Poly(N-vinylimidazole) as an efficient catalyst for acetylation of alcohols, phenols, thiols and amines under solvent-free conditions

Article abstract of DOI:10.1039/c2ra21295a

Poly(N-vinylimidazole) is able to promote instantaneous quantitative acetylation of a variety of functionalized alcohols, phenols, thiols and amines with acetic anhydride at room temperature under solvent-free conditions. This new method consistently has excellent yields and the catalyst can be reused and recovered several times. Furthermore, the reaction can even be carried out on a larger scale. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2ra21295a

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Poly(N-vinylimidazole) as an efficient catalyst for
acetylation of alcohols, phenols, thiols and amines
under solvent-free conditions
Cite this: RSC Advances, 2013, 3, 99
Nader Ghaffari Khaligh*
Received 11th May 2012,
Poly(N-vinylimidazole) is able to promote instantaneous quantitative acetylation of
a variety of
Accepted 25th October 2012
functionalized alcohols, phenols, thiols and amines with acetic anhydride at room temperature under
solvent-free conditions. This new method consistently has excellent yields and the catalyst can be reused
and recovered several times. Furthermore, the reaction can even be carried out on a larger scale.
DOI: 10.1039/c2ra21295a
www.rsc.org/advances
It is worth noting that in several cases, the use of solvent-
free conditions is necessary for the success of the process that
can be dramatically slow or practically unfeasible using an
organic solvent.22,23
1. Introduction
Organic bases are catalysts for a wide range of reactions.1–5
The development of heterogeneous catalysts has become a
major area of research in synthetic organic chemistry due to
some potential advantages of these materials over homoge-
neous systems, such as simplified recovery, reusability,
enhanced selectivity and reactivity, easy product isolation
and incorporation in continuous reactors and microreactors.6
The imidazole ring is a structural fragment of the side
chain in most amino acids (i.e., histamine, histidine, etc.),
which constitutes a part of almost all enzymes and is partly
responsible for their catalytic activity. Imidazole is known to
catalyze a number of biochemical reactions,7,8 therefore, the
catalytic behaviors of monomeric9 and polymeric10 imidazole
was extensively studied while simulating some biological
processes.11,12 Imidazole-containing macromolecules catalyze
the hydrolysis of p-nitrophenyl acetate,11 Michael addition of
thiols to electron-deficient alkenes13 and aza-Michael reaction
in water14 acting as base catalysts.
Poly(N-vinylimidazole) (PVIm) is a weak base (pKBH+ = 5–6).15
Its peculiar features in comparison with aliphatic amines are
negligible protonation within the neutral pH range and a high
capacity for hydrogen bonding.16,17 Poly(N-vinylimidazole) is
known to form complexes with such metal ions as Cu(II),10
Zn(II),18 Cd(II),19 Ag(I)10 and Hg(II).20 The synthesized
poly(N-vinylimidazole) with trimethoxysilyl terminal groups
was chemically anchored (grafted) on magnetic nanoparticles
that were to be used for the removal and recovery of heavy
metals from industrial effluents.21a The practical applications
of PVIm are numerous, ranging from dyestuffs, catalysts,
corrosion inhibitors, ion exchange resins to their utility in
quenching media and metal ion complexations.21b
The protection of alcohols, phenols, thiols and amines by the
formation of esters and amides is one of the most important and
widely used transformations in organic chemistry.24 The protec-
tion of such functional groups is often necessary during the course
of various transformations in a synthetic sequence, especially in
the construction of polyfunctional molecules, such as nucleosides,
carbohydrates, steroids and natural products.25,26 A variety of
procedures are routinely performed for the preparation of acetyl
derivatives, including homogeneous or heterogeneous catalysts,
such as DMAP and 4- pyrrolidinopyridine,27 TMEDA,28 Bu3P,29
iodine,30 p-toluenesulfonic acid,31 alumina,32 montmorillonite
K-10 and KSF,33 zeolite HSZ-360,34 zirconium sulfophenyl
phosphonate,35 phosphomolybdic acid (PMA),36 acetonyltriphe-
nylphosphonium bromide,37 Sc(OTf)3,38 trimethylsilyl tri-
fluoromethanesulfonate (TMSOTf),39 Cu(OTf)2,40 copper(II) tetra-
fluoroborate,41a Mg(NTf2)2,41b In(OTf)3,42 Ce(OTf)3,43 silver tri-
flate,44 magnesium bromide,45 bismuth(III) salts,46 Mg(ClO4)2,47a
HClO4–SiO2,47b HBF4–SiO2,47c BiOClO4?xH2O,47d ferric perchlorate
adsorbed on silica gel,48 zinc chloride,49 cobalt chloride,50 RuCl3,51
InCl3,52 ZrCl4,53 TaCl5,54 Cp2ZrCl2 and cerium polyoxometa-
55
late.56 However, some of the reported methods for the acetylation
suffer from one or more of the disadvantages, such as drastic
reaction conditions, expensive catalysts, hygroscopicity and
thermal instability of the catalysts, use of halogen-containing
solvents and catalysts, long reaction times and low yields.
