Synthesis of Nα-protected amino acid-derived selenocarbamates employing isocyanates as key intermediates

Article abstract of DOI:10.1080/00397910903318641

A simple protocol for the synthesis of Nα-urethane-protected N-alkyl-Se-alkyl selenocarbamate derivatives of amino acids has been described. The reaction of Nα-urethane-protected amino alkyl isocyanates with selenating agent LiAlHSeH and subsequent coupli

Full text of DOI:10.1080/00397910903318641

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α
Synthesis of N -Protected Amino
Acid–Derived Selenocarbamates
Employing Isocyanates as Key
Intermediates
Vommina V. Sureshbabu a & Shankar A. Naik a
a Peptide Research Laboratory, Department of Studies in Chemistry ,
Central College Campus, Bangalore University , Bangalore, India
Published online: 09 Aug 2010.
To cite this article: Vommina V. Sureshbabu & Shankar A. Naik (2010) Synthesis of N α -Protected
Amino Acid–Derived Selenocarbamates Employing Isocyanates as Key Intermediates, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
40:18, 2676-2685, DOI: 10.1080/00397910903318641
To link to this article: http://dx.doi.org/10.1080/00397910903318641
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Synthetic Communications1, 40: 2676–2685, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903318641
SYNTHESIS OF Na-PROTECTED AMINO ACID–DERIVED
SELENOCARBAMATES EMPLOYING ISOCYANATES
AS KEY INTERMEDIATES
Vommina V. Sureshbabu and Shankar A. Naik
Peptide Research Laboratory, Department of Studies in Chemistry, Central
College Campus, Bangalore University, Bangalore, India
A simple protocol for the synthesis of Na-urethane-protected N-alkyl-Se-alkyl selenocarba-
mate derivatives of amino acids has been described. The reaction of Na-urethane-protected
amino alkyl isocyanates with selenating agent LiAlHSeH and subsequent coupling with an
alkyl halide yielded the title compounds in good yield and purity. All the selenocarbamates
obtained have been characterized by 1H NMR, 13C NMR, and mass spectral studies.
Keywords: LiAlHSeH; Na-protected amino alkyl isocyanates; selenium; selenocarbamates
INTRODUCTION
Organoselenium chemistry has continued to attract considerable attention
because of its pivotal role in the synthesis of a large number of biologically active
compounds (e.g., selenocarbohydrates,[1] selenoamino acids,[2] and selenopeptides[3]).
More important, organoselenium compounds have emerged as an exceptional class
of structures that exemplify a role in biochemical processes, serving as important
therapeutic compounds ranging from antiviral and anticancer agents to naturally
occurring food supplements.[4] Compared to sulfur compounds, the corresponding
selenium analogs appear to be much more active in cancer prevention.[5]
Selenium derivatives also exhibit antioxidant and radioprotecting activity[6] and
are free radical scavengers.[7] Selenoproteins occur widely in nature, and a number of
them have been isolated and characterized.[8] Typically in these biomolecules, the
selenium occurs as selenocysteine,[9] considered the 21st amino acid genetically coded
in DNA. The analogs of selenium include selenoamides,[10] selenocarbamates,[11]
isoselenocyanates,[12] and selenoureas.[13] These are essential building blocks for the
preparation of a number of biologically relevant peptides, such as heterocycles.[14]
Among the series of carbamate derivatives, there exists mainly one carbamate of
the type given in Fig. 1. These are prepared by the reaction of isocyanates, isothiocya-
nates, and isoselenocyanates with alcohol and thiol. N-Alkyl-Se-alkyl selenocarba-
mates have been reported but are scarce, having very few methods of preparation.[11]
Received June 1, 2009.
Address correspondence to Vommina V. Sureshbabu, Department of Studies in Chemistry, Central
College Campus, Bangalore University, Dr. B. R. Ambedkar Veedhi, Bangalore 560 001, India. E-mail:
sureshbabuvommina@rediffmail.com, hariccb@hotmail.com, hariccb@gmail.com
2676

