C-C bond formation by radical cyclization: Regioselective synthesis of [6,6] pyrano pyran system

Article abstract of DOI:10.1080/00397910701266125

A number of 4-aryloxymethyl pyrano[3,2-c][1]benzopyran-5-(2H)-ones 3(a-f) were prepared by refluxing 4-chloromethyl pyrano[3,2-c][1]benzopyran-5-(2H)-ones 1(a,b) with o-bromophenols 2(a-c) in acetone in the presence of anhydrous potassium carbonate and so

Full text of DOI:10.1080/00397910701266125

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C‐C Bond Formation by Radical Cyclization:
Regioselective Synthesis of [6,6] Pyrano
pyran System
K. C. Majumdar a & M. Jana a
a Department of Chemistry , Kalyani, India
Published online: 22 May 2007.
To cite this article: K. C. Majumdar & M. Jana (2007) C‐C Bond Formation by Radical Cyclization:
Regioselective Synthesis of [6,6] Pyrano pyran System, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 37:10, 1735-1745, DOI:
10.1080/00397910701266125
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Synthetic Communicationsw, 37: 1735–1745, 2007
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701266125
C-C Bond Formation by Radical
Cyclization: Regioselective Synthesis of [6,6]
Pyrano pyran System
K. C. Majumdar and M. Jana
Department of Chemistry, Kalyani, India
Abstract: A number of 4-aryloxymethyl pyrano[3,2-c][1]benzopyran-5-
(2H)-ones 3(a–f) were prepared by refluxing 4-chloromethyl pyrano[3,2-
c][1]benzopyran-5-(2H)-ones 1(a,b) with o-bromophenols 2(a–c) in
acetone in the presence of anhydrous potassium carbonate and sodium
iodide in 70–80% yield. Compounds 3(a–f) were then subjected to radical
cyclization by refluxing with tri-n-butyltin chloride, sodium cyanoborohy-
dride, and azobisisobutyronitrile (AIBN) in dry benzene for 4–5 h to give a
cis-diastereomeric mixture of [6,6]pyranopyran derivatives in 80–85% yield.
Keywords: azobisisobutyronitrile, o-bromophenol, 6-endo cyclization, radical cycliza-
tion, sodium cyanoborohydride, tri-n-butyltin chloride
INTRODUCTION
In recent years, radical cyclization has emerged as a valuable tool for the con-
struction of carbo- and heterocyclic compounds, including natural products.[1]
An understanding of the kinetic and the structural information of these
reactive intermediates paved the way for development of the modern
synthetic radical chemistry.[2] During our work on the synthesis of hetero-
cycles by the application of sigmatropic rearrangements,[3] we recently
observed the unusual formation of [6,6]pyranopyrans in case of substrates
containing 5-hydroxypyrimidine[4] and 3-hydroxy coumarin[5] in the second
Claisen rearrangement step. The generation and subsequent reactions of
Received October 16, 2006
Address correspondence to K. C. Majumdar, Department of Chemistry, Kalyani
741 235, W.B., India. E-mail: kcm_ku@yahoo.co.in
1735

1736
K. C. Majumdar and M. Jana
radicals formed from aryl halides using tri-n-butyltinhydride and azobisisobu-
tyronitrile (AIBN) is now well established.[6] Aryl radical cyclization
normally has a high 5-exo/6-endo ratios, indicating a stronger preference
for exo cyclization than alkyl radicals. However, this preference was
reversed in cyclizations involving stabilized radicals.[7] Coumarin and its
derivatives are reported[8] to possess various physiological activities.
3-Alkyl and 4-alkyl coumarins are well known[9] for their hypnotic, insectici-
dal, and antifungal activities, their anticoagulant effect on blood, and diuretic
properties. Various heterocyclic compounds have been synthesized using
4-hydroxy coumarin or its derivatives and analogues as starting materials.
