Download Reaction of orthoesters with alcohols in the presence of acidic

Indian Journal of Chemistry
Vol. 44B, August 2005, pp. 1686-1692
Reaction of orthoesters with alcohols in the presence of acidic catalysts: A study
H M Sampath Kumar*, Sipak Joyasawal, B V S Reddy, P Pawan Chakravarthy, A D Krishna, & J S Yadav
Organic Chemistry Division I, Indian Institute of Chemical Technology, Hyderabad 500007, India
Email: [email protected]
Received 30 September 2003; accepted (revised) 9 April 2005
Allylic and benzylic alcohols are converted into corresponding unsymmetrical ethers when reacted with various
orthoesters in the presence of montmorillonite KSF at ambient temperature. A detailed study has been undertaken to
examine the mechanism and generality of these reactions with regard to various acidic catalysts, which reveal interesting
competitive reactions mainly O-acetylation, together with trace amount of dimerized product. The type of the side product
and their relative quantity depends upon the nature of the catalyst employed. Furthermore, the low yields of the Claisen
rearrangement product obtained from allylic alcohols under heating is rationalized due to the formation of some of these
products.
Keywords: Allylic alcohols, benzylic alcohols, unsymmetrical ethers, orthoesters, montmorillonite KSF, O-acetylation,
Claisen rearrangement
IPC: Int.Cl.7 C 07 C 31/00, C 07 C 69/00
Claisen-Johnson orthoester rearrangement1 is an
important C-C bond forming reaction, which yields
vinylic esters. This reaction being heat promoted is
generally low yielding. The poor conversion is usually
attributed to the inactivation of the catalyst or
decomposition of the orthoester and to compensate
these losses, the reaction is generally conducted for
long reaction time heating at high temperature (with
excess of orthoester in order to improve the yield).
Even though reaction of orthoesters with various
carbonyl functions such as ketones, aldehydes,
carboxylic as well as sulphonic acids were studied in
detail in the past, 5, 6 this study pertaining to alcohols is
only limited to Claisen-Johnson rearrangement.
However, to our knowledge, there has not been any indepth study regarding the nature of the side products or
the details of the competitive reaction associated with
this important transformation. As a part of ongoing
activity to prepare dimethyl, vinyl carboxylate
derivatives required for the synthesis of various
pyrethroid analogs, we have been screening a number
of solid acid catalysts to evolve an eco-friendly
protocol2 for the Claisen-Johnson rearrangement.
During our pursuit, we found serendipitously that
various primary (allylic and benzylic) alcohols undergo
rapid O-alkylation in the presence of Montmorillonite
KSF at room temperature.3 This reaction was found to
be general with regard to various orthoesters and
alcohols, even though it was found to be selective with
regard to allylic and benzylic alcohols. Our strategy
was successfully applied to the total synthesis of
natural products by Mori et al4. which proved the
utility of this protocol as an attractive alternative to the
existing O-alkylation protocols.
⎯⎯⎯⎯⎯⎯
IICT Communication No 020402
All the reactions were conducted at ambient
temperature by stirring the alcohol with stoichio
Results and Discussion
In the context of our earlier observations, we studied
various acids such as, silicon dioxide, clay, BF3,
amberlyst-15 as an effort to evolve a high yielding and
environmental friendly protocol and found that with the
change in the catalyst, there has been a significant
change in the yields as well as the nature of products
formed (Scheme I). The formation of the side products
in case of orthoester reaction at RT will be important
and particularly interesting in case of Claisen-Johnson
orthoester rearrangement at elevated temperature, as it
would influence the yields and hence the results of our
studies in this regard is presented here.
First the outcome of this reaction of the orthoesters
with alcohols at room temperature in the presence of
various catalysts is presented in Table I.