AMINO ACID–DERIVED SELENOCARBAMATES
2677
Figure 1. Types of N-alkyl-O,S,Se-alkyl selenocarbamates.
Selenocarbamates of type 2 have been prepared by the reaction of an amine
and carbon monoxide with elemental selenium, followed by treatment with an alkyl
halide. An alternate route is the coupling of an isocyanate with a selenating agent to
form the selenolate ion, which on treatment with alkyl=aryl halide gives the desired
selenocarbamate.[15] Our group has reported the preparation of isocyanates derived
from Fmoc-a-amino acids (i.e., Na-Fmoc-amino alkyl isocyanates[16]) obtained
by the Curtius rearrangement of Na-Fmoc amino acid azides.[17] These key
intermediates were employed in the synthesis of dipeptidyl ureas, oligo-a-peptidyl
ureas,[18] retro-inverso peptides[19] employing Goldschmidt–Wick type reaction,
N-formamides,[20] and isonitriles,[21] which are useful synthons in multicomponent
reactions. With an aim to demonstrate newer applications of Fmoc-a-amino alkyl
isocyanates, we envisaged the synthesis of selenocarbamates by the coupling of
isocyanate with an appropriate selenating agent and alkyl=aryl halide. The amino
acid–derived isocyanates are stable, isolable as solids, and reactive species. The
choice of selenating agent played a key role in the synthesis. The alkali metal salts
of hydrogen selenide,[22] which can be prepared in situ by the reaction of elemental
selenium with a reducing agent such as Li, Na, and NaBH4, have often been utilized
as selenating agents, but the applicability of these reagents is limited to the synthesis
of a certain class of molecules. These reagents were tried for the preparation of
selenocarbamates but met with limited success. The difficulty in isolation and the
feasibility of the reaction solvent prompted us to look for another alternative. For
example, the reaction of N-Fmoc-a-amino isocyanate with sodium selenide obtained
by the reaction of NaBH4 with elemental selenium in ethanol resulted in ethanolysis
of the isocyanate, leading to O-ethyl-N-alkylcarbamate. A mild and efficient selenat-
ing agent that suits our protocol was LiAlHSeH, prepared by the reaction of lithium
aluminium hydride with elemental selenium. A wide range of applications have
been demonstrated using this reagent, ranging from mono and diselenides,
Scheme 1. Synthesis of N-protected a-amino alkyl isocyanates: (i) NMM, ECF, aq. NaN3, THF, ꢀ10 ꢁC,
10 min; (ii) 65 ꢁC, 30 min.

2678
V. V. SURESHBABU AND S. A. NAIK
Scheme 2. Synthesis of selenating agent: (i) THF, 0 ꢁC, argon atm., 30 min.
selenoanhydrides, substituted selenoureas, and selenolactones to selenoamides.[23] It
is noteworthy that the reagent is prepared in tetrahydrofuran (THF), and the
neucleophillic attack by the solvent itself is circumvented as in case of other selenat-
ing agents. The aprotic conditions employed for the reaction using LiAlHSeH favor
the formation of desired carbamate derivatives bearing different alkyl groups.
RESULTS AND DISCUSSION
Na-Fmoc-amino acid was converted to its corresponding acyl azide by treating
the generated mixed anhydride with aqueous sodium azide. The mixed anhydride
was obtained by the reaction of acid with N-methylmorpholine (NMM) and ethyl
chloroformate. The Curtius rearrangement of the corresponding acyl azide under
thermal conditions resulted in the isocyanate. The formation of isocyanate was
further confirmed by the appearance of a sharp peak characteristic of carbonyl
stretching vibrational frequency at around 2250 cmꢀ1. All the isocyanates were
obtained as crystalline solids in excellent yield (Scheme 1).
The key point in our protocol was the choice of appropriate selenating agent.
The initial studies centered around the use of alkali metal salts of hydrogen selenide
such as sodium hydrogen selenide because they are the most commonly employed
selenating agents.[24] Attempts to couple isocyanate with these selenating agents in
turn resulted in O-alkyl-N-alkylcarbamates. The use of these reagents requires either
alcohol or water as the reaction solvent because of their solubility. In a test reaction
using ethanol as the solvent, the reaction of Na-Fmoc amino alkyl isocyanate with
NaHSe obtained by reaction of elemental selenium with NaBH4 resulted in
ethanolysis of the isocyanate. Upon consideration of the solvent factor and the ease
of isolation of the product, we inferred that LiAlHSeH is the suitable selenating
agent for our protocol. Along with isocyanate synthesis, the selenating agent LiAlH-
SeH was prepared simultaneously by the reaction of LiAlH4 with elemental selenium
in dry THF under an argon atmosphere (Scheme 2). The formation of heterogeneous
grayish suspension confirmed the complete consumption of selenium, and the
reagent was used directly.
In an initial study, Fmoc-Leu-NCO 5c was added to an anhydrous THF
solution of LiAlHSeH 7. The reaction mixture was stirred at 0 ꢁC initially and for
Scheme 3. Synthesis of N-urethane-protected N-alkyl-Se-alkyl selenocarbamates: (i) 7, THF, 0 ꢁC to rt, 3 h.

