A facile route to N-acetyl α,β-unsaturated γ-lactam derivatives using ethyl acetamidocyanoacetate and dialkyl acetylenedicarboxylate in the presence of triphenylphosphine

Article abstract of DOI:10.1016/j.tetlet.2008.01.063

The three component reaction of ethyl acetamidocyanoacetate with dialkyl acetylenedicarboxylates and triphenylphosphine leads to phosphorus ylides in good yields. These ylides undergo a 1,2-proton shift, loss of triphenylphosphine, and subsequent intramol

Full text of DOI:10.1016/j.tetlet.2008.01.063

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 1824–1827
A facile route to N-acetyl a,b-unsaturated c-lactam derivatives
using ethyl acetamidocyanoacetate and dialkyl acetylenedicarboxylate
in the presence of triphenylphosphine
Sakineh Asghari *, Mahmood Tajbakhsh, Vali Taghipour
Department of Chemistry, University of Mazandaran, PO Box 453, Babolsar, Iran
Received 17 October 2007; revised 1 January 2008; accepted 10 January 2008
Available online 15 January 2008
Abstract
The three component reaction of ethyl acetamidocyanoacetate with dialkyl acetylenedicarboxylates and triphenylphosphine leads to
phosphorus ylides in good yields. These ylides undergo a 1,2-proton shift, loss of triphenylphosphine, and subsequent intramolecular
amidation leads to the formation of N-acetyl a,b-unsaturated c-lactams.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Ethyl acetamidocyanoacetate; Dialkyl acetylenedicarboxylate; Phosphorus ylides; N-Acetyl a,b-unsaturated c-lactam
Five membered cyclic lactams have been used success-
fully in routes to various alkaloids1–3 and are suitable pre-
cursors for unusual c-amino acids such as statine and its
analogues.4,5 There are also many examples of pyrroline-
containing natural products with interesting pharmacolog-
ical activities, such as antihypertensive,6 antitumor, and
antibiotic activities.7,8 3-Pyrrolin-2-ones are also important
structural units of the related indolocarbazole alkaloids
(+)-staurosporine9 and (+)-K252a,10 which are strong
kinase inhibitors widely used as molecular tools. On the
other hand, 3,4-diaryl- and 1,3,4-triaryl-3-pyrrolin-2-ones,
have been shown to be a potential new type of selective
COX2 inhibitor.11,12 Moreover, the a,b-unsaturated c-
butyrolactam moiety can be utilized as a Michael acceptor
for a variety of nucleophiles.13 Therefore, the synthesis of
3-pyrrolin-2-ones is currently receiving considerable
attention.14
ylphosphine. Thus, the reaction of a strong CH-acid such
as 1 with an acetylenic ester 2 in the presence of triphenyl-
phosphine leads to a stable phosphorus ylide 3. These
ylides are converted into N-acetyl a,b-unsaturated c-lactam
derivatives 4 in boiling toluene18 (Scheme 1).
On the basis of the chemistry of trivalent phosphorus
nucleophiles,19–21 it is reasonable to assume that N-acetyl
a,b-unsaturated c-lactam 4 results from the initial addition
of triphenylphosphine to the acetylenic ester and sub-
sequent protonation of the reactive 1:1 adduct by 1, fol-
lowed by the attack of the conjugate base of 1 on vinyltri-
phenylphosphonium cation 5 to generate phosphorane 3
which in turn is converted into betaine 6 by a 1,2-proton
transfer. Loss of triphenylphosphine and cyclization of
betaine 6 leads to compound 4 (Scheme 2).
The structures of compounds 3a–c were deduced from
1
their elemental analysis and their MS, IR, H, 13C, and
As part of our current studies on the development of
new routes to heterocyclic systems,15–17 we report on the
reaction between ethyl acetamidocyanoacetate 1 and di-
alkyl acetylenedicarboxylates 2 in the presence of triphen-
31P NMR spectra. Ylide 3a could have been formed as a
pair of diastereomers, but according to the NMR analysis
only one diastereomer appeared to have been formed.
However, we did not succeed in determining the stereo-
chemistry of ylide 3a.
The 1H and 13C NMR spectra of 3b were consistent with
the presence of two isomers. The ylide moiety is strongly
*
Corresponding author. Tel.: +98 1125246000; fax: +98 1125242002.
E-mail address: s.asghari@umz.ac.ir (S. Asghari).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.01.063