Therefore, the introduction of new methods and catalysts for the
preparation of esters and amides is still in demand.
It is clear that green chemistry not only requires the use of
environmentally benign reagents and solvents, but also the
recovery and reuse of the catalyst. One way to overcome the
problem of recyclability of the traditional reagents is to
chemically anchor the reactive center onto a large surface
Department of Chemistry, College of Science, University of Guilan, Rasht, 41335-
19141, Iran. E-mail: ngkhaligh@guilan.ac.ir; ngkhaligh@gmail.com;
Fax: +982166934046; Tel: +982166431738
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area solid carrier.57 In these types of solids, the reactive
centers are highly mobile, similar to homogeneous reagents,
and at the same time these species have the advantage of
being recyclable in the same fashion as heterogeneous
reagents. Functional polymers have the potential advantages
of small molecules with the same functional groups.58
Following our current interest based on the use of solid
catalysts under solvent-free conditions,59 in this paper, we are
describing our work on the successful use of non-toxic,
Table 1 Reaction of 4-chlorobenzyl alcohol and Ac2O in the presence of Im,
MIm and PVIm at room temperature under solvent-free conditionsa
Entry Catalyst
Amount (mg) Time (min) Yield (%)b
1
2
3
4
—
—
50
50
6 h
10
5
5
N. R.
92
94
94
Imidazole
N-methylimidazole
Poly(N-vinylimidazole) 50
a
b
Reaction conditions: substrate, 1.0 mmol; Ac2O, 1.0 mmol. GC-
MS yield.
environmentally
benign
and
inexpensive
poly(N-vinylimidazole) (PVIm) as a solid nucleophile catalyst
for the acetylation of alcohols, phenols, thiols and amines with
acetic anhydride at room temperature under solvent-free
conditions. The novelty of this work is the use for the first
time of PVIm for the acetylation of alcohols, phenols, thiols
and amines with acetic anhydride. The results could be useful
in finding new applications for this polymer.
a
heterogeneous
catalyst,
also
as
compared
to
N-methylimidazole and imidazole, poly(N-vinylimidazole) is
highly stable and easy to handle.
In order to standardize the reaction, different amounts of
PVIm were used to get 4-chlorobenzyl acetate from 4-chlor-
obenzyl alcohol and acetic anhydride at room temperature
under solvent-free conditions, and the results are summarized
in Table 2. It was clear that the reaction can be carried out in
the presence of a catalytic amount of PVIm and the yields were
excellent. It was found that the reaction was not possible
without catalyst.
After these preliminary experiments, a catalytic amount of
PVIm (20 mg) was used to convert a variety of functionalized
alcohols, phenols, thiols and amines with acetic anhydride
into respective esters and amides at room temperature under
solvent-free conditions, and the results are presented in
Table 3.
2. Results and discussion
In continuation of our ongoing research program on the
development of new catalysts and methods for organic
transformations,59–61 here we wish to report the environmen-
tally benign synthesis of poly(N-vinylimidazole) (PVIm) for
acetylation of alcohols, phenols, thiols and amines at room
temperature under solvent-free conditions (Scheme 1). PVIm
was synthesized using a standard free-radical polymerization,
as reported earlier.62,63
The results incorporated in Table 3 demonstrate the
generality and scope of PVIm during the acetylation of
structurally diverse alcohols and phenols. The reaction could
be carried out with 1 equiv. of Ac2O at room temperature in 5–
35 min. The compounds containing both electron withdrawing
and electron donating groups reacted equally efficiently under
the standard reaction conditions to give the acetylated
products in excellent yields. The reaction conditions were
mild enough not to induce any damage to moieties like the
methoxyl group (Table 3, entry 9), which often underwent
cleavage in the presence of strong acids or certain Lewis acids.
