Ring-closing metathesis toward the synthesis of 2,5-dihydrofuran and 2,5-dihydropyrrole skeletons from Baylis-Hillman adducts

Article abstract of DOI:10.1016/j.tetlet.2004.02.047

We have developed an efficient method for the synthesis of 2, 5-dihydrofuran and 2,5-dihydropyrrole skeletons from the simply modified Baylis-Hillman adducts via RCM reaction.

Full text of DOI:10.1016/j.tetlet.2004.02.047

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2805–2808
Ring-closing metathesis toward the synthesis of 2,5-dihydrofuran
and 2,5-dihydropyrrole skeletons from Baylis–Hillman adducts
Jeong Mi Kim,a Ka Young Lee,a Sangku Leeb and Jae Nyoung Kima,*
aDepartment of Chemistry and Institute of Basic Science, Chonnam National University, Kwangju 500-757, South Korea
bKorea Research Institute of Bioscience and Biotechnology, 52 Oun, Yusong, Taejon 305-333, South Korea
Received 15 December 2003;revised 30 January 2004;accepted 6 February 2004
Abstract—We have developed an efficient method for the synthesis of 2,5-dihydrofuran and 2,5-dihydropyrrole skeletons from the
simply modified Baylis–Hillman adducts via RCM reaction.
Ó 2004 Elsevier Ltd. All rights reserved.
Ring-closing metathesis (RCM) reaction is a powerful
tool in modern chemistry due to its wide applicability in
synthetic organic chemistry.1 By using the RCM reac-
tion tremendous cyclic compounds have been elegantly
constructed including carbocyclic and heterocyclic
ring.1–4
HX
X
SN2
OAc
RCM
EWG
EWG
SN2 via consecutive SN2'-SN2'
RCM by Grubbs catalyst
XH = OH, NHTs
EWG = COOEt, CN, COMe
Crowe and Goldberg have reported the cross-metathesis
(CM) between acrylonitrile and various terminal ole-
fins.2 Other p-conjugated olefins such as enones and
enoic esters failed in cross-metathesis with less reactive
old-fashioned catalyst. Recently, Grubbs and co-work-
ers have reported the first successful results on the
intermolecular cross-metathesis and intramolecular
ring-closing metathesis with more reactive ruthenium
catalyst for the a-functionalized olefin system bearing
ester, aldehyde, benzoyl, or acetyl group.3 More
recently, ring-closing metathesis reaction of a-function-
alized olefins have been studied extensively by many
research groups.4
Figure 1.
as an ester or nitrile functionality. As mentioned above
Grubbs and co-workers have reported the CM reaction
and RCM reaction by using such an olefinic double
bond.3 Thus, if we could introduce another double bond
at the benzylic position of the Baylis–Hillman
adducts5e;9 we could prepare the desired RCM products
(Scheme 1).
To realize our idea we prepared O-allyl derivative 1a
of the Baylis–Hillman adduct via the well-known
During the studies on the Baylis–Hillman and related
chemistry5 we envisioned that we could construct some
useful ring systems by using the Baylis–Hillman adducts
as the starting materials including 2,5-dihydropyrrole6;8
and 2,5-dihydrofuran7;8 as shown in Figure 1.
X
X
Grubbs cat (5-7%)
CH2Cl2
reflux, 15 min. - 4 h
EWG
EWG
The Baylis–Hillman adducts have one olefinic double
bond bearing an electron-withdrawing substituent such
X = O, NTs
EWG = COOEt, CN, COMe
N Mes
Cl
Mes
N
Grubbs type II catalyst
Keywords: Ring-closing metathesis;2,5-Dihydrofuran;2,5-Dihydro-
pyrrole;Baylis–Hillman adducts.
Ru
Cl
Ph
PCy3
* Corresponding author. Tel.: +82-62-530-3381;fax: +82-62-530-3389;
e-mail: kimjn@chonnam.ac.kr
Scheme 1.