Recently we reported[10] the synthesis of [6,6]pyranothiopyrano by
the application of sequential Claisen rearrangement followed by pyridine
hydrotribromide–mediated regioselective 6-endo cyclization. However,
[3,3] sigmatropic rearrangement of 4-aryloxymethyl pyrano[3,2-c]coumarin
derivatives failed to afford any cyclic products.[11] We, therefore, explored
the possibility of whether the [6,6]pyranopyran derivatives can be synthesized
by tri-n-butyl tin hydride and AIBN-induced radical cyclization of appropriate
substrates 3(a–f). Herein we report the results.
RESULTS AND DISCUSSION
4-Chloromethylpyrano [3,2-c][1]benzopyran-5-(2H)-ones 1(a,b) were
refluxed with o-bromophenols 2(a–c) in acetone in the presence of
anhydrous potassium carbonate and sodium iodide (Finkelstein condition) to
afford a number of 4-aryloxymethyl pyrano[3,2-c][1] benzopyran-5-(2H)-
ones 3(a–f) (Scheme 1), which have been utilized as the starting material
Scheme 1. Reagents and Conditions: i) Me2 C55O/K2CO3; NaI, 4-5 h.

C-C Bond Formation by Radical Cyclization
1737
Scheme 2. Reagents and Conditions: i) Me2 C55O/K2CO3, Reflux, 20 h ii) Chloro-
benzene, Reflux, 16 h.
of this study (Scheme 1). Compounds 1(a,b) in turn were prepared by the
following literature procedure,[12] that is, by alkylation of 4-hydroxycoumarin,
followed by Claisen rearrangement (Scheme 2).
Compounds 3(a–f) were characterized from their elemental analyses
and spectral data. IR spectrum of the compound 3a showed carbonyl absorp-
1
tion at 1700 cm21. The high field H NMR (300-MHz) spectrum of the
compound 3a exhibited a two-proton doublet at d 4.96 for -OCH2, two-
proton doublet at d 5.13 for another -OCH2, and one proton triplet at d
6.14 for the vinylic proton among other signals for aromatic protons.
The substrate 3a was then refluxed in benzene under a nitrogen atmosphere
with tri-n-butyl tin chloride and sodium cyanoborohydride in the presence of a
catalytic amount of azobisisobutyronitrile (AIBN) for 4 h to give cyclic
product 7a. Thin-layer chromatography (TLC) indicated a single product.
1
However, H NMR revealed that it was actually a mixture of two compounds.
In the aromatic region, the protons displayed chemical shifts at nearly similar
positions whereas in the aliphatic region they showed different chemical shifts.
The positions, splitting patterns, and coupling constants of the protons at the
ring junctures implies that they are diastereomeric in nature, both with cis ring
juncture,[13] as their coupling constant is less than 8 Hz. We tried our best to
separate these two products, but all our efforts miserably failed. A few years
ago, we reported the pyranopyran system obtained by thermal Claisen rearrange-
ment with cis-fusion. However, there we obtained only one cis diastereomer.[14]
We tried to obtain only one diastereomer by changing the reaction condition.
At first, the reaction was carried out by refluxing a suspension of the
compounds 3(a–f) (0.46 mmol). Bu3SnH (0.52 mmol) and AIBN (0.28 mmol)
in dry benzene (12 ml) were refluxed for 4–5 h under a nitrogen atmosphere.
In all the cases, diastereomeric mixtures were obtained except in case of 7a.
n
Next the compounds 3(a–f) (0.46 mmol), Bu3SnCl (1.8 mmol), Na(CN)BH3
(3.7 mmol), and AIBN (0.12 mmol) were refluxed in dry benzene for 4–5 h
under a nitrogen atmosphere. Here too, a mixture of diastereomers was

1738
K. C. Majumdar and M. Jana
Figure 1. cis-[6,6]fused compounds.
obtained except in the case of 7a. We therefore failed to produce a single
diastereo isomer. However, in one case (7a), nearly pure (95:5) one diastereomer
was obtained.