KUMAR et al.: REACTION OF ORTHOESTERS WITH ALCOHOLS IN THE PRESENCE OF ACIDIC CATALYSTS
O
OH
R
R'C ( O R ") 3
C atalyst
R
O R'' +
R
R
O
+
R
O
1
2
R = V in yl, A ryl o r n -B u tyl
R'= H , C H 3
R "= C H 3 , C 2 H 5 , C 3 H 7
C atalyst: M o n tm o rillo n ite-K S F , K 1 0 , B F 3 .E t 2 O , S iO 2 , am b erlyst-1 5
3
Scheme I
Table I ⎯ Reaction of orthoesters with alcohols in the presence of various acid catalysts at rt
Conditiona
Entry
1
2
Alcohol
Time
1
2
3
12hr
12hr
12hr
12hr
12hr
4hr
80
83
65
13
35
75
5
15
20
5
5
4
5
45
4
1
MMKSF
8hr
MMKSF
12hr
MMKSF
8hr
MMKSF
12hr
MMK10
12hr
12hr
SiO2
SiO2(μw)c 5min
BF3.O(Et)2
6hr
Amberlyst-15 12hr
90
72
93
51
76
15
5
5
2
10
4
15
3
35
82
71
41
6
4
15
5
7
Orthoester* Catalyst*
TEOA
MMKSF
TMOA
MMKSF
TEOA
MMK10
TEOA
SiO2
TEOA Amberlyst-15
TEOA
BF3.O(Et)2
C6H5CH2OH
TEOA
TEOF
TMOA
TPOF
TEOA
TEOA
TEOF
TEOA
TEOA
C6H5CH=CHCH2OH
Product Yield (%)b
3
CH3(CH2)3CH2OH
TEOA
SiO2
12hr
10
10
35
4
CH2=CH-CH2OH
TMOA
MMKSF
8hr
86c
5
-
5
(CH3)2C=CH-CH2OH
TMOA
MMKSF
10hr
81c
4
-
TMOA
MMKSF
8hr
62
10
-
TMOA
MMK10
12h r
10
62
-
TMOA
MMKSF
12hr
92
-
-
TEOA
MMKSF
12hr
90
-
-
TMOA
MMKSF
12hr
84
-
-
TEOA
SiO2
12hr
75
-
10
TEOA
SiO2
12hr
78e
-
6
6
OH
O
OH
7
8
C
H
C C
H H2
O
CH2OH
O
9i
CH2OH
S
OH
j
10
Ph
OH
11d
MeO
a. All reactions are carried out at room temperature.
b. Yield of isolated product after chromatography. c. Based on GC analysis of the reaction mixture.
c. Pulsed irradiation in domestic microwave oven (BPL BMO 700T) using Pyrex test tube.
d. Optically pure (absolute configuration S), Optical rotation [α ]D25 -31.0 [c 1.5 CHCl3].
e. Optically pure (absolute configuration S), Optical rotation [α]D25 -4.5 [c 2.2 CHCl3].
* TEOA; Triethylorthoacetate, TMOA: Trimethylorthoacetate, TPOF: Tripropylorthoformate
TEOF: Triethylorthoformate, MMKSF: Montmorillonite KSF, MMK10: Montmorillonite K10
1687
INDIAN J. CHEM., SEC B, AUGUST 2005
1688
Plausible mechanism
HO
EtO
OEt
OEt
Alcohol, Catalyst
H3C
H3C
EtOH
OEt
O
+
i
H3C
OEt
O
Ph
+
EtOH o Et2O
O
+
O Et
Et
HO
H3C
+
CH3CO2Et
ii
OEt
Scheme II ⎯ Reaction of orthester withalcohols at rt in the presence of acidic catalysts
metric quantity of orthoester together with appropriate
solid catalysts under N2. The products were isolated
and characterized after chromatography. We found
that Montmorillonite catalysts (both KSF and K10)
gave excellent yields of unsymmetrical ethers 1 and
as stated before, this reaction is found to be general
with regard to various orthoesters but selective with
respect to various primary allylic and benzylic
alcohols. However, when SiO2 is used as the catalyst,
we could isolate corresponding O-acetylated
compound 3 as major product (in case of primary
alcohols) together with minor quantity of dimeric
ethers 2 and unsymmetrical ethers 1 and this catalyst
was effective in case of primary saturated (nonbenzylic and allylic) as well as secondary alcohols
also. As O-acetylation with orthoester is not hitherto a
known reaction, we were curious to explore this to
find out whether such a transformation could be
useful as a synthetic methodology and the results are
summarized in Table I.