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.02.047

2806
J. M. Kim et al. / Tetrahedron Letters 45 (2004) 2805–2808
Table 1. RCM reaction of Baylis–Hillman adducts
Entry
1
B–H adducts 1
Conditions
Products 2
Yields (%)
99
O
O
Catalyst (5 mol %) CH2Cl2 reflux, 15 min
COOEt
2a
COOEt 1a
O
O
O
O
2
3
Catalyst (5 mol %) CH2Cl2 reflux, 15 min
Catalyst (5 mol %) CH2Cl2 reflux, 15 min
88
98
2b
CN
1b
CN
COMe
2c
1c
1d
1e
COMe
COOEt
COMe
Ts
N
Ts
Ts
N
N
4
5
Catalyst (7 mol %) CH2Cl2 reflux, 4 h
Catalyst (5 mol %) CH2Cl2 reflux, 4 h
98
99
COOEt
COMe
2d
2e
Ts
N
consecutive SN20–SN20 reaction using DABCO.5e;9 With
this compound in hand we examined the RCM reaction
by using the commercial second generation Grubbs
catalyst (5 mol %) in dichloromethane (reflux, 15 min)
and obtained the 2,5-dihydrofuran 2a in quantitative
yield (99%).10
can occur via the metal carbene intermediate, generated
at the N-allyl moiety initially. However, the starting
material 1g, which has the E-configuration, gave also the
corresponding RCM product, N-tosyl-3-ethoxycar-
bonyl-2,5-dihydropyrrole 2g without any problem irre-
spective of the double bond configuration.
Thus, we prepared similar analogs 1b–e and prepared
the RCM products 2b–e in excellent yields. We wish to
report herein the results. As shown in Scheme 1 and
Table 1, 2,5-dihydrofuran derivatives 2b and 2c were
synthesized irrespective of the EWG including ester,
nitrile, and acetyl group. As a next trial, we prepared the
nitrogen analog 1d and 1e and examined the RCM
reaction. Fortunately we could obtain the corresponding
2,5-dihydropyrroles 2d and 2e in good yields.
In conclusion, we have developed an efficient method for
the synthesis of 2,5-dihydrofuran and 2,5-dihydropyr-
role backbone from the simply modified Baylis–Hillman
adducts via RCM reaction.
Acknowledgements
This work was supported by the grant (R05-2003-000-
10042-0) from the Basic Research Program of the Korea
Science & Engineering Foundation. Spectroscopic data
was obtained from the Korea Basic Science Institute,
Kwangju branch.
It is interesting to note that the RCM reaction of pri-
mary analog 1f afforded the corresponding N-tosyl-3-
cyano-2,5-dihydropyrrole 2f in 80% yield (Scheme 2).
This compound was generated by the elimination of
styrene moiety (isolated as trans-stilbene by CM in 79%
yield) during the RCM reaction.11 We thought at first
that the configuration around the double bond in such a
primary compound is very important for the successful
RCM reaction. The starting material 1f has Z-configu-
ration around the double bond and the RCM reaction
References and notes
1. For recent reviews, see: (a) Furstner, A. Angew. Chem.,
Int. Ed. 2000, 39, 3013;(b) Trnka, T. M.;Grubbs, R. H.
Acc. Chem. Res. 2001, 34, 18.
2. Crowe, W. E.;Goldberg, D. R. J. Am. Chem. Soc. 1995,
117, 5162.
3. Chatterjee, A. K.;Morgan, J. P.;Scholl, M.;Grubbs,
R. H. J. Am. Chem. Soc. 2000, 122, 3783.
4. Since Grubbs and co-workers have reported the CM and
RCM reactions of the a-functionalized olefins3 many
research groups reported on the reactions involving the
disubstituted electron-deficient alkenes, see: (a) Hekking,
K. F. W.;van Delft, F. L.;Rutjes, F. P. J. T. Tetrahedron
2003, 59, 6751;(b) Morgan, J. P.;Grubbs, R. H. Org. Lett.
2000, 2, 3153;(c) Randl, S.;Buschmann, N.;Connon,
Ts
N
N
N
catalyst (10 mol%)
CH2Cl2
CN Ts
2f (80%)
2g (96%)
reflux, 24 h
CN
1f (Z)
catalyst (10 mol%)
CH2Cl2
reflux, 2 h
COOEt
Ts
N
COOEt
Ts
1g (E/Z = 9:1 mixture)
Scheme 2.

J. M. Kim et al. / Tetrahedron Letters 45 (2004) 2805–2808
2807
S. J.;Blechert, S. Synlett 2001, 1547;(d) Alcaide, B.;
Almendros, P.;Alonso, J. M.;Aly, M. F.;Redondo, M. C.