Compound 7a was characterized from its elemental analysis and spectro-
scopic data. The IR spectrum of the compound 7a (for the mixture) showed
1
carbonyl absorption at 1703 cm21. H NMR of the compound 7a showed
the following major signals: d 2.18 (s, 3H, CH3), 2.26 (s, 3H, CH3), 2.42
(s, 3H, CH3) 3.22–3.26 (dt, ring juncture proton, 1H, J ¼ 3.8 Hz,
J ¼ 11.5 Hz), 3.38–3.42 (dt, ring juncture proton, 1H, J ¼ 3.8 Hz, J ¼ 11 Hz),
3.71–3.78 (ddꢀt, 1H, J ¼ 11 Hz), 4.12–4.20 (ddꢀt, 1H, J ¼ 11.5 Hz), 4.61–
4.66 (dd, 1H, J ¼ 3.9 Hz, J ¼ 11.2 Hz), 4.72–4.80 (dd, 1H, J ¼ 3.8 Hz,
1
J ¼ 10.5 Hz), 6.7–7.36 (m, 4H, ArH), 7.58 (s, 1H, ArH). The H NMR data
for the other diastereomers that are formed in other cases are presented in the
experimental section.
The cis-stereochemistry of the major diastereoisomer may be derived
1
1
from the comparison of H NMR data with the H NMR data analysis result
of our earlier published[14] cis-product (Fig. 1).
The minor diastereoisomer may also possess cis-stereochemistry as
indicated by low coupling constants of the ring juncture protons. The two dia-
stereoisomers may be represented by Fig. 2 and Fig. 3.
The generality of the reaction was tested by subjecting five other sub-
strates 3(b–f) under the same reaction conditions to give products 7(b–f)
(Scheme 3). The formation of products 7a–f from 3a–f may be easily
explained by the generation of an aryl radical 8 in the tri-n-butyltinchloride,
sodium cyanoborohydride, and azobisisobutyronitrile mediated reaction.
Figure 2. Another cis-diastereomer of compound 7.

C-C Bond Formation by Radical Cyclization
1739
Figure 3.
The aryl radical 8 may undergo cyclization by two different modes, a 6-endo
trig cyclization[15] to afford the heterocyclic radical 10 (pathway a) or a 5-exo
trig cyclization[16] to give the spiroheterocyclic radical 9 (not isolated,
pathway b). The possibility of the formation of heterocyclic radical 10 via
spirocylic radical 9 by a neophyl rearrangement[17] cannot be ruled out
(Scheme 4).
It is known that radical cyclizations leading to six-membered rings
are usually less general than cyclization leading to five-membered rings.
Six-membered ring-forming reactions are also slower than five-membered
ring-formation reactions and are subject to competitive formation of
reduced uncyclized by-products. However, appropriately substituted
5-hexenyl radicals are known to undergo 6-endo cyclization to give
six-membered rings. It is interesting to note that a regioselectively six-
membered heterocyclic ring is formed in all the cases studied at present.
Scheme 3. Reagents and conditions: i) Bu3SnCl, Na(CN)BH3; AIBN, C6H6, reflux,
N2 atmosphere, 4–5 h.

1740
K. C. Majumdar and M. Jana
Scheme 4.
In conclusion, the reaction is mild, regioselective, but not stereoselective.
This is an attractive and simple methodology for the synthesis of a [6, 6]pyr-
anopyran ring system.
EXPERIMENTAL
Melting points were determined in an open capillary and are uncorrected. UV
absorption spectra were recorded in EtOH on a Shimadzu UV-2401PC spec-
trophotometer (lmax in nm) and IR spectra in KBr discs on a Perkin Elmer L
120 000A apparatus (nmax in cm21). 1H NMR spectra were run in CDCl3 with
TMS as an internal standard on a Bruker DPX 300-MHz instrument at the
Indian Institute of Chemical Biology, Kolkata (chemical shifts in ppm).