Thus the reaction of alcohols and orthoester with
various acid catalysts give following products at
ambient temperature; unsymmetrical ether 1, Oacetylated compound 3, dimeric ethers 2. Formation
of all the above products can be easily explained by a
mechanism involving a cationic intermediate (i, ii)
generated by the action of the acidic catalyst on the
orthoester. The unsymmetrical ether formation is
possible through a horizontal transfer of alkyl group
to the alcohol-oxygen, whereas the O-acetylated
product is possibly formed by the nucleophilic attack
of the alcoholic oxygen on the positively charged
carbon of the cationic intermediate (ii) as shown in
Scheme II. This could be established by allowing the
optically pure secondary alcohol (11, absolute
configuration S) with orthoester in the presence of
silicon dioxide which formed corresponding ether
with retention of configuration as determined by
chemical correlation (absolute configuration of the
product was determined by chemical correlation with
a literature precedent7 that compounds with close
structural resemblance and having same sign of
optical rotation should have same absolute
configuration). However, in case of secondary
alcohols, the acetylated product could be isolated in
only trace amounts.
As the relative abundance of these products mainly
depends upon the nature of the catalyst used, the
product formation can be correlated to the degree of
stabilization of the positively charged intermediate on
the solid acids. For instance, predominant formation
of the unsymmetrical ether in case of Montmorillonite
catalysts can only be explained due to the stabilization
of the positively charged intermediate in the inter
lamellar layer of the smectite clay, whereas the poor
binding in case of silicon dioxide which allows the
nucleophilic attack by the alcohol on the positively
charged carbon center of the intermediate (i) followed
by the loss of alkyl group either to the substrate
(forms unsymmetrical ether) or ethanol (forms diethyl
ether) may result in the formation of O-acetylated
product as shown in Scheme II.
When these reactions were conducted in the
presence of acidic ion exchange resin i.e., amberlyst-15
or Lewis acid like BF3.Et2O, a mixture of
unsymmetrical ether and dimeric ether were found to
be the major components of the product mixture with
only a trace amounts of the corresponding acetylated
product (<5-7%). Formation of the dimeric ethers in
significant proportions may be attributed to the
formation of allylic and benzylic cations by the action
of strong Lewis acid or ion exchange resin which in
turn undergoes nucleophilic attack by the other
alcoholic oxygen to form dimeric ether. However,
when TEOF was used, the transformation was
relatively clean as no acetylated compound was
KUMAR et al.: REACTION OF ORTHOESTERS WITH ALCOHOLS IN THE PRESENCE OF ACIDIC CATALYSTS
formed. Further, this reaction could be conveniently
accelerated at elevated temperature (conventional or
microwave irradiation inw the presence of SiO2) which
gave corresponding unsymmetrical ether in high yields.
Since the above reactions viz., O-alkylation, Oacetylation and dimerization occur at ambient
temperature conditions, this could pose a problem
particularly when we attempt Claisen-Johnson
orthoester rearrangement involving allylic alcohols
employing any such catalysts. It has been disclosed by
other groups that carboxylic and sulfonic acids form
corresponding esters when refluxed with orthoesters.5
As we carry out Claisen rearrangement at higher
temperature thresholds, utility of carboxylic acids (eg,
butyric acid) as catalysts would be disadvantageous.
Because these catalysts get inactivated (in the form of
esters) rapidly and this in turn demands addition of
aliquots of catalysts at regular intervals in order to
take the reaction to forward direction. Even though
use of phenol is found to be an attractive option in this
regard, our continued interest in finding alternative
catalysts to derive an eco-friendly experimental
protocol for this transformation employing reusable
solid catalysts such as Montmorillonites or silicon
R'
CH3C(OEt)3
R
OH
R'
dioxide, prompted us to attempt Claisen-Johnson
orthoester rearrangement of some representative
allylic alcohols using these catalysts. The kind of
products derived from such an effort is clearly evident
as we could isolate mixture of products together with
Claisen rearranged product, which is in conformity
with our earlier observation (Scheme III). We could
also isolate ethyl ether 3 together with the expected
products, i.e., unsymmetrical terminal ether 4, dimeric
ether 5, claisen ester 1 and acetylated product 2. Thus
the complex mixture of the products formed clearly
renders the above protocol unsuitable for this purpose
(Table II).