AMINO ACID–DERIVED SELENOCARBAMATES
2679
Table 1. Synthesis of N-urethane-protected amino acid–derived N-alkyl-Se-alkyl selenocarbamates
Entry
Product
Mp (ꢁC) Yield (%) Mass [M þ Naþ] calcd.=obsd.
8a
153
142
72
70
427.0537=427.0530
455.0850=455.0841
8b
8c
8d
165
157
75
78
469.1=469.1a
469.1006=469.1025
8e
8f
182
145
72
69
503.0=503.0a
561.0905=561.0892
8g
151
134
75
72
545.1319=545.1310
381.0693=381.0685
8h
(Continued )

2680
V. V. SURESHBABU AND S. A. NAIK
Table 1. Continued
Entry
Product
Mp (ꢁC) Yield (%) Mass [M þ Naþ] calcd.=obsd.
8i
128
74
491.0850=491.0837
8j
136
149
71
80
305.0380=305.0374
381.0693=381.0690
8k
aCorresponds to ES-MS of the compound.
an hour more at room temperature under an argon atmosphere. Methyl iodide in
THF was added to the reaction mixture at the same temperature. The reaction
mixture was further stirred for another 2 h. The evaporation of the solvent followed
by a simple workup resulted in the isolation of Fmoc-Leu-NH-CO-Se-CH3 8c in 75%
yield. The crude residue was purified by column chromatography to obtain 8c as a
white crystalline solid. The generality of this reaction was demonstrated by preparing
a series of selenocarbamates with other amino acid–derived isocyanates including a
set with Boc- and Z-protection and different alkyl halide to get the desired selenocar-
bamates in good yields (Scheme 3, Table 1).
All the selenocarbamates were obtained as crystalline solids and were charac-
1
terized by mass (MS), H NMR, 13C NMR, and 77Se NMR spectroscopic analysis.
1
The H NMR spectra of the selenocarbamates showed field shift of methyl protons
bonded to the selenium atom at around d 2–2.5 ppm compared to its oxygen analog
at d 3.5–3.8 ppm. A similar change was observed in 13C NMR spectra of the
prepared compounds wherein methyl carbon had a value of d 6–8 ppm. The synthesis
of selenocarbamate was further confirmed by 77Se NMR analysis of the samples
wherein a single peak was observed at around 310–330 ppm in accordance with
similar reports in literature.
CONCLUSION
The reaction of Na-urethane-protected amino alkyl isocyanates with a selenat-
ing agent yielded the selenolate ion, which on treatment with an alkyl halide resulted
in Na-protected N-alkyl-Se-alkyl selenocarbamate derivatives of amino acids. The