Synlett 2001, 773;(e) Royer, F.;Vilain, C.;Elkaim, L.;
Grimaud, L. Org. Lett. 2003, 5, 2007;(f) Louie, J.;
Bielawski, C. W.;Grubbs, R. H. J. Am. Chem. Soc. 2001,
123, 11312;(g) Anand, R. V.;Baktharaman, S.;Singh, V.
Tetrahedron Lett. 2002, 43, 5393;(h) Rajesh, S.;Banerji,
B.;Iqbal, J. J. Org. Chem. 2002, 67, 7852;(i) Alcaide, B.;
Almendros, P.;Alonso, J. M.;Redondo, M. C. J. Org.
Chem. 2003, 68, 1426;(j) Chen, X.;Wiemer, D. F. J. Org.
Chem. 2003, 68, 6597.
Synthesis of N-allyl derivative 1d: The reaction of tosyl-
amide and allyl bromide gave the N-allyltosylamide in
53% yield (K2CO3, acetone, reflux, 7 h). To a stirred
solution of the corresponding Baylis–Hillman acetate
(300 mg, 1.2 mmol) in aqueous THF (4 mL, THF/H2O,
3:1) was added DABCO (163 mg, 1.45 mmol) and stirred
for 10 min at rt. To the reaction mixture was added N-
allyltosylamide (255 mg, 1.2 mmol) and heated to 50–60 °C
for 3 days. After usual workup and column chromato-
graphic purification process (hexane/ethyl acetate, 4:1) we
could obtain 220 mg of crude product as the mixture of
Baylis–Hillman alcohol and desired product 1d (in a ratio
of 2:5 by 1H NMR). The two compounds have very
similar mobility on TLC and we cannot separate them
easily. Thus, we converted the Baylis–Hillman alcohol into
its acetate again (Ac2O, DMAP, CH2Cl2, rt, 4 h) and
separate the desired product 1d in 31% isolated yield
(149 mg). The spectroscopic data of prepared compound
1d are listed below.
5. Our recent publications on the Baylis–Hillman chemistry:
(a) Lee, K. Y.;Kim, J. M.;Kim, J. N. Tetrahedron Lett.
2003, 44, 6737;(b) Kim, J. N.;Kim, J. M.;Lee, K. Y.
Synlett 2003, 821;(c) Im, Y. J.;Lee, C. G.;Kim, H. R.;
Kim, J. N. Tetrahedron Lett. 2003, 44, 2987;(d) Lee,
K. Y.;Kim, J. M.;Kim, J. N.
Kim, J. N.;Lee, H. J.;Lee, K. Y.;Gong, J. H.
Synlett 2003, 357;(e)
Synlett
2002, 173;(f) Lee, K. Y.;Kim, J. M.;Kim, J. N.
Tetrahedron 2003, 59, 385, and further references cited
therein.
1d (31%): 1H NMR (CDCl3) d 1.08 (t, J ¼ 7:1 Hz, 3H),
2.43 (s, 3H), 3.79–3.84 (m, 2H), 3.98–4.12 (m, 2H), 4.80 (s,
1H), 4.84 (d, J ¼ 7:3 Hz, 1H), 5.20–5.26 (m, 1H), 5.70 (s,
1H), 6.11 (s, 1H), 6.43 (s, 1H), 7.00–7.28 (m, 7H), 7.69 (d,
J ¼ 8:3 Hz, 2H).
6. For the synthesis of 2,5-dihydropyrrole derivatives, see: (a)
Xu, Z.;Lu, X. J. Org. Chem. 1998, 63, 5031;(b) Xu, Z.;
Lu, X. Tetrahedron Lett. 1997, 38, 3461.
7. For the synthesis of 2,5-dihydrofurans, see: (a) Hojo, M.;
Ohkuma, M.;Ishibashi, N.;Hosomi, A. Tetrahedron Lett.
1993, 34, 5943;(b) Hojo, M.;Ishibashi, N.;Hosomi, A.
Synlett 1996, 234;(c) Tiecco, M.;Testaferri, L.;Santi, C.
Eur. J. Org. Chem. 1999, 797.
8. For the synthesis of dihydrofurans and dihydropyrroles by
RCM, see: (a) Wakamatsu, H.;Blechert, S. Angew. Chem.,
Int. Ed. 2002, 41, 794;(b) Wakamatsu, H.;Blechert, S.