In conclusion, we have for the first time unravelled
a series of reactions that are associated with the
interaction of orthoesters with primary and secondary
alcohols in the presence of various acid catalysts. OAcetylation is another important transformation
observed for the first time by the interaction of the
alcohols and orthoesters. These reactions, particularly
O-alkylations and O-acetylation could be serious side
reactions when Claisen Johnson orthoester
rearrangement is attempted in the presence of various
acid catalysts.
O
R
R
+
EtO C
Catalyst, reflux 2
2
R
OEt
R
+
O
R'
4
R'
5
Scheme III
Table II ⎯ Result of Claisen-Johnson orthoester rearrangement in the presence of
various acid catalysts under conventional heating
Entry
R
R'
Alcohols
OH
R
R'
R'
R
R'
+
O
1
Catalyst= Montmorillonite-KSF and K10, SiO2
R,R'=H,CH3, Ph
1689
Conditions
Product yield (%)
Orthoester* Catalyst* Time
1
2
3
4
5
1
R=H,R'=C6H5
R=R'=CH3
TEOA
TEOA
MMKSF
MMKSF
6hr
5hr
24
22
5 23 35 3
7 21 32 5
2
R=H, R'=C6H5
R=R'= CH3
TEOA
TEOA
MMK10
MMK10
8hr
6hr
16
13
8 27 34 4
5 24 34 6
3
R=H, R'=C6H5
R=R'=CH3
TEOA
TEOA
SiO2
SiO2
6hr
6hr
*. TEOA: Triethylorthoacetate. MMKSF: Montmorillonite-KSF
MMK10: Montmorillonite-K10
3 32 10 23 3
4 28 8 27 5
OEt
3
1690
INDIAN J. CHEM., SEC B, AUGUST 2005
Experimental Section
Infrared spectra were recorded on a Perkin-Elmer
Infrared 683 spectrophotometer with NaCl optics.
Proton magnetic resonance spectra were recorded on a
Varian GEMINI-200, AVANCE-300, Varian UNITY400 NMR spectrometer in CDCl3. In the 1H NMR
spectra tetramethylsilane was used as an internal
reference. Chemical shifts were reported in ppm
downfield from TMS and were given on the δ scale.
Mass measurements were carried out on a MicroMass
VG70-70H mass spectrometer operating at 70 eV using
direct inlet system and were given in the mass units
(m/z). Unless otherwise stated, all non-aqueous
reactions were performed under an atmosphere of
nitrogen in flame dried glass equipped with stir bar and
a rubber septum. Standard inert atmosphere techniques
were used in handling all air and moister-sensitive
reagents. Reactions were monitored by analytical thin
layer chromatography (TLC) using 0.25-mm E.Merk
precoated silicagel plate (60F254). The spots were
detected using UV light (254 nm), blowing I2 or by
dipping into anisaldehyde/sulfuric acid or βnaphthol/sulfuric acid solution followed by charring on
a hot plate. Product purification by flash column
chromatography was performed on silica gel (100-200
mesh). Solutions in organic solvents were dried over
anhydrous sodium sulfate and solvents were stripped
off on a Buchi rotary evaporator connected to water
aspirator. Trace solvent was removed on a vacuum
pump. TEOF, TEOA, TMOA, TPOF, Benzyl alcohol,
cinnamyl alcohol, Silica Gel, MMK10, MMKSF etc.
are commercially available and used after drying.
(a) Reaction of orthoesters with alcohols at rt.;
O-alkylation/acetylation of alcohols. General procedure. Mixture of alcohol (1 mole), orthoester 2
moles and catalyst (30% m/m) was stirred at rt under
N2 for 4-12 hr. The reaction was monitored by TLC
and after completion of reaction, the reaction mixture
was filtered and catalyst was washed with DCM (10
mL), the filtrate was concentrated in vacuo and the
products were separated and purified by column
chromatography on silica gel to afford the products.
Wherever, BF3.Et2O was used as the catalyst, the
crude product was isolated by quenching the reaction
mass with water (10 mL) followed by extraction with
ether (2×10 mL) and evaporation of the solvent.