Sterically hindered and electron deficient alcohols (Table 3,
entries 5, 6 and 13–16) were efficiently acetylated under
solvent-free conditions. Excellent selectivity was observed in
that secondary and tertiary alcohols did not experience any
competitive dehydration (Table 3, entries 9–16). In order to
To begin with, a comparative study with imidazole (Im) and
N-methylimidazole (MIm), instead of poly(N-vinylimidazole)
(PVIm) was carried out. The acetylation of 4-chlorobenzyl
alcohol by Ac2O with equivalent ratio was carried out in the
presence of monomeric and polymeric acetylating agents at
room temperature under solvent-free conditions (Table 1). As
shown, after stirring for 6 h at room temperature in the
absence of acetylating agents, the reaction did not proceed as
monitored by TLC (Table 1, entry 1). On the other hand, in the
presence of a certain amount of the Im, MIm and PVIm
acetylating agents at room temperature, the reaction was
completed with 100% conversion in 10, 5 and 5 min,
respectively. When the acetylating agent was changed from
imidazole to N-methylimidazole, the reaction time was
improved (Table 1, entries 2, 3), however, when replacing
N-methylimidazole with poly(N-vinylimidazole), no significant
improvement was observed in the reaction time (Table 1,
entries 3, 4). According to the results in Table 1, it can be
concluded that both acetylating agents were effective for the
acetylation of benzyl alcohol at room temperature, but PVIm is
Table 2 Reaction of 4-chlorobenzyl alcohol and Ac2O in the presence of
different amounts of PVIm at room temperature under solvent-free conditions
Entry
Amount of PVIm (mg)
Time (min)
Yield (%)a
1
2
3
4
5
6
—
2
5
10
20
30
6 h
30
25
10
5
N. R.
36
42
78
94
5
95
Scheme 1 Acetylation of alcohols, phenols, thiols and amines at room
a
temperature under solvent-free conditions.
GC-MS yield.
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Table 3 PVIm catalyzed acetylation of alcohols, phenols, thiols and aminesa
Entry
1
Substrate
Product
Time (min)
5
Yield (%)b
98
2
5
96
3
4
5
5
5
95
94
88
30
6
35
83
7
8
5
8
94
95
9
8
8
92
92
10
11
8
94
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Table 3 (Continued)
12
25
86
13
14
28
20
88
96
15
16
20
12
89
95
17
18
18
25
94
90
19
20
20
30
90
84c
21
20
87c
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Table 3 (Continued)
22
5
93c
23
45
92d
24
25
2
4
90
96
26
27
4
2
86
93
28
29
4
90
20
93c
30
31
8
98c
15
98c
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Table 3 (Continued)
32
4
98c
33
34
35
10
8
94
96
10
96c
36
6
88
a
b
c
Reaction conditions: substrate, 1.0 mmol; Ac2O, 1.0 mmol; catalyst, 20 mg. Isolated yield of the corresponding acetylated product. Ac2O,
d
2.0 equiv.; isolated yield of the di-acetate. Ac2O, 3.0 equiv.; isolated yield of the tri-acetate.
elucidate the mildness and stereospecificity of the PVIm
catalyzed acetylation of alcohols, the reaction of optically
active l(2)-Menthol and R(+)-Borneol were studied (Table 3,
entries 12 and 16). We observed that l(2)-Menthol ([a]D=
249.0u, c = 10 in 95% ethanol, 99% e.e.) and R(+)-Borneol
([a]D= +37.9u, c = 5 in ethanol, 99% e.e.) react enantioselectivity
with retention of the configuration on the benzylic center to
provide l(2)-Menthyl acetate ([a]D = 273.0u, neat, 98% e.e.)
and (+)-Bornyl acetate ([a]D = +41.5u, neat, 96% e.e.). The
highest optical purity was obtained in an enantiopure form in
high yield and under high regioselectivity using PVIm at room
temperature under solvent-free conditions. The optical rota-
tion of the product was determined and compared with that
reported from Aldrich.
Also, we extended the use of PVIm for direct acetylation of
amines (Table 3, entries 24–28) and thiols (Table 3, entries 33–
36) with Ac2O. The superiority of PVIm was further established
by the fact that direct acetylation of amines and thiols with 1.0
equiv. Ac2O could be achieved in 2–12 min at room
temperature (86–96% yields) under the catalytic influence of
20 mg PVIm. Our protocol has some advantage because the
two groups were acetylated during the reaction conditions
(Table 3, entries 29–32, 35) and furyl mercaptane was
transformed smoothly to the corresponding acetate derivative
(Table 3, entry 36).
in the literature (Table 4). It is clear that based on Table 4,
some methods are superior in terms of reaction time, catalyst
amount, or product yield. However, here we describe one or
more of the limitations of the existing protocols, such as the
use of halogenated catalysts, use of hazardous materials and
use of costly catalysts (e.g. the triflates). Although the triflates,
in general, are claimed as the most effective catalysts, they lead
to competitive side reactions (e.g. dehydration and rearrange-
ment etc.) for acid sensitive substrates.