AMINO ACID–DERIVED SELENOCARBAMATES
2681
isocyanate precursors were prepared by the Curtius rearrangement of corresponding
acyl azides. All the selenocarbamates have been isolated as stable solids and fully
characterized.
EXPERIMENTAL
All solvents were freshly distilled before use. Amino acids were used as received
from Sigma-Aldrich Company. Melting points were taken on a Buchi model 150
melting-point apparatus in open capillaries and are uncorrected. Infrared (IR)
spectra were recorded on a Nicolet model impact 400 D Fourier transform (FT)–
1
IR spectrometer (KBr pellets, 3 cmꢀ1 resolution). H and 13C NMR spectra were
recorded at 400 and 100 MHz respectively, with tetramethylsilane (TMS) as internal
standard. The 77Se chemical shifts were expressed in d values deshielded with respect
to neat Me2Se in CDCl3. Mass spectra (MS) were recorded on a MALDI-TOF
(Kratos) mass spectrometer and liquid chromatographic–mass spectroscopic
(LC-MS) instrument [LC-Agilent-2625, MS-Waters quderpole atmospheric pressure
ionization (API)]. The sonic bath was of German make (35 KHz, Elma, T 310=H).
The microwave reaction was carried out in an LG MS 194A microwave oven
producing microwave irradiation with a frequency of 2,450 MHz. The microwave
reaction was specifically carried out at 60% of the total power output, which would
correspond to an average power of 720 W. Unless otherwise mentioned, all amino
acids used have L-configuration. Thin-layer chromatography (TLC) was carried
out using the precoated silica-gel G254 plates. LiAlHSeH 7 were prepared according
to a previously reported method.
Briefly, the preparation of LiAlHSeH 7 is as follows: lithium aluminum
hydride (0.38 g, 10.0 mmol) was added to a solution of selenium powder (0.80 g,
10.0 mmol) in dry THF (100 ml) at 0 ꢁC under an argon atmosphere. The mixture
was stirred for 30 min. The black selenium powder was consumed in less than
10 min. The reaction mixture became a heterogeneous grayish suspension. The
reagent was formed in situ and was then ready for further reaction.
General Procedure for the Synthesis of Na-Urethane-Protected
Amino Alkyl Isocyanates, 5
To a stirred solution of Fmoc=Boc=Z-a-amino acid (10 mmol) in THF, 1.2 ml
(11.0 mmol) of N-methylmorpholine (NMM) and 1.05 ml (11.0 mmol) of ethyl
chloroformate were added at ꢀ10 ꢁC. The mixture was stirred for 5 min, and sodium
azide (0.98 g, 15.0 mmol) in a minimum amount of water was added. The stirring was
continued for 20 min or until completion of the reaction. THF was then evaporated
under vacuum, and the residue was dissolved in CH2Cl2. The organic extract was
washed with 5% Na2CO3, 10% citric acid, water, and brine and dried over anhydrous
sodium sulfate. CH2Cl2 was removed under reduced pressure, and the residue was
dissolved in 15.0 ml of toluene. The solution was refluxed for 30 min. Complete con-
version of acid azide (IR peak at 2145 cmꢀ1) to isocyanate (IR peak at 2225 cmꢀ1
)
was confirmed, and toluene was removed in vacuo. The resulting analytically pure
isocyanate was used in the next step.