Angew. Chem., Int. Ed. 2002, 41, 2403;(c) Yang, C.;
Murray, W. V.;Wilson, L. J. Tetrahedron Lett. 2003, 44,
1783.
9. For the introduction of nucleophile in an SN2 fashion via
using the DABCO salt concept, see: (a) Basavaiah, D.;
Kumaragurubaran, N.;Sharada, D. S. Tetrahedron Lett.
2001, 42, 85;(b) Im, Y. J.;Kim, J. M.;Mun, J. H.;Kim,
J. N. Bull. Korean Chem. Soc. 2001, 22, 349;(c) Basavaiah,
D.;Jaganmohan, R.;Satyanarayana, T. Chem. Rev. 2003,
103, 811, and further references cited therein.
10. Synthesis of allyl ether 1a: To a stirred solution of the
corresponding Baylis–Hillman acetate (500 mg, 2 mmol) in
THF (3 mL) was added DABCO (452 mg, 4 mmol) and
stirred for 10 min at rt. To the reaction mixture was added
allyl alcohol (1.5 mL) and heated to 50–60 °C for 3 days.
After usual workup process and column chromatographic
purification process (hexane/ether, 30:1) we could obtain
the desired compound 1a in 71% isolated yield (350 mg).
Other compounds 1b and 1c were synthesized by using the
same experimental procedure. The spectroscopic data of
prepared compounds are listed below.
Synthesis of N-allyl derivative 1e: To a stirred solution of
the corresponding Baylis–Hillman acetate (400 mg,
1.835 mmol) in aqueous THF (4 mL, THF/H2O, 3:1) was
added DABCO (247 mg, 2.2 mmol) and stirred for 10 min
at rt. To the reaction mixture was added tosylamide
(314 mg, 1.835 mmol) and heated to 60–70 °C for 20 h.
After usual workup and column chromatographic purifi-
cation process (hexane/CH2Cl2/ether, 10:10:1) we could
obtain 252 mg of the Baylis–Hillman adduct derived from
N-tosylimine in 42% yield.5e To the reaction mixture of
this compound (130 mg, 0.395 mmol) and allyl bromide
(72 mg, 0.593 mmol) in DMF (2 mL) was added K2CO3
(82 mg, 0.593 mmol) and stirred at room temperature for
1 h. After usual workup and column chromatographic
purification process (hexane/ether, 3:1) we could obtain
the desired product 1e in 103 mg (71%).
1e: 1H NMR (CDCl3) d 2.29 (s. 3H), 2.43 (s, 3H), 3.82 (d,
J ¼ 6:9 Hz, 2H), 4.81–4.84 (m, 1H), 4.87 (s, 1H), 5.22–5.35
(m, 1H), 6.00 (d, J ¼ 1:5 Hz, 1H), 6.12 (s, 1H), 6.32 (d,
J ¼ 1:5 Hz, 1H), 6.93–6.97 (m, 2H), 7.18–7.28 (m, 5H),
7.67 (d, J ¼ 8:4 Hz, 2H); 13C NMR (CDCl3) d 21.49,
26.31, 48.99, 60.99, 117.68, 127.50, 127.76, 127.88, 128.44,
128.61, 129.40, 134.39, 137.19, 137.79, 143.17, 147.44,
198.03.
Synthesis of N-allyl derivative 1f: To a stirred solution of
the corresponding Baylis–Hillman acetate (100 mg,
0.498 mmol) and N-allyltosylamide (158 mg, 0.747 mmol)
in DMF (2 mL) was added K2CO3 (103 mg, 0.747 mmol)
and stirred at room temperature for 1 h. After usual
workup and column chromatographic purification process
(hexane/ether, 4:1) we could obtain the desired product 1f
in 106 mg (60%). Starting material 1g was prepared
similarly in 40% yield.
1a (71%): 1H NMR (CDCl3) d 1.22 (t, J ¼ 7:2 Hz, 3H),
3.96 (dt, J ¼ 5:4 and 1.5 Hz, 2H), 4.09–4.20 (m, 2H), 5.13–
5.29 (m, 3H), 5.85–5.98 (m, 2H), 6.31 (s, 1H), 7.23–7.37
(m, 5H); 13C NMR (CDCl3) d 14.31, 60.67, 69.85, 78.38,
116.90, 124.81, 127.65, 127.82, 128.26, 134.59, 139.68,
141.52, 165.84.