We are currently engaged in mechanistic studies to under-
stand the precise role of PVIm. Although the actual mechan-
ism of the reaction was not clear, based on the literature64–66
and FTIR spectra of PVIm (bottom) and the initial reaction of
PVIm with acetic anhydride to generate an intermediate PVIm–
Ac (top), the mechanism shown in Scheme 2 was selected as
the most probable one.
The acetic anhydride-imidazole and acetic anhydride-
N-methylimidazole systems proceed entirely by a nucleophilic
and general base catalysis, respectively.66 It seems that the
acetic anhydride-poly(N-vinylimidazole) system also reacts
entirely via a nucleophilic route, with the intermediate
formation of the acetylimidazolinium-anion ion-pair inter-
mediate (I). This was followed by an essentially irreversible
attack of an alcohol, phenol, thiol or amine to the inter-
mediate (I) to generate an ester, thioester or amide product
and to regenerate PVIm. PVIm can be used in high enough
amounts to serve as both the proton scavenger (III) and the
catalyst (I).
We have compared the obtained results in the acetylation of
benzyl alcohol with acetic anhydride catalyzed by PVIm, with
some heterogeneous and homogeneous catalysts, as reported
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Table 4 Comparison of PVIm with other catalysts for the acetylation of benzyl alcohol at room temperature
Entry
Catalyst
Solvent
Catalyst (mol%)
Time (min)
Yield (%)a
Ref.
1
2
3
4
5
6
7
8
9
I2
Neat
Neat
Neat
CH2Cl2
CH3CN
Neat
Neat
CH3CN
Neat
10
1
60
60
30
12
15
240
60
30
600
5
99
90
84
97
98
100
98
96
85
93
98
30
montmorillonite KSF
zeolite HSZ-360
Cu(OTf)2
Ce(OTf)3
Mg(ClO4)2
CoCl2
RuCl3
InCl3
Cp2ZrCl2
PVIm
20 mg
20 mg
2.5
1
1
0.5
5
0.1
1
20 mg
33
34b
40
43
47
50
51
52
55
10
11
Neat
Neat
This work
a
b
Isolated yield. Temperature 60 uC.
In the acetic anhydride-poly(N-vinylimidazole) system the
spectral evidence for intermediate formation was seen (Fig. 1).
PVIm (20 mg) was treated with Ac2O (1.0 mmol) in the absence
The PVIm spectrum contains bands of stretching vibrations
of imidazole rings (1516, 1422, 1285 and 1233 cm21),
stretching vibrations of azole C–H (1080 cm21) and bending
vibrations of heterocycles (914, 825 and 747 cm21).67 The most
important distinction between these two spectra was the
development of a band at 1714 cm21 following the functiona-
lization of imidazole groups of PVIm with acetyl groups upon
reaction with acetic anhydride. The imidazole units of PVIm
corresponding to the ring deformation suffered a shift towards
a higher wave number, (1013, 877, 761 cm21) when the
imidazole ring was acetylated with acetic anhydride. This
of
a substrate at room temperature under solvent-free
conditions and magnetic stirring. After 5 min, the mixture
was diluted with Et2O (25 ml) and the catalyst allowed to settle
down. The supernatant ethereal solution was decanted off, the
catalyst washed with Et2O (2 6 10 ml) and dried at room
temperature under a vacuum for 2 h. The FTIR spectra of the
acetylimidazolinium-anion ion-pair intermediate (top), was
compared with FTIR spectra of PVIm (bottom).
Scheme 2 The plausible mechanism.
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interaction increased the stiffness of the associated ring and
consequently more energy was required to deform the
aromatic cycle, reflected in a higher wave number value.