Silica gel (60–120 mesh) was obtained from Spectrochem, India. Extracts
were dried over anhydrous sodium sulfate. Petroleum ether refers to the
fraction boiling between 608C and 808C.
General Procedure for the Preparation of Compounds 3(a–f)
Compounds (1a,b) (1 mmol) in acetone (100 ml) were refluxed with several
o-bromophenols 2(a–c) (1 mmol) in the presence of anhydrous potassium
carbonate (1 g) and a catalytic amount of Nal for 3–4 h. The reaction
mixture was then cooled and filtered, and the solvent was removed. The
residual mass was subjected to column chromatography over silica gel

C-C Bond Formation by Radical Cyclization
1741
using petroleum ether–ethyl acetate (19:1) as eluant to give compounds 3(a–f),
which were then recrystallized from chloroform.
Data
Compound 3a: Yield: 74%; white solid; mp 1598C; IR (KBr): nmax ¼ 1725,
1
1571, 1467, 1276 cm21; UV (EtOH): lmax ¼ 206, 280, 351 nm; H NMR
(CDCl3, 300 MHz): dH ¼ 2.25 (s, 3H, CH3), 2.28 (s, 3H, CH3), 2.41 (s, 3H,
CH3), 4.96 (d, 2H, J ¼ 2.1 Hz, OCH2), 5.13 (d, 2H, J ¼ 1.9 Hz, OCH2),
6.14 (t, 1H, J ¼ 1.9 Hz, 55CH), 6.91–6.94 (1H, dt, J ¼ 1.2 Hz, J ¼ 7.6,
ArH), 7.16–7.35 (m, 4H, ArH); MS: m/z ¼ 426, 428 (Mþ, Mþ2). Anal.
calcd. for C22H19O4Br: C, 61.83; H, 4.45%. Found C, 61.58; H, 4.21%.
Compound 3b: Yield: 72%; white solid; mp 908C; IR (KBr): nmax ¼ 1699,
1
1608, 1454, 1271 cm21; UV (EtOH): lmax ¼ 208, 283, 344 nm; H NMR
(CDCl3, 300 MHz): dH ¼ 2.36 (s, 3H, CH3), 2.41 (s, 3H, CH3), 4.94
(d, 2H, J ¼ 2.1 Hz, OCH2), 5.16 (d, 2H, J ¼ 1.8 Hz, OCH2), 6.12 (t, 1H,
J ¼ 1.9 Hz, 55CH), 7.26–7.57 (m, 5H, ArH), 7.79–7.81 (d, 1H, J ¼ 6 Hz,
ArH); MS: m/z ¼ 413, 415 (Mþ, Mþ2). Anal. calcd. for C21H17O4Br: C,
60.87; H, 4.12%. Found C, 60.76; H, 4.16%.
Compound 3c: Yield: 70%; white solid; mp 1158C; IR (KBr): nmax ¼ 1699,
1
1601, 1567, 1496 cm21; UV (EtOH): lmax ¼ 217, 289, 344 nm; H NMR
(CDCl3, 300 MHz): dH ¼ 2.27 (s, 3H, CH3), 2.41 (s, 3H, CH3), 5.10
(d, 2H, J ¼ 2.1 Hz, OCH2), 5.15 (d, 2H, J ¼ 1.8 Hz, OCH2), 6.07 (t, 1H,
J ¼ 1.9 Hz, 55CH), 6.91–6.94 (dt, 1H, J ¼ 1.2 Hz, J ¼ 7.5 Hz, ArH),
7.04–7.07 (1H, dd, J ¼ 1.2 Hz, J ¼ 8.1 Hz, ArH), 7.18–7.36 (m, 3H,
ArH), 7.58 (dd, 1H, J ¼ 1.3 Hz, J ¼ 7.8 Hz, ArH); MS: m/z ¼ 413, 415
(Mþ, Mþ2). Anal. calcd. for C21H17O4Br: C, 60.87; H, 4.12%; Found C,
60.57; H, 4.09%.