Benzyl ethyl ether: 1H NMR (200 MHz, CDCl3): δ
1.25 (t, J= 8.0 Hz, 3H, OCH2CH3), 3.55 (q, J = 8.0
Hz, 2H, OCH2CH3), 4.50 (s, 2H, benzylic CH2) 7.207.40 (m, 5H, aromatic H); IR: 1230, 1087 cm –1; mass
(m/z): 136 (M+).
Dibenzyl ether: 1H NMR (200 MHz, CDCl3): δ
4.58 (s, 4H, benzylic CH2), 7.20-7.40 (m, 10H,
aromatic H); IR: 1113 cm –1; mass (m/z): 198 (M+).
Benzyl acetate: 1H NMR (400 MHz, CDCl3): δ
2.08 (s, 3H, COCH3), 5.10 (s, 2H, benzylic CH2),
7.20-7.40 (m, 5H, aromatic H); IR: 1741,1220, 1027
cm –1; mass (m/z): 150 (M+).
Benzyl methyl ether: 1H NMR (200 MHz,
CDCl3): δ 3.30 (s, 3H, OCH3), 4.55 (s, 2H, benzylic
CH2), 7.20-7.40 (m, 5H, aromatic H); IR: 1225, 1077
cm –1; mass (m/z): 122 (M+).
Ethyl 3-phenyl- (E)-2-propenyl ether: 1H NMR
(300 MHz, CDCl3): δ 1.20 (t, J = 8.0 Hz, 3H,
OCH2CH3), 3.50 (q, 8.0 Hz, 2H, OCH2CH3), 4.15 (d,
J = 7.2 Hz, 2H, allylic CH2), 6.22-6.30 (m, 1H,
olefinic H), 6.50 (d, J = 17.4 Hz, 1H, olefinic H),
7.20-7.40 (m, 5H, aromatic H); IR: 1625, 1203, 1070
cm.–1; mass (m/z): 162 (M+).
Methyl 3-phenyl- (E)-2-propenyl ether: 1H NMR
(200 MHz, CDCl3): δ 3.40 (s, 3H, OCH3), 4.15 (d, J =
7.2, 2H, allylic CH2), 6.20-6.40 (m, 1H, olefinic H),
6.60 (d, J = 17.4 Hz, 1H, olefinic H), 7.20-7.40 (m,
5H, aromatic H); IR: 1641, 1200, 1065 cm.–1; mass
(m/z): 148 (M+).
Propyl 3-phenyl- (E)-2-propenyl ether: 1H NMR
(200 MHz, CDCl3): δ 1.0 (t, J = 7.6 Hz, 3H,
OCH2CH2CH3), 1.60-1.80 (m, 2H, OCH2CH2CH3),
3.44 (t, J = 7.6 Hz, 2H, OCH2CH2CH3), 4.15 (d, J =
7.2 Hz, 2H, allylic CH2), 6.20-6.40 (m, 1H, olefinic
H), 6.60 (d, J = 17.4 Hz, 1H, olefinic H), 7.20-7.40
(m, 5H, aromatic H); IR: 1635, 1203, 1070 cm.–1;
mass (m/z): 176 (M+).
3-Phenyl- (E)-2-propenyl acetate: 1H NMR (300
MHz, CDCl3): δ 2.25 (s, 3H, COCH3), 4.10 (d, J = 7.2
Hz, 2H, allylic CH2), 6.22-6.30 (m, 1H, olefinic H),
6.60 (d, J = 17.4 Hz, 1H, olefinic H), 7.15-7.35 (m,
5H, aromatic H); IR: 1736, 1635, 1236, 1027 cm –1;
mass (m/z): 176 (M+).
Pentyl acetate: 1HNMR (200MHz, CDCl3): δ
0.90-1.40 (m, 9H, OCH2CH2CH2CH2CH3), 2.00 (s,
3H, COCH3), 4.05 (t, J = 7 Hz, 2H, OCH2); IR:
1740,1235,1050 cm.–1; mass (m/z): 130 (M+).