1f: 60%; 1H NMR (CDCl3) d 2.41 (s, 3H), 3.93 (d,
J ¼ 6:3 Hz, 2H), 4.11 (s, 2H), 5.19–5.26 (m, 2H), 7.12 (s,
1H), 7.29 (d, J ¼ 8:1 Hz, 2H), 7.41–7.44 (m, 3H), 7.67–
7.71 (m, 2H), 7.74 (d, J ¼ 8:1 Hz, 2H).
1b (50%): 1H NMR (CDCl3) d 4.00 (dt, J ¼ 5:7 and
1.5 Hz, 2H), 4.93 (s, 1H), 5.20–5.34 (m, 2H), 5.85–5.98 (m,
1H), 6.00–6.01 (m, 2H), 7.32–7.42 (m, 5H); 13C NMR
(CDCl3) d 69.74, 80.09, 116.86, 117.74, 125.12, 127.00,
128.75 (2C), 130.35, 133.64, 137.28.
1g: 40%; 1H NMR (CDCl3) d 1.31 (t, J ¼ 7:2 Hz, 3H), 2.40
(s, 3H), 3.74 (d, J ¼ 6:3 Hz, 2H), 4.19 (q, J ¼ 7:2 Hz, 2H),
4.24 (s, 2H), 4.90–4.97 (m, 2H), 5.45–5.56 (m, 1H), 7.21 (d,
J ¼ 8:1 Hz, 2H), 7.39–7.41 (m, 5H), 7.54 (d, J ¼ 8:1 Hz,
2H), 7.80 (s, 1H).
1
1c (71%): H NMR (CDCl3) d 2.81 (s, 3H), 3.91–3.94 (m,
2H), 5.13–5.29 (m, 2H), 5.41 (s, 1H), 5.84–5.97 (m, 1H),
6.17 (s, 2H), 7.21–7.37 (m, 5H); 13C NMR (CDCl3)
d 26.30, 69.79, 77.18, 116.85, 125.03, 127.36, 127.65,
128.25, 134.58, 140.09, 149.58, 198.40.
Typical procedure for the synthesis of 2,5-dihydrofuran
derivative 2a: To a stirred solution of allyl ether 1a
(123 mg, 0.5 mmol) in dichloromethane (50 mL) was added

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J. M. Kim et al. / Tetrahedron Letters 45 (2004) 2805–2808
Grubbs catalyst (21 mg, 0.025 mmol, 5 mol %) and heated
4.55 (m, 1H), 5.72–5.75 (m, 1H), 6.77 (q, J ¼ 2:1 Hz, 1H),
7.13 (d, J ¼ 8:1 Hz, 2H), 7.18-7.25 (m, 5H), 7.41 (d,
J ¼ 8:1 Hz, 2H); 13C NMR (CDCl3) d 13.82, 21.42, 54.85,
60.76, 68.97, 127.06, 127.78, 127.93, 128.19, 129.41,
135.39, 135.58, 135.98, 129.43, 143.22, 161.76;HRMS
calcd for C20H21NO4S 371.1191, found 371.1187.
to reflux for 15 min. After removal of the solvent
and column chromatographic purification process
(hexane/ether, 2:1) we could obtain the 2,5-dihydrofuran
derivative 2a, 108 mg (99%). Other compounds were
synthesized by using the same experimental procedure.
The spectroscopic data of prepared compounds are listed
below.
2e (99%): white solid, mp 155–156 °C;IR (KBr) 1674,
1342, 1165 cmꢀ1; 1H NMR (CDCl3) d 2.16 (s, 3H), 2.36 (s,
3H), 4.43 (ddd, J ¼ 17:7, 5.7, and 2.4 Hz, 1H), 4.51–4.59
(dt, J ¼ 17:7 and 2.4 Hz, 1H), 5.76–5.79 (m, 1H), 6.66–
6.68 (m, 1H), 7.13 (d, J ¼ 8:4 Hz, 2H), 7.21 (s, 5H), 7.42
(d, J ¼ 8:4 Hz, 2H); 13C NMR (CDCl3) d 21.41, 26.99,
55.06, 68.78, 127.05, 127.61, 127.88, 128.26, 129.43,
135.08, 135.42, 139.47, 143.27, 143.91, 192.91;HRMS
calcd for C19H19NO3 S 341.1086, found 341.1081.