Compound 3d: Yield: 75%; white solid; mp 1208C; IR (KBr): nmax ¼ 1698,
1601, 1495, 1255 cm21; UV (EtOH): lmax ¼ 206, 283, 310, 323 nm; 1H NMR
(CDCl3, 300 MHz): dH ¼ 2.28 (s, 3H, CH3), 5.13 (d, 2H, J ¼ 2.03 Hz,
OCH2), 5.16 (d, 2H, J ¼ 1.8 Hz, OCH2) 6.08 (t, 1H, J ¼ 1.9 Hz, 55CH),
6.84–6.91 (dt, 1H, J ¼ 1.2 Hz, J ¼ 7.6 Hz, ArH), 7.03–7.11 (dd, 1H,
J ¼ 1.1 Hz, J ¼ 8.2 Hz, ArH), 7.26–7.58 (m, 4H, ArH) 7.78–7.81 (d, 1H,
J ¼ 7.8 Hz, ArH); MS: m/z ¼ 398, 400 (Mþ, Mþ2). Anal. calcd. for
C20H15O4Br: C, 60.15; H, 3.76%. Found C, 60.06; H, 3.54%.
Compound 3e: Yield: 70%; white solid; mp 1818C; IR (KBr): nmax ¼ 1709,
1619, 1565, 1497, 1448, 1202 cm21; UV (EtOH): lmax ¼ 211, 283, 340 nm;
1H NMR (CDCl3, 300 MHz): dH ¼ 2.41 (s, 3H, CH3), 5.11 (d, 2H,
J ¼ 2.1 Hz, OCH2), 5.19 (d, 2H, J ¼ 1.8 Hz, OCH2), 6.10 (t, 1H,

1742
K. C. Majumdar and M. Jana
J ¼ 1.9 Hz, 55CH), 6.86–6.90 (dt, 1H, J ¼ 1.2 Hz, J ¼ 7.5 Hz, ArH), 7.17–
7.21 (dd, 1H, J ¼ 1.1 Hz, J ¼ 8.1 Hz, ArH) 7.33–7.35 (m, 2H, ArH), 7.50–
7.54 (dd, 1H, J ¼ 1.2 Hz, J ¼ 7.9 Hz, ArH), 7.58 (d, 1H, J ¼ 6 Hz, ArH);
MS: m/z ¼ 398, 400 (Mþ, Mþ2). Anal. calcd. for C20H15O4Br: C, 60.15; H,
3.76%. Found C, 60.35; H, 3.54%.
Compound 3f: Yield: 70%; white solid; mp 1608C; IR (KBr): nmax ¼ 1702,
1
1611, 1480, 1031 cm21; UV (EtOH): lmax ¼ 205, 273, 281 nm; H NMR
(CDCl3, 300 MHz): dH ¼ 5.13 (d, 2H, J ¼ 2.1 Hz, OCH2), 5.19 (d, 2H,
J ¼ 1.8 Hz, OCH2), 6.10 (t, 1H, J ¼ 1.9 Hz, 55CH), 6.82–6.88 (dt, 1H,
J ¼ 1.2 Hz, J ¼ 7.6 Hz, ArH), 7.03–7.06 (dd, 1H, J ¼1.1 Hz, J ¼ 8.2 Hz,
ArH), 7.26–7.32 (m, 3H, ArH) 7.52–7.58 (m, 2H, ArH), 7.78–7.81 (dd,
1H, J ¼ 1.2 Hz, J ¼ 7.9 Hz, ArH); MS: m/z ¼ 384, 386 (Mþ, Mþ2). Anal.
calcd for C19H13O4Br: C, 59.22; H, 3.38%. Found C, 59.40; H, 3.26%.