S. Asghari et al. / Tetrahedron Letters 49 (2008) 1824–1827
1825
O
O
EtO2C
O
CH2Cl2
PPh3
H
NC
RO2C
CO2R
+
+
r.t.
N
H
1
NC
EtO2C
NC
NH
NH
2
-
EtO2C
EtO2C
O
OEt
EtO2C
EtO2C
O
H
+
O
+
N
Ph3P
OEt
Ph3P
O
EtO2C
NC
N
-
Toluene
reflux
NC
RO2C
O
3Z-isomer
3E-isomer
CO2R
H
RO2C
H
Scheme 3.
PPh3
3
4
tional) isomers. From the spectrum it was seen that the
methine proton appeared as a pair of symmetrical doublets
with unequal intensities at d 3.18 ppm (3JPH 18.5 Hz) and d
3.28 ppm (3JPH 18.5 Hz) at 25 °C. Raising the temperature
also led to the gradual collapse of these resonances to a sin-
gle doublet. The NH proton was observed as a pair of sing-
lets at d 9.63 and d 10.17 ppm. The 31P NMR spectrum of
3b exhibited two singlets at d 25.68 and d 26.28 ppm. The
13C NMR of 3b exhibited two doublets of unequal intensi-
ties at d 40.09 (1JPC 124.0 Hz, P@C) for the E-isomer and
at d 41.27 (1JPC 124.0 Hz, P@C) for the Z-isomer and
two doublets at d 49.71 (2JPC 13.4 Hz), and d 50.70 (2JPC
13.9 Hz) for the methine carbons of the E- and Z-isomers,
respectively.
2-4
a
R
Me
Et
b
c
t-Bu
Scheme 1.
conjugated with the adjacent carbonyl group, and the rota-
tion about the partial double bond in isomers (E)-3b and
(Z)-3b is slow on the NMR time scale at ambient temper-
ature. Assignment of the (Z)-configuration to the major
1
geometric isomer is based on the H chemical shift of the
1
The H NMR of 4a exhibited a triplet at d 1.36 ppm
OR moiety, which is expected to be shielded as a result
of the anisotropic effect of the phenyl groups (Scheme 3).
The 1H NMR spectrum of 3b at room temperature
(25 °C) exhibited two pairs of sharp singlets at d 2.00 and
d 2.03 ppm with an intensity of 42:58 for the methyl groups
of the E- and Z-isomer, respectively. Increasing the temper-
ature resulted in coalescence of their acetyl resonances. At
60 °C, a fairly broad singlet was observed for the acetyl
group. This observation was attributed to the temperature
dependent equilibrium between the geometrical (rota-
(3JHH 7.3 Hz) and a multiplet at d 4.14–4.23 ppm for the
ethoxy group, a singlet at d 3.82 ppm for the methoxy
group and a singlet at d 6.95 ppm for the olefinic proton.
The 13C NMR spectrum of 4a displayed twelve distinct res-
1
onances in agreement with the lactam structure. The H
and 13C NMR spectra of 4b and 4c were similar to those
of 4a except for the ester groups, which exhibited charac-
teristic resonances with appropriate chemical shifts (see
Ref. 18).
O
EtO2C
CO2R
CO2R
+
+
H
NC
O
Ph3P
EtO2C
Ph3P
H
N
..
PPh3
-
H
-
+
N
NC
RO2C
CO2R
RO2C
CO2R
H
5
O
H
N
EtO2C
NC
H
O
EtO2C
O
N
RO2C
NH
..
NC
RO2C
H
1,2-proton transfer
-PPh3
OR
O
NC
-
CO2R
H
RO2C
CO2R
PPh3
RO2C
+
PPh3
6
3
O
EtO2C
N
O
-ROH
NC
RO2C
H
4
Scheme 2.

