,
76.6, 73.5, 73.0, 68.8, 65.5, 46.2, 40.3, 35.0, 29.3; HRMS: mass cal-
culated for C14H13O4 (MþH)þ 245.0813, observed (MþH)þ
245.0812; IR (CHCl3): nmax 1759,1716 cmꢀ1
1H NMR (200 MHz, CDCl3):
d 7.52 (m, 2H, Aromatic), 7.39 (m, 3H,
Aromatic), 5.79 (m, 1H, Alkene CH), 5.61 (s, 1H, CHPh), 5.47 (m, 1H,
Alkene CH), 4.55 (m, 1H, CHOH), 4.32 (dd, 1H, J¼10.8, 4.4 Hz, CH2O),
3.76e3.57 (m, 2H, CHeO, CH2O), 2.59e2.49 (m, 2H, CH2, CH),
4.1.7. (ꢂ) (1S,3S,6R)-6-(Hydroxymethyl) cyclohex-4-ene-1,3-diol
(10). To a solution of mixture 7,8 (3.3 g, 13.5 mmol) in dry THF
(400 mL) at ꢀ15 ꢁC, LAH (1.5 g, 40.5 mmol) was added and the
resulting mixture was stirred at the same temperature for 2 h. The
reaction mixture was then quenched cautiously with ethyl acetate
(50 mL) followed by saturated aqueous solution of Na2S04, to pre-
cipitate out aluminium salts. After the filtration, filtrate was con-
centrated in vacuo, to result epimeric mixture of 9, 10 and the other
regioisomer 11. The mixture was purified by silica gel chromatog-
raphy (MeOH/EtOAc¼1:99) to give pure 9 (15%) and a mixture of 10
and 11 in 85% yield, which was used further without separation.
1.89e1.73 (m, 1H, CH2);13C NMR (50MHz,CDCl3):
d138.0, 132.8, 129.0,
128.4, 126.2, 124.8, 102.17, 76.5, 70.7, 67.7, 40.0, 38.3. LCMS: mass cal-
culated for C14H16O3Na (MþNa)þ 255.1, observed (MþNa)þ 255.3.
4.2. Enzymatic resolution reactions
4.2.1. Enzymatic hydrolysis of racemic 3a. To a solution of racemic
compound 3a (500 mg,1.43 mmol) in 10 mL acetonitrile, was added
5 mL of 1 M phosphate buffer with pH 7.2 and 20% (w/w) of C.
cylindracea lipase (CCL) stirred for 4 days at 45 ꢁC. Enzyme was
filtered, filtrate was concentrated in vacuo and subjected for col-
umn chromatography (pet. ether/EtOAc¼91:9) to yield enantiopure
1H NMR (200 MHz, CDCl3) of compound 9:
d 5.86e5.72(m, 2H,
Alkene CH), 4.38e4.32 (m, 1H, allylic CHeO), 4.14e4.08 (m, 1H,
CH2O), 3.87e3.79 (m, 1H, CH2O), 3.68e3.60 (m, 1H, CHeO), 2.50 (m,
1H, Allylic CH), 2.11e2.02 (m, 1H, CH2), 1.88e1.73 (m, 1H, CH2); 13
C
3b (45%, 98% ee) [
a]
20 ꢀ26.1ꢁ (c 1.05g/100 mL in CHCl3).
D
NMR (50 MHz, CDCl3):
d 130.5, 127.9, 66.1, 65.4, 61.7, 41.5, 35.3.
LCMS: mass calculated for C7H12O3Na (MþNa)þ 167.06, observed
4.2.2. Enzymatic acylation of racemic 3b. To a solution of racemic
compound 3b (500 mg, 1.64 mmol) in 15 mL diethyl ether, was
added vinyl acetate (0.15 mL, 1.64 mmol) and 20% (w/w) of C.
cylindracea lipase (CCL) stirred for 3 h at rt. Enzyme was filtered,
filtrate was concentrated in vacuo and subjected for column chro-
matography (pet. ether/EtOAc¼91:9) to yield enantiopure 3a (48%,
(MþNa)þ 167.17.
4.1.8. (ꢂ) (4aR,7S,8aS)-2-Phenyl-4a,7,8,8a-tetrahydro-4H-benzo[d]
[1,3]dioxin-7-ol (12). Mixture of compounds 10 and 11 (3g,
21 mmol) was dissolved in dry dioxane, and benzaldehyde dime-
thylacetal (6 mL, 27 mmol), PTSA (200 mg,1.05 mmol) was added to
it slowly and the reaction was stirred at rt for 24 h. Reaction mixture
was quenched with ice and stirred for 30 min. Extracted with EtOAc
three times, combined organic layer was washed with water and
brine solution, dried over Na2SO4 and concentrated in vacuo, fol-
lowed by silica gel chromatography (pet. ether/EtOAc¼88:12) to
afford (ꢂ)-12 in 70% yield.
98% ee). [
a]
20 ꢀ8.0ꢁ (c 1.01g/100 mL in CHCl3).