General Procedure for the Preparation of Compounds 7(a–f) by
Radical Cyclization
n
A suspension of the compound 3a–f (0.46 mmol), Bu3SnCl (1.8 mmol),
Na(CN)BH3 (3.7 mmol), and AIBN (0.12 mmol) in 10 ml of dry benzene
was refluxed for 4–5 h under a nitrogen atmosphere. The solvent was evapor-
ated under reduced pressure, and the residue was taken in water (10 ml) and
was extracted with CHCl3 (3 ꢀ 10 ml). The combined organic extract was
washed with brine and dried (Na2SO4). Evaporation of the solvent furnished
a residue, which was magnetically stirred with a saturated solution of
potassium fluoride (5 ml) for 24 h. It was then extracted with CHCl3
(3 ꢀ 10 ml) and was washed with water for several times, and dried
(Na2SO4). The residual mass after removal of the solvent (CHCl3) was
subjected to column chromatography over silica gel using petroleum ether–
ethylacetate (97:3) as eluant to give cyclized products 7(a–f), which were
then recrystallized from chloroform-petroleum ether.
Data
Compound 7a: Yield: 82%; diastereomeric ratio (A : B) 95:5; white solid; mp
272–2758C; IR (KBr): nmax ¼ 1703, 1630, 1584, 1486 cm21; UV (EtOH):
1
lmax ¼ 207, 273, 284 nm; H NMR (CDCl3, 300 MHz) of the diastereomer
(A): dH ¼ 2.18 (s, 3H, CH3), 2.26 (s, 3H, CH3), 2.42 (s, 3H, CH3), 3.22–
3.26 (dt, ring juncture proton, 1H, J ¼ 3.8 Hz, J ¼ 11.5 Hz), 3.38–3.42 (dt,
ring juncture proton, 1H, J ¼ 3.8 Hz, J ¼ 11 Hz), 3.71–3.78 (ddꢀt, 1H,
J ¼ 11 Hz), 4.12–4.20 (ddꢀt, 1H, J ¼ 11.5 Hz), 4.61–4.66 (dd, 1H,
J ¼ 3.9 Hz, J ¼ 11.2 Hz), 4.72–4.80 (dd, 1H, J ¼ 3.8 Hz, J ¼ 10.5 Hz),
6.7–7.36 (m, 4H, ArH), 7.58 (s, 1H, ArH).

C-C Bond Formation by Radical Cyclization
1743
Compound 7b: Yield: 79%; diastereomeric ratio (A : B) 47:53; white solid;
mp 106–1108C; IR (KBr): nmax ¼ 1704, 1613, 1572, 1403 cm21; UV
(EtOH): lmax ¼ 208, 271, 281, 294 nm.1H NMR (CDCl3, 300 MHz) of the
diastereomer (A): dH ¼ 2.26 (s, 3H, CH3), 2.30 (s, 3H, CH3) 3.23–3.25
(m, ring juncture proton, 1H, overlap with other peaks), 3.40–3.46 (dt, ring
juncture proton, 1H, J ¼ 3.8 Hz, J ¼ 11 Hz), 3.72–3.79 (ddꢀt, 1H,
J ¼ 11 Hz), 4.13–4.20 (ddꢀt, 1H, J ¼ 11 Hz), 4.63–4.67 (dd, 1H,
J ¼ 3.8 Hz, J ¼ 11 Hz), 4.71–4.77 (dd, 1H, J ¼ 3.8 Hz, J ¼ 10.6 Hz),
6.70–7.81 (m, 6H, ArH).
1H NMR of other diastereomer (B): dH ¼ 2.26 (s, 3H, CH3), 2.30 (s, 3H,
CH3), 3.00–3.09 (dt, ring juncture proton, 1H, J ¼ 3.8 Hz, J ¼ 11 Hz), 3.23–
3.25 (m, ring juncture proton, 1H, overlap with other peaks), 3.94–3.96
(ddꢀt, 1H, J ¼ 10.8 Hz), 4.17–4.21 (ddꢀt, 1H, J ¼ 11 Hz), 5.13–5.18
(dd, 1H, J ¼ 3.8 Hz, J ¼ 10.6 Hz), 5.19–5.37 (dd, 1H, J ¼ 3.8 Hz,
J ¼ 10.5 Hz), 6.70–7.81 (m, 6H, ArH).