Additionally, the intensity of the band at about 1555 cm21, as
shown in Fig. 1, corresponded to positively charged nitrogen
atoms. Combining the obvious and strong OH bands at 2595
and 1966 cm21 in Fig. 1, we can infer that hydrogen bonding
formed between the PVIm and acetic acid at equilibrium
(Scheme 2). Furthermore, there was change of positions of the
absorption peaks from 1670–1284 cm21 for PVIm–Ac (top) in
comparison with PVIm (bottom) in Fig. 1, which implies that
the acetyl groups of PVIm–Ac were ionized.
starting benzyl alcohol, phenol and thiophenol were recovered
at the end of the reaction. This may be considered as a useful
practical achievement in the acetylation of amines in the
presence of alcohols, phenols and thiols. Also benzyl alcohol
was acetylated in the presence of phenol and thiophenol in
85 : 15% and 84 : 16% yields, respectively (Table 5, entry 4, 5).
Lastly, phenol was acetylated in the presence of thiophenol in
62 : 38% yield (Table 5, entry 6). On the other hand, the intra-
chemoselective acetylation is shown in Table 3 (Entries 29–32).
Results showed that the phenol, primary and secondary alcohol,
and amine and aniline moiety were acetylated with together.
In addition, we also checked the chemoselectivity of this
protocol. In Table 5, we demonstrate the inter-chemoselective
acetylation of benzyl alcohol, phenol, thiophenol and aniline
together. The results showed that aniline was acetylated
selectively in the presence of benzyl alcohol, phenol and
thiophenol (Table 5, entries 1, 2, 3). It was evident that only
aniline was acetylated to acetanilide, while almost all of the
3. Experimental
3.1. General remarks
Chemicals were purchased from Merck, Aldrich and Fluka
Chemical Companies and used without further purification.
N-vinylimidazole was obtained from Aldrich Chemical Co. and
Fig. 1 FTIR spectra for PVIm (bottom) and PVIm–Ac (top).
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Table 5 Competitive acetylation of benzyl alcohol, phenol and aniline catalyzed
by PVIma
Table 5 (Continued)
6
5
62
38
Entry Substrate
1
Product
Time (min) Yield (%)b
5
5
5
100
0
a
Reaction conditions: substrate, 1.0 mmol; Ac2O, 1.0 mmol; catalyst,
2
100
0
b
20 mg. Determined by GC-MS of the corresponding acetylated
product.
was distilled under reduced pressure at 55 uC just prior to use.
Azoisobutyronitrile (AIBN; BDH) (Fluka) was recrystallized
from ethanol just before use. The purity determination of the
products was accomplished by TLC on silica gel polygram SIL
G/UV 254 plates. The MS were measured under GC (70 eV)
conditions. The IR spectra were recorded on a Perkin Elmer
781 and Bruker Vector 22 Spectrophotometer. In all of the
cases the 1H NMR spectra were recorded with a Bruker Avance
400 or 300 MHz instrument. Chemical shifts are reported in
parts per million in CDCl3 with tetramethylsilane as an
internal standard. 13C NMR data were collected on a Bruker
Avance 100 or 75 MHz instrument.
3
100
0
3.2. Typical procedure of acetylation
4
5
85
15
The substrate (alcohol, phenol, thiol or amine; 1.0 mmol) was
treated with Ac2O (1.0 mmol) in the presence of PVIm (20 mg)
at room temperature under solvent-free conditions and
magnetic stirring. After completion of the reaction, as
indicated by TLC, the mixture was diluted with Et2O (25 ml)
and the catalyst allowed to settle down. The supernatant
ethereal solution was decanted off, the catalyst washed with
Et2O (2 ml) and the combined ethereal solution concentrated
under a vacuum to afford the product identical (mp, IR, 1H
and 13C NMR and GCMS) to an authentic sample of acetylated
product. An important advantage of polymeric catalysts is that
they can readily be separated from the products due to a
considerable difference in the properties of high- and low-
molecular compounds. As a result, such catalysts may be used
repeatedly, which is profitable from the economic viewpoint
and is environmentally benign. Thus, the use of
poly(N-vinylimidazole) is promising from the viewpoint of
‘‘green chemistry’’.68 The recovered catalyst was dried at 50 uC
under a vacuum for 8 h. The recovered catalyst, after drying,
was reused for four more consecutive acetylation reactions of
benzyl alcohol (1.0 mmol) affording 98, 97, 97 and 96% yields
in 2, 3, 3 and 5 min, respectively (Scheme 3).
5
5
84
16
One of the difficulties of most of the heterogeneous basic
catalysts arises from rapid poisoning by CO2 and H2O
contained in the environment and the reactants. To be active
catalysts, they need pretreatment at a specific temperature to
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1
remove adsorbed CO2 and H2O. To obtain full capabilities of
PVIm, the reaction system should be kept free of impurities.