D
4.2.3. (þ)(4aR,7S,8aS) 7-O-acetyl-2-phenyl-4a,7,8,8a-tetrahydro-4H-
benzo[d][1,3]dioxin-7-ol (15). To a solution of compound (ꢂ)-12
(500 mg, 2.15 mmol) in 15 mL diethyl ether, was added vinyl acetate
(0.2 mL, 2.15 mmol) and 20% (w/w) of CCL stirred for 3 h at rt.
Enzyme was filtered, filtrate was concentrated in vacuo and sub-
jected to column chromatography (pet. ether/EtOAc¼94:6) to yield
15 (44%, >98% ee) and enriched ent-12 (56%, 87% ee). 1H NMR
1H NMR (200 MHz, CDCl3):
d 7.55e7.50 (m, 2H, Aromatic),
7.41e7.36 (m, 3H, Aromatic), 5.89e5.83 (m, 1H, Alkene CH), 5.65 (s,
1H, CHPh), 5.53 (dd, 1H, J¼9.7, 1.5 Hz, Alkene CH), 4.41 (m, 1H,
CHOH), 4.34 (dd, 1H, J¼10.7, 4.5 Hz, CH2O), 3.89e3.83 (m, 1H,
CHeO), 3.70 (t, 1H, J¼11.4 Hz, CH2O), 2.48e2.46 (m, 1H, OH),
2.25e2.16 (m, 1H, CH), 2.03e1.87 (m, 2H, CH2); 13C NMR (50 MHz,
(200 MHz, CDCl3):
d 7.55(m, 2H, Aromatic), 7.39 (m, 3H, Aromatic),
5.89e5.81 (m, 1H, Alkene CH), 5.66 (s, 1H, CHPh), 5.46 (m, 1H, Al-
kene CH), 4.33 (dd, 1H, J¼10.8, 4.5 Hz), 3.94e3.81 (m, 1H), 3.73 (t,
1H, J¼11.2 Hz), 2.56e2.45 (m, 1H), 2.26e2.18 (m, 1H), 2.09 (s, 3H)
20
CDCl3):
d
138.1, 130.2, 128.9, 128.3, 126.8, 126.1, 102.3, 75.3, 70.6,
2.07e1.94(m, 1H); [
a]
ꢀ150.4ꢁ (c 1.1g/100 mL in CHCl3) LCMS:
D

M. Varada et al. / Tetrahedron 67 (2011) 5744e5749
5749
mass calculated for C16H18O4Na (MþNa)þ 297.1103, observed
(MþNa)þ 297.1081.
6. Wang, J.; Verbeure, B.; Luyten, I.; Lescrinier, E.; Froeyen, M.; Hendrix, C.;
Rosemeyer, H.; Seela, F.; Van Aerschot, A.; Herdewijn, P. J. Am. Chem. Soc. 2000,
122, 8595e8602.
7. (a) Wang, J.; Verbeure, B.; Luyten, I.; Froeyen, M.; Hendrix, C.; Rosemeyer, H.;
Seela, F.; Van Aerschot, A.; Herdewijn, P. Nucleosides, Nucleotides Nucleic Acids
2001, 20, 785e788; (b) Wang, J.; Verbeure, B.; Luyten, I.; Froeyen, M.; Hendrix,
C.; Rosemeyer, H.; Seela, F.; Van Aerschot, A.; Herdewijn, P. Nucleic Acids Res.
2001, 29, 4941e4947.
Acknowledgements
V.M. and V.K. thank CSIR, New Delhi for Junior Research Fel-
lowship, V.A.K. thanks CSIR, New Delhi for research grant
(NWP0036).
8. Kalota, A.; Karabon, L.; Swider, C. R.; Viazovkina, E.; Elzagheid, M.; Damha, M. J.;
Gewirtz, A. M. Nucleic Acids Res. 2006, 34, 451e461.
9. Nauwelaerts, K.; Fisher, M.; Froeyen, M.; Lescrinier, E.; Van Aerschot, A.; Xu, D.;
DeLong, R.; Kang, H.; Juliano, R. L.; Herdewijn, P. J. Am. Chem. Soc. 2007, 129,
9340e9348.
Supplementary data
10. Wang, J.; Herdewijn, P. J. Org. Chem. 1999, 64, 7820e7827.
11. Wang, J.; Morral, J.; Hendrix, C.; Herdewijn, P. J. Org. Chem. 2001, 66,
8478e8482.
1H, 13C, LCMS, HRMS analysis of relevant compounds, chiral
HPLC and 1H NMR with chiral shift reagents. Supplementary data
associated with this article can be found, in the online version, at
doi:10.1016/j.tet.2011.06.003.
12. Wislicenus, W. Chem. Ber. 1887, 20, 2930e2934.
13. Danishefsky, S.; Kitahara, T. J. Am. Chem. Soc. 1974, 96, 7807e7808.
14. (a) Gunjal, A. D.; Erande, N.; D’Costa, M.; Kumar, V. A. Nucleic Acids Symp. Ser.
2008, 191e192; (b) Erande, N. D.; Gunjal, A. D.; Fernandes, M.; Kumar, V. A.
Chem. Commun. 2011, 4007e4009.
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