Compound 7c: Yield: 76%; diastereomeric ratio (A : B) 50:50; white solid;
mp 245–2508C; IR (KBr): nmax ¼ 1709, 1631, 1584, 1500 cm21; UV
(EtOH): lmax ¼ 207, 274, 285 nm; 1H NMR (CDCl3, 300 MHz) of the
diastereomer (A): dH ¼ 2.30 (s, 3H, CH3), 2.42 (s, 3H, CH3), 3.20–3.27
(m, ring juncture proton, 1H, overlap with other peaks), 3.41–3.46 (dt, ring
juncture proton, 1H, J ¼ 3.8 Hz, J ¼ 11 Hz), 3.72–3.80 (dd ꢀ t, 1H,
J ¼ 10.8 Hz), 4.13–4.20 (ddꢀt, 1H, J ¼ 11 Hz), 4.61–4.67 (dd, 1H,
J ¼ 3.8 Hz, J ¼ 11 Hz), 4.70–4.75 (dd, 1H, J ¼ 3.8 Hz, J ¼ 10.6 Hz), 6.8–
7.6 (m, 6H, ArH).
dH 1H NMR of other diastereomer (B): ¼ 2.30 (s, 3H, CH3), 2.42 (s, 3H,
CH3) 2.98–3.07 (dt, ring juncture proton, 1H, J ¼ 3.8 Hz, J ¼ 11 Hz), 3.20–
3.27 (m, ring juncture proton, 1H, overlap with other peaks), 3.89–3.97
(ddꢀt, 1H, J ¼ 10.8 Hz), 4.16–4.24 (ddꢀt, 1H, J ¼ 11 Hz), 5.18–5.23
(dd, 1H, J ¼ 3.8 Hz, J ¼ 10.6 Hz), 5.67–5.72 (dd, 1H, J ¼ 3.8 Hz,
J ¼ 10.5 Hz), 6.8–7.6 (m, 6H, ArH).
Compound 7d: Yield: 80%; diastereomeric ratio (A : B) 50:50; white
;
solid; mp 167–1708C; IR (KBr): nmax ¼ 1703, 1615, 1501, 1404 cm21
1
UV (EtOH): lmax ¼ 205, 272, 282 nm; H NMR (CDCl3, 300 MHz) of the
diastereomer (A): dH ¼ 2.30 (s, 3H, CH3), 3.20–3.30 (m, ring juncture
proton, 1H, overlap with other peaks), 3.41–3.47 (dt, ring juncture
proton, 1H, J ¼ 3.8 Hz, J ¼ 11 Hz), 3.73–3.80 (ddꢀt, 1H, J ¼ 11 Hz),
4.14–4.21 (dtꢀt, 1H, J ¼ 11 Hz), 4.63–4.69 (dd, 1H, J ¼ 1.2 Hz,
J ¼ 11 Hz), 4.71–4.76 (dd, 1H, J ¼ 1 Hz, J ¼ 11 Hz ), 6.8–7.84 (m, 7H,
ArH).
1H NMR of other diastereomer (B): dH ¼ 2.30 (s, 3H, CH3), 3.00–3.08
(dt, ring juncture proton, 1H, J ¼ 3.8 Hz, J ¼ 11 Hz), 3.20–3.30 (m, ring
juncture proton, 1H, overlap with other peaks), 3.90–3.97 (ddꢀt, 1H,
J ¼ 10.8 Hz), 4.18–4.26 (ddꢀt, 1H, J ¼ 11 Hz), 5.20–5.25 (dd, 1H,

1744
K. C. Majumdar and M. Jana
J ¼ 3.8 Hz, J ¼ 10.6 Hz), 5.67–5.72 (dd, 1H, J ¼ 3.8 Hz, J ¼ 10.5 Hz), 6.8–
7.84 (m, 7H, ArH).