Notably, we have noticed that the conversion of benzyl
alcohol to the respective acetate product can be carried out
even on a larger scale (10 mmol) in 82% yield for 10 min
without any difficulty using 20 mg of PVIm. Therefore, it
indicates that a large-scale reaction was possible using the
same amount of catalyst. It is important to point out that the
present method was much cleaner and did not involve any
chromatographic separation for a large-scale reaction. The
facts that heterogeneous catalysts have good mechanical
stability, can be easily handled as they are invariably low
toxic, non-corrosive free flowing powder and are easily
separated from the reaction mixture through filtration and
reused, make them suitable for industrial applications.
f) Table 2, entry 32: IR (neat) n = 3320, 1730, 1682 cm21; H
NMR (CDCl3, 400 MHz): d = 2.16 (s, 3H), 2.36 (s, 3H), 7.14–7.17
(m, 2H), 7.22–7.25 (m, 1H), 7.39 (br s, 1H), 8.11 (d, J = 7.6 Hz,
1H) ppm; 13C NMR (CDCl3, 100 MHz): d = 20.6, 22.7, 117.8,
120.7, 123.9, 125.9, 130.7, 140.5, 167.0, 167.9 ppm.
h) Table 2, entry 35: IR (neat) n = 1765, 1679 cm21; 1H NMR
(400 MHz , CDCl3): d = 2.33 (s, 3H), 2.45 (s, 3H), 7.24 (d, J = 8.0
Hz, 1H), 7.33 (t, J = 7.6 Hz, 1H), 7.50 (t, J = 7.6 Hz, 1H) 7.53 (d, J
= 7.6 Hz, 1H) ppm; 13C-NMR (100 MHz, CDCl3): d = 20.7, 26.2,
120.7, 125.9, 130.2, 134.7, 143.5, 167.0, 171.9 ppm.
i) Table 2, entry 36: IR (neat) n = 1696 cm21; H NMR (400
1
MHz, CDCl3): d = 2.36 (s, 3H), 4.29 (s, 2H), 6.01 (d, J = 2.8 Hz,
1H), 6.23 (dd, J = 2.8 and 0.8 Hz, 1H), 7.22 (d, J = 0.8, 1H) ppm;
13C-NMR (100 MHz, CDCl3): d = 22.4, 29.7, 107.2, 110.7, 141.7,
152.7, 194.3 ppm.
3.3. The spectral data of some representative and new
products
4. Conclusions
1
a) Table 2, entry 14: IR (neat) n = 1738, 1697 cm21; H NMR
PVIm is a new and highly efficient catalyst for acetylation of
alcohols, phenols, thiols and amines. The low cost, ease of
handling and, with the increasing environmental concern,69
the solvent-free conditions employed in the present method
make it ‘‘environmentally friendly’’ and therefore useful for
industrial applications. The methodology of chemical effi-
ciency, simplified experimental procedures and the minimiza-
tion of the amount of organic solvent used which is to
facilitate the separation of the product from the heterogeneous
catalyst.
(CDCl3, 400 MHz): d = 2.23 (s, 3H), 6.84 (s, 1H), 7.32–7.53 (m,
8H), 7.95 (d, J = 8.8 Hz, 2H); 13C (CDCl3, 100 MHz) d = 20.1,
77.2, 128.2, 128.3, 128.6, 128.8, 133.1, 133.1, 134.1, 169.9, 193.3
ppm.
b) Table 2, entry 16: IR (neat) n = 1726 cm21; [a]25 = 238u
(neat); 1H NMR (CDCl3, 400 MHz): d = 0.82 (s, 3H), 0.88 (s, 3H),
0.92 (s, 3H), 1.26 (m, 4H), 1.72 (m, 2H), 2.08 (s, 3H), 2.36 (m,
1H), 4.89 (d, J = 9.8 Hz, 1H); 13C (CDCl3, 100 MHz) d = 13.4,
18.7, 19.6, 21.2, 27.0, 28.0, 36.7, 44.8, 47.7, 48.6, 79.8, 171.3
ppm.
c) Table 2, entry 19: mp 70–72 uC; IR (KBr) n = 1756 cm21; 1H
NMR (CDCl3, 400 MHz): d = 2.38 (s, 3H), 7.25 (d, J = 8.8 Hz,
1H), 7.46 (m, 2H), 7.55 (s, 1H), 7.79 (m, 3H); 13C NMR (CDCl3,
100 MHz) d = 21.2, 118.5, 121.1, 125.7, 126.5, 127.6, 127.7,
129.4, 131.4, 133.7,148.3, 169.6 ppm.
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