2682
V. V. SURESHBABU AND S. A. NAIK
General Procedure for the Synthesis of Na-Urethane-Protected
N-Alkyl-Se-alkyl Selenocarbamates, 8
Na-Protected amino alkyl isocyanate (1.1 g, 10 mmol) was added to the THF
solution of LiAlHSeH (10 mmol) at 0 ꢁC. The reaction was stirred at the same tem-
perature for 1 h. Methyl iodide (0.6 ml, 10 mmol) was added to the reaction mixture
and stirred at room temperature for 2 h. THF was evaporated under reduced press-
ure and extracted into ethyl acetate. The organic layer was washed with water and
brine, dried over anhydrous sodium sulfate, and evaporated to dryness. The residue
was purified by column chromatography on silica gel with ethyl acetate=hexane
(20%) to obtain the selenocarbamate derivative as a stable white solid. All the
1
prepared compounds were characterized by H NMR, 13C NMR, 77Se NMR, and
MS techniques.
Characterization Data for Na-Urethane-Protected N-Alkyl-Se-alkyl
Selenocarbamates, 8a–k
(S)-(9H-Fluoren-9-yl)methyl 1-(Methylselanylcarbonyl)ethylcarbamate,
8a. IR (KBR): 1693, 1647 cmꢀ1; Rf (20% EtOAc=hexane): 0.3; 1H NMR (d, CDCl3):
1.26 (d, J ¼ 6.4 Hz, 3H), 2.25 (s, 3H), 4.35–4.49 (m, 3H), 5.12 (br, 1H), 5.63 (m, 1H),
6.42 (br, 1H), 7.20–7.32 (m, 4H), 7.52 (d, J ¼ 7.2 Hz, 2H), 7.71 (d, J ¼ 7.2 Hz, 2H);
13C NMR (CDCl3, d): 6.23, 19.89, 47.77, 56.99, 67.27, 120.54, 125.56, 127.61,
128.26, 141.83, 144.23, 155.92, 162.53.
(S)-(9H-Fluoren-9-yl)methyl 2-Methyl-1-(methylselanylcarbonyl)propyl-
1
carbamate, 8b. IR (KBR): 1693, 1652cmꢀ1; Rf (20% EtOAc=hexane): 0.45; H
NMR (d, CDCl3): 0.98 (d, J ¼ 6.8 Hz, 6H), 2.35 (s, 3H), 3.57 (m, 1H), 4.23 (t, J ¼ 6.8 Hz,
1H), 4.41–4.51 (m, 2H), 4.92 (br, 1H), 5.56 (br, 1H), 6.21 (br, 1H), 7.22–7.35
(m, 4H), 7.48 (d, J ¼ 7.2 Hz, 2H), 7.69 (d, J ¼ 7.6 Hz, 2H); 13C NMR (CDCl3, d):
6.30, 19.89, 29.93, 47.77, 57.01, 67.27, 120.52, 125.62, 127.52, 128.32, 141.82, 144.35,
156.40, 163.12.
(S)-(9H-Fluoren-9-yl)methyl 3-Methyl-1-(methylselanylcarbonyl)butyl-
carbamate, 8c. IR (KBR): 1693, 1649 cmꢀ1; Rf (20% EtOAc=hexane): 0.5; 1H
NMR (d, CDCl3): 0.84 (d, J ¼ 4.4 Hz, 6H), 1.60–1.69 (m, 3H), 2.17 (s, 3H), 4.14 (t,
J ¼ 6 Hz, 1H), 4.33 (d, J ¼ 7.2 Hz, 2H), 5.05 (br, 1H), 5.61 (br, 1H), 6.35 (br, 1H),
7.21–7.34 (m, 4H), 7.48 (d, J ¼ 7.2 Hz, 2H), 7.68 (d, J ¼ 7.6 Hz, 2H); 13C NMR
(CDCl3, d): 6.35, 22.78, 25.33, 43.26, 47.61, 59.84, 67.30, 120.50, 125.51, 127.58,
128.23, 141.79, 144.28, 155.97, 163.26.
(S)-(9H-Fluoren-9-yl)methyl 2-Methyl-1-(methylselanylcarbonyl)butyl-
carbamate, 8d. IR (KBR): 1693, 1655cmꢀ1; Rf (20% EtOAc=hexane): 0.5; 1H
NMR (d, CDCl3): 0.89–0.95 (m, 6H), 1.52–1.58 (m, 3H), 2.10 (s, 3H), 4.22 (t, J ¼ 6.4 Hz,
1H), 4.40–4.51 (m, 2H), 5.01 (br, 1H), 5.58 (br, 1H), 6.35 (br, 1H), 7.20–7.32
(m, 4H), 7.52 (d, J ¼ 7.2 Hz, 2H), 7.72 (d, J ¼ 7.6 Hz, 2H); 13C NMR (CDCl3, d):
6.32, 15.91, 25.70, 36.21, 47.71, 47.79, 55.56, 67.20, 120.53, 125.55, 127.60, 128.24,
141.85, 144.23, 156.32, 163.12.