1
2a (99%): IR (KBr) 1720, 1261 cmꢀ1; H NMR (CDCl3)
d 1.51 (t, J ¼ 7:2 Hz, 3H), 4.00–4.17 (m, 2H), 4.86 (ddd,
J ¼ 15:9, 3.9, and 1.8 Hz, 1H), 5.01 (ddd, J ¼ 15:9, 6.3,
and 1.8 Hz, 1H), 5.90–5.94 (m, 1H), 7.00 (q, J ¼ 1:8 Hz,
1H), 7.25–7.36 (m, 5H); 13C NMR (CDCl3) d 13.90, 60.51,
75.26, 86.88, 127.10, 128.11, 128.24, 135.91, 138.25,
140.81, 162.23;HRMS calcd for C 13H14O3 218.0943,
found 218.0947.
1
2f (80%): mp 161–162 °C;IR (KBr) 2233 cm ꢀ1; H NMR
1
2b (88%): IR (KBr) 2229 cmꢀ1; H NMR (CDCl3) d 4.91
(CDCl3) d 2.45 (s, 3H), 4.24–4.32 (m, 4H), 6.52–6.53 (m,
(ddd, J ¼ 15:9, 4.2, and 1.8 Hz, 1H), 5.03 (ddd, J ¼ 15:9,
6.3, and 1.8 Hz, 1H), 5.82–5.87 (m, 1H), 6.88 (q,
J ¼ 1:8 Hz, 1H), 7.32–7.44 (m, 5H); 13C NMR (CDCl3)
d 76.00, 87.61, 113.15, 116.04, 126.34, 128.85, 129.01,
138.22, 142.75;HRMS calcd for C 11H9NO 171.0684,
found 171.0688.
1H), 7.36 (d, J ¼ 8:4 Hz, 2H), 7.71 (d, J ¼ 8:4 Hz, 2H); 13
C
NMR (CDCl3) d 21.51, 54.56, 55.19, 110.73, 112.81,
127.36, 130.10, 133.20, 141.83, 144.40;HRMS calcd for
C12H12N2O2S 248.0619, found 248.0623.
2g (96%): mp 103–104 °C;IR (KBr) 1712, 1346,
1
1161 cmꢀ1; H NMR (CDCl3) d 1.27 (t, J ¼ 7:2 Hz, 3H),
2c (98%): IR (KBr) 1763, 1685 cmꢀ1
;
1H NMR (CDCl3)
2.43 (s, 3H), 4.18 (q, J ¼ 7:2 Hz, 2H), 4.28 (s, 4H), 6.58 (s,
1H), 7.33 (d, J ¼ 8:4 Hz, 2H), 7.73 (d, J ¼ 8:4 Hz, 2H);
13C NMR (CDCl3) d 14.07, 21.47, 53.47, 55.38, 60.92,
127.40, 129.87, 131.90, 133.73, 135.71, 143.79,
162.05;HRMS calcd for C 14H17NO4S 295.0878, found
295.0871.
d 2.27 (s, 3H), 4.90 (ddd, J ¼ 16:5, 3.3, and 2.1 Hz, 1H),
5.06 (ddd, J ¼ 16:5, 6.0, and 2.1 Hz, 1H), 5.96–5.99 (m,
1H), 6.91 (q, J ¼ 2:1 Hz, 1H), 7.26–7.35 (m, 5H); 13C
NMR (CDCl3) d 27.56, 75.26, 86.96, 127.00, 128.11,
128.35, 138.17, 140.80, 143.96, 193.43;HRMS calcd for
C12H12O2 188.0837, found 188.0835.
11. For the RCM reaction involving the styryl moiety, see: (a)
Maishal, T. K.;Sarkar, A. Synlett 2002, 1925;(b) Maity,
B. C.;Swamy, V. M.;Sarkar, A. Tetrahedron Lett. 2001,
42, 4373.
2d (98%): IR (KBr) 1720, 1346, 1161 cmꢀ1 1H NMR
;
(CDCl3) d 1.09 (t, J ¼ 7:2 Hz, 3H), 2.36 (s, 3H), 3.93–4.10
(m, 2H), 4.37 (ddd, J ¼ 16:8, 5.7, and 2.1 Hz, 1H), 4.48–