Compound 7e: Yield: 70%; diastereomeric ratio (A : B) 45 : 55; white solid;
mp 225–2288C; IR (KBr): nmax ¼ 1703, 1632, 1581, 1463, 1443 cm21; UV
1
(EtOH): lmax ¼ 205, 274, 282, 306 nm; H NMR (CDCl3, 300 MHz) of the
diastereomer (A): dH ¼ 2.42 (s, 3H, CH3), 3.22–3.36 (m, ring juncture
proton, 1H, overlap with other peaks), 3.43–3.50 (dt, ring juncture proton,
1H, J ¼ 3.8 Hz, J ¼ 11 Hz), 3.82–3.89 (ddꢀt, 1H, J ¼ 11 Hz), 4.13–4.21
(ddꢀt, 1H, J ¼ 11 Hz), 4.61–4.67 (dd, 1H, J ¼ 1.3 Hz, J ¼ 3.3 Hz), 4.89–
4.94 (dd, 1H, J ¼ 3.24 Hz, J ¼ 6 Hz), 6.76–7.60 (m, 7H, ArH).
1H NMR of other diastereomer (B): dH ¼ 2.42 (s, 3H, CH3), 3.00–3.09
(dt, ring juncture proton, 1H, J ¼ 3.8 Hz, J ¼ 11 Hz), 3.22–3.36 (m, ring
juncture proton, 1H, overlap with other peaks), 4.01–4.09 (ddꢀt, 1H,
J ¼ 10.8 Hz), 4.16–4.24 (ddꢀt, 1H, J ¼ 11 Hz), 5.18–5.23 (dd, 1H,
J ¼ 3.8 Hz, J ¼ 10.6 Hz), 5.85–5.90 (dd, 1H, J ¼ 3.8 Hz, J ¼ 10.5 Hz),
6.76–7.60 (m, 7H, ArH).
Compound 7f: Yield: 75%; diastereomeric ratio (A:B) 50:50; white solid; mp
158–1618C; IR (KBr): nmax ¼ 1710, 1626, 1492, 1405 cm21; UV (EtOH):
1
lmax ¼ 205, 273, 282, 306, 319 nm; H NMR (CDCl3, 300 MHz) of the dia-
stereomer (A): dH ¼ 3.24–3.35 (m, ring juncture proton, 1H, overlap with
other peaks), 3.43–3.48 (dt, ring juncture proton, 1H, J ¼ 3.8 Hz,
J ¼ 11 Hz), 3.76–3.84 (ddꢀt, 1H, J ¼ 10.8 Hz), 4.15–4.23 (ddꢀt, 1H,
J ¼ 11 Hz), 4.64–4.69 (dd, 1H, J ¼ 3.67 Hz, J ¼ 11 Hz), 5.70–5.75 (dd,
1H, J ¼ 3.4 Hz, J ¼ 10.8 Hz), 6.9–7.8 (m, 8H, ArH).
1H NMR of other diastereomer (B) dH: ¼ 3.01–3.10 (dt, ring juncture
proton, 1H, J ¼ 3.8 Hz, J ¼ 11 Hz), 3.24–3.35 (m, ring juncture proton,
1H, overlap with other peaks), 3.94–4.01 (ddꢀt, 1H, J ¼ 10.8 Hz), 4.19–
4.27 (ddꢀt, 1H, J ¼ 11 Hz), 5.22–5.27 (dd, 1H, J ¼ 3.7 Hz, J ¼ 10.6 Hz),
5.70–5.75 (dd, 1H, J ¼ 3.8 Hz, J ¼ 10.5 Hz), 6.9–7.8 (m, 8H, ArH).
ACKNOWLEDGMENTS
We thank the CSIR ( New Delhi) for financial assistance. One of us (M. J.) is
grateful to the University of Kalyani for a senior research fellowship. We also
thank B. Roy for comments.
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