Ethyl pentyl ether: 1H NMR (200 MHz, CDCl3): δ
0.90-1.50 (m, 9H, CH3(CH2)3CH2O), 1.20 (t, J = 8.0
Hz, 3H,OCH2CH3), 3.50 (t, J = 7.4 Hz, 2H,
OCH2(CH2)3CH3), 3.60 (q, J = 8.0 Hz, 2H, OCH2CH3);
IR: 1155, 1090 cm –1; mass (m/z): 116 (M+).
Allyl methyl ether: 1H NMR (200 MHz, CDCl3): δ
3.30 (s, 3H, CH3O), 3.74 (d, J = 7 Hz, 2H, allylic
CH2), 4.90-5.10 (m, 2H, olefinic H), 5.60-5.80 (m,
KUMAR et al.: REACTION OF ORTHOESTERS WITH ALCOHOLS IN THE PRESENCE OF ACIDIC CATALYSTS
1H, olefinic H); IR: 1645, 1208, 1078 cm.–1; mass
(m/z): 72 (M+).
Methyl 3-methyl-2-butenyl ether: 1H NMR (200
MHz, CDCl3): δ 1.50 (s, 3H, vinylic CH3), 1.60 (s,
3H, vinylic CH3), 3.30 (s, 3H, OCH3), 3.80 (d, 8 Hz,
2H, allylic CH2), 5.10-5.20 (m, 1H, olefinic H); IR:
1675, 1215, 1090 cm.–1; mass (m/z): 100 (M+).
1-[4-Methoxy-(Z)-2-butenyloxymethyl] benzene:
1
H NMR (200 MHz, CDCl3): δ 3.30 (s, 3H, CH3O),
4.00 (dd, J= 8.2, 16.8 Hz, 4H, allylic CH2O), 4.45 (s,
2H, benzylic CH2), 5.70-5.90 (m, 2H, olefinic H),
7.20-7.40 (m, 5H, aromatic H); IR: 1640, 1210,1125,
1085, 1034 cm.–1; mass (m/z): 192 (M+).
3,7-Dimethyl-(2E)-2,6-octadienyl methyl ether:
1
H NMR (300 MHz, CDCl3): δ 1.60 (s, 9H, vinylic
CH3), 2.00 (m, 4H, CH2CH2), 3.20 (s, 3H, CH3O),
3.80 (d, J = 8 Hz, 2H,allylic CH2), 5.00 (t, J = 7 Hz,
1H, olefinic H ), 5.25 (t, J = 8 Hz, 1H, olefinic H); IR:
1675, 1660, 1205, 1105 cm.–1; mass (m/z): 168 (M+).
Benzo[d][1,3]dioxol-5-ylmethyl methyl ether:
1
H NMR (200 MHz, CDCl3): δ 3.30 (s,3H, CH3O),
4.40 (s, 2H, benzylic CH2), 5.90 (s, 2H, OCH2O), 6.72
(s, 2H, aromatic H), 6.82 (s,1H, aromatic H); IR:
1272, 1196, 1050 cm.–1; mass (m/z): 166 (M+).
Benzo[d][1,3]dioxol-5-ylmethyl ethyl ether : 1H
NMR (200 MHz, CDCl3): δ 1.20 (t, J = 8.0 Hz, 3H,
OCH2CH3), 3.48 (q, J = 8.0 Hz, 2H, OCH2CH3), 4.40
(s, 2H, benzylic CH2), 5.90 (s, 2H, OCH2O), 6.72 (s,
2H, aromatic H), 6.82 (s,1H, aromatic H); IR: 1247,
1189, 1099 cm.–1; mass (m/z): 180 (M+).
Methyl 2-thienylmethyl ether: 1H NMR (200
MHz, CDCl3): δ 3.44 (s, 3H, CH3O), 4.40 (s, 2H,
CH2O), 6.75 (m, 2H, aromatic H), 7.05 (m, 1H,
aromatic H); IR: 1208, 109°cm. -1; mass (m/z): 128
(M+).