AMINO ACID–DERIVED SELENOCARBAMATES
2683
(S)–(9H-Fluoren-9-yl)methyl 1-(Methylselanylcarbonyl)-2-phenylethyl-
1
carbamate, 8e. IR (KBR): 1693, 1650 cmꢀ1; Rf (20% EtOAc=hexane): 0.45; H
NMR (d, CDCl3): 2.12 (s, 3H), 2.85 (br, 2H), 4.20 (t, J ¼ 6.8 Hz, 1H), 4.40 (d, J ¼ 4 Hz,
2H), 5.12 (br, 1H), 5.82 (m, 1H), 6.42 (br, 1H), 7.18–7.72 (m, 13H); 13C NMR (CDCl3,
d): 6.30, 47.66, 48.03, 53.62, 67.37, 120.55, 125.52, 127.70, 128.24, 129.16, 129.49,
129.75, 136.58, 141.83, 144.20, 156.02, 162.72.
(S)-Benzyl
3-(((9H-Fluoren-9-yl)methoxy)carbonyl)-3-(methylselanyl-
carbonyl)propanoate, 8f. IR (KBR): 1693, 1657 cmꢀ1; Rf (20% EtOAc=hexane):
1
0.35; H NMR (d, CDCl3): 2.24 (s, 3H), 2.73 (d, J ¼ 4 Hz, 2H), 4.42–4.50 (m, 3H),
5.18 (s, 2H), 5.28 (br, 1H), 6.15 (m, 1H), 6.42 (br, 1H), 7.18–7.76 (m, 13H); 13C
NMR (CDCl3, d): 6.46, 47.17, 47.88, 55.31, 67.07, 73.42, 120.06, 125.03, 127.13,
127.80, 128.39, 128.60, 128.71, 135.18, 141.34, 143.68, 155.47, 163.23, 170.31.
(S)-(9H-Fluoren-9-yl)methyl 1-(Benzylselanylcarbonyl)-3-methylbutyl-
carbamate, 8g. IR (KBR): 1695, 1647 cmꢀ1; Rf (20% EtOAc=hexane): 0.5; 1H
NMR (d, CDCl3): 0.93 (d, J ¼ 5.4 Hz, 6H), 1.45 (q, 2H), 1.62 (m, 1H), 4.20 (s,
2H), 4.39–4.49 (m, 2H), 4.75 (d, J ¼ 7.6 Hz, 1H), 5.15 (br, 1H), 5.65 (m, 1H), 6.35
(br, 1H), 7.16–7.72 (m, 13H); 13C NMR (CDCl3, d): 22.68, 23.40, 25.72, 28.20,
41.29, 47.77, 49.82, 67.22, 120.53, 125.54, 127.60, 128.32, 128.54, 128.65, 135.25,
141.34, 143.68, 155.60, 170.51.
(S)-Benzyl 3-Methyl-1-(methylselanylcarbonyl)butylcarbamate, 8h. IR
1
(KBR): 1701, 1650 cmꢀ1; Rf (20% EtOAc=hexane): 0.4; H NMR (d, CDCl3): 0.95
(d, J ¼ 7.6 Hz, 6H), 1.37 (m, 2H), 1.65 (m, 1H), 2.25 (s, 3H), 5.11 (s, 2H), 5.25 (br,
1H), 5.43 (m, 1H), 6.39 (br, 1H), 7.32–7.55 (m, 5H); 13C NMR (CDCl3, d): 6.22,
22.53, 23.40, 41.39, 49.75, 49.91, 67.50, 128.57, 128.81, 128.98, 129.16, 136.50,
155.92, 163.55.
(S)-Benzyl 1-(Benzylselanylcarbonyl)-2-phenylethylcarbamate, 8i. IR
(KBR): 1701, 1648 cmꢀ1; Rf (20% EtOAc=hexane): 0.45; 1H NMR (d, CDCl3):
2.67 (s, 2H), 2.92 (d, J ¼ 7.6 Hz, 2H), 5.10 (br, 1H), 5.34 (s, 2H), 6.10 (m, 1H, 6.42
(br, 1H), 7.18–7.36 (m, 15H); 13C NMR (CDCl3, d): 28.18, 43.51, 65.68, 66.31,
127.19, 127.52, 128.13, 128.30, 128.52, 128.66, 128.95, 129.22, 129.40, 136.25,
136.41, 140.87, 155.89, 163.15.
(S)-tert-Butyl 1-(Methylselanylcarbonyl)ethylcarbamate, 8j. IR (KBR):
1
1692, 1652 cmꢀ1; Rf (20% EtOAc=hexane): 0.35; H NMR (d, CDCl3): 1.42 (s, 9H),
1.56 (d, J ¼ 4 Hz, 3H), 2.14 (s, 3H), 4.97 (br, 1H), 5.18 (br, 1H), 5.65 (m, 1H); 13C
NMR (CDCl3, d): 6.32, 21.72, 28.32, 57.25, 80.03, 156.01, 163.12.
(S)-tert-Butyl
1-(Methylselanylcarbonyl)-2-phenylethylcarbamate,
8k. IR (KBR): 1692, 1658 cmꢀ1; Rf (20% EtOAc=hexane): 0.4; 1H NMR (d, CDCl3):
1.44 (s, 9H), 2.21 (s, 3H), 2.96 (d, J ¼ 6.4 Hz, 2H), 5.29 (br, 1H), 5.86 (m, 1H), 6.26
(br, 1H), 7.12–7.36 (m, 5H); 13C NMR (CDCl3, d): 6.36, 28.41, 42.35, 65.37, 80.20,
127.08, 128.85, 129.14, 136.35, 154.91, 163.05.

2684
V. V. SURESHBABU AND S. A. NAIK
ACKNOWLEDGMENTS
The authors thank the Department of Science and Technology (Grant No.
SR=S1=OC-26=2008) and the Council of Scientific and Industrial Research,
Government of India, for financial support.
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