Ethyl 1-phenylethyl ether: 1H NMR (200 MHz,
CDCl3): δ 1.22 (t, J = 8.0 Hz, 3H, OCH2CH3), 1.50 (d,
J = 7.2 Hz, 3H, CHCH3), 3.38(q, J = 8.0 Hz, 2H,
OCH2CH3), 4.50 (q, J = 7.2 Hz, 1H, benzylic H),
7.20-7.30 (m, 5H,aromatic H); IR: 1225,1086 cm.–1;
mass (m/z): 150 (M+).
1-Phenylethyl acetate: 1H NMR (200 MHz,
CDCl3): δ 1.56 (d, J = 7.2 Hz, 3H, CHCH3), 2.11 (s,
3H, COCH3), 5.88 (q, J = 7.2 Hz, 1H, benzylic CH),
7.20-7.30 (m, 5H,aromatic H); IR: 1735, 1261, 1040
cm.–1; mass (m/z): 164 (M+).
2-[1-Ethoxy-(1S)-ethyl]-6-methoxynapthalene:
1
H NMR (200 MHz, CDCl3): δ 1.22 (t, J = 8.0 Hz,
3H, OCH2CH3), 1.50 (d, J = 8.2 Hz, 3H, benzylic CH3 ),
3.38 (q, J = 8.0 Hz, 2H, OCH2CH3), 3.68 (s, 3H,
1691
OCH3), 4.50 (q, J = 8.2 Hz, 1H, benzylic CH), 7.407.56 (m, 3H, aromatic H), 7.62-7.82 (m, 3H, aromatic
H); IR: 1270, 1179, 1101, 1020 cm.–1; mass (m/z):
230 (M+); [α]D25 -4.5 (c= 2.2 CHCl3).
b) Reaction of orthoesters with allylic alcohols
at elevated temperature; Claisen-Johnson orthoester rearrangement. General procedure: Mixture
of allylic alcohol (1 mole), orthoester (3 moles) and
catalyst (30% m/m) was heated at 100°C under N2 for
4-8 hr. The reaction was followed by TLC and after
completion of reaction, the reaction mixture was
filtered and catalyst was washed with DCM (10 mL),
the combined filtrate was concetrated in vacuo and
the products were separated and purified by column
chromatography on silica gel.
Ethyl 3-phenyl-4-pentenoate: 1H NMR (200
MHz, CDCl3): δ 1.18 (t, J = 8.0 Hz, 3H, OCH2CH3),
2.65 (dd, J = 5.2, 9 Hz, 2H, CH2CO2Et), 3.84 (q, J = 9
Hz, 1H, benzylic CH), 4.05 (q, J = 8.0 Hz, 2H,
OCH2CH3), 5.00-5.20 (m, 2H, olefinic H), 5.90-6.10
(m, 1H, olefinic H), 7.15-7.30 (m, 5H, aromatic H);
IR: 1740, 1642, 1235, 1030 cm.–1; mass (m/z): 204
(M+).
Ethyl 2-phenylallyl ether: 1H NMR (300 MHz,
CDCl3): δ 1.22 (t, J = 8 Hz, 3H, OCH2CH3), 3.5 (q, J
= 8 Hz, 2H, O CH2CH3), 4.7 (d, J = 8 Hz, 1H,
benzylic CH), 5.05 (d, J = 18 Hz, 1H, olefinic H),
5.22 (d, J = 8 Hz, 1H, olefinic H), 5.80-6.00 (m, 1H,
olefinic H), 7.25-7.35 (m, 5H, aromatic H); IR: 1650,
1200, 1057 cm.–1; mass (m/z): 162 (M+).
Ethyl 3,3-dimethyl-4-pentenoate: 1H NMR (200
MHz, CDCl3): δ 1.10 (s, 6H, (CH3)2C), 1.18 (t, J = 8
Hz, 3H, OCH2CH3), 2.22 (s, 2H, CH2C=O), 4.05 (q, J
= 8 Hz, 2H, OCH2CH3), 4.85 (d, 1H, J = 7.5 Hz,
olefinic H), 4.95 (d, 1H, J = 15 Hz, olefinic H), 5.82
(dd, J = 7,16 Hz, 1H, olefinic H); IR:1742, 1645,
1220, 1034 cm.–1; mass (m/z): 156 (M+).
Acknowledgement
One of the authors (S J) thanks CSIR, New Delhi
for the award of fellowship.
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