1826
S. Asghari et al. / Tetrahedron Letters 49 (2008) 1824–1827
1
230 mesh) column chromatography using hexane/ethyl acetate (1:5)
The structural assignments made on the basis of the H
and 13C NMR spectra of compounds 4b and 4c were
supported by the measurement of their IR spectra. The
carbonyl regions of the spectra exhibited distinct absorp-
tion bands for each compound. The mass spectra of
4b–4c displayed molecular ion peaks and other fragmenta-
tions involved with the loss of the ester moiety.
as eluent. The solvent was removed to afford product 3a as a white
powder, yield 1.1 g (80%), mp 193–195 °C. IR (KBr) (mmax, cmÀ1):
3474 (NH), 2378 (CN), 1766, 1736 (C@O), 1692 (C@C); 1H NMR
3
(500.1 MHz, CDCl3): dH 1.03 (3H, t, JHH 7 Hz, CH3), 2.06 (3H, s,
3
CH3), 3.04 (3H, s, OCH3), 3.29 (1H, d, JPH 18.5 Hz, CH), 3.69 (3H,
s, OCH3), 3.99–4.04 (2H, m, OCH2), 7.45–7.77 (15H, m, arom), 9.63
(1H, s, NH); 13C NMR (125.8 MHz, CDCl3): dC 13.70 (CH3), 23.24
1
2
(CH3), 41.57 (d, JPC 123.9 Hz, P@C), 44.64 (OCH3), 50.60 (d, JPC
In conclusion, the method presented here carries the
advantage of being performed under neutral conditions
and requires no activation or modification of the reactants.
We anticipate that the reactions described herein represent
a simple process in the synthesis of polyfunctionalized a,b-
unsaturated c-lactams of interest.
3
14.0 Hz, CH), 52.3 (OCH3), 62.64 (OCH2), 62.97 (d, JPC 4.0 Hz,
quaternary carbon), 116.75 (CN), 125.78 (d, JPC 93.5 Hz, Cipso),
128.75 (d, 3JPC 12.3 Hz, Cmeta), 132.39 (Cpara), 134.14 (d, 2JPC 8.3 Hz,
1
2
Cortho), 164.55 (C@O, amide), 170.08 (C@O, ester), 170.91 (d, JPC
5.3 Hz, C@O ester), 172.42 (d, JPC 12.2 Hz, C@O ester); 31P NMR
3
(202.5 MHz, CDCl3): dP 26.24; MS (EI), m/z (%): 574 (1), 278 (5), 262
(2), 239 (30), 221 (91), 175 (44), 57 (100), 43 (21). Anal. Calcd for
C31H31N2O7P (574.57): C, 64.80; H, 5.44; N, 4.88. Found: C, 64.86;
H, 5.49; N, 4.83.
References and notes
Compound 3b: White powder, mp 164–166 °C, yield 85%, IR (KBr)
(mmax, cmÀ1): 3459 (NH),1762, 1740 (C@O), 1681 (C@C); 1H NMR
1. Casiraghi, G.; Spanu, P.; Rassu, G.; Pinna, L.; Ulghri, F. J. Org.
Chem. 1994, 59, 2906–2909.
2. Griffart, B. D.; Langois, N. Tetrahedron Lett. 1994, 35, 119–122.
3. Bergman, J.; Pelcman, B. Tetrahedron Lett. 1978, 28, 4441–4444.
4. Ma, D.; Ma, J.; Ding, W.; Dal, L. Tetrahedron: Asymmetry 1996, 7,
2365–2370.
5. Jouin, P.; Castro, B. J. Chem. Soc., Perkin Trans. 1 1987, 1177–1182.
6. Omura, S.; Iwai, Y.; Hiraho, A.; Nakagawa, A.; Awaya, J.; Tsuchiya,
H.; Takahashi, Y.; Masuma, R. J. Antibiot. 1977, 30, 275–282.
7. Kaneko, T.; Wong, H.; Okamoto, K. T.; Clardy, J. Tetrahedron Lett.
1985, 26, 4015–4018.
8. Nettleton, D. E.; Doyle, T. W.; Krishnan, B.; Matsumoto, G. K.;
Clardy, I. Tetrahedron Lett. 1985, 26, 4011–4014.
9. (a) Furusaki, A.; Hashiba, N.; Matsumoto, T.; Hirano, A.; Iwai, Y.;
Omura, S. J. Chem. Soc., Chem. Commun. 1978, 800–801; (b)
Furusaki, A.; Hashiba, N.; Matsumoto, T.; Hirano, A.; Iwai, Y.;
Omura, S. Bull. Chem. Soc. Jpn. 1982, 55, 3681–3685.
10. (a) Sezaki, M.; Sasaki, T.; Nakazawa, T.; Takeda, U.; Iwata, M.;
Watanabe, T.; Koyama, M.; Kai, F.; Shomura, T.; Kojima, M. J.
Antibiot. 1985, 38, 1439–1444; (b) Kase, H.; Iwahashi, K.; Matsuda,
Y. J. Antibiot. 1986, 39, 1059–1065; (c) Nakanishi, S.; Matsuda, Y.;
Iwahashi, K.; Kase, H. J. Antibiot. 1986, 39, 1066–1071.
11. Shen, F.; Bai, A.-P.; Gue, Z.-R.; Cheng, G.-F. Acta Pharmacol. Sin.
2002, 762–768.
12. Bosch, J.; Roca, T.; Catena, J.-L.; Llorens, O.; Perez, J.-J.; Lagunas,
C.; Fernandez, A. G.; Miquel, I.; Fernandez-Serrat, A.; Ferrerons, C.
Bioorg. Med. Chem. Lett. 2000, 10, 1745–1748.
3
(500.1 MHz, CDCl3): (3b-Z, 58%) dH 0.4 (3H, t, JHH 7.1 Hz, CH3),
3
3
1.01 (3H, t, JHH 7.1 Hz, CH3), 1.24 (3H, t, JHH 7.2 Hz, CH3), 2.03
3
(3H, s, CH3), 3.28 (1H, d, JPH 18.5 Hz, CH), 3.45–3.53 (2H, m,
OCH2), 3.98–4.02 (2H, m, OCH2), 4.20–4.26 (2H, m, OCH2), 7.45–
7.76 (15H, m, arom), 9.63 (1H, s, NH); (3b-E, 42%): dH 0.34 (3H, t,
3JHH 7.1 Hz, CH3), 1.09 (3H, t, JHH 7.1 Hz, CH3), 1.25 (3H, t, JHH
7.3 Hz, CH3), 2.00 (3H, s, CH3), 3.18 (1H, d, 3JPH 18.5 Hz, CH), 3.67–
3.73 (2H, m, OCH2), 3.90–3.97 (2H, m, OCH2), 4.08–4.12 (2H, m,
OCH2), 7.45–7.76 (15H, m, arom), 10.17 (1H, s, NH); 13C NMR
(125.8 MHz, CDCl3) (3b-Z): dC 13.66, 13.75, and 14.18 (3CH3), 23.21
3
3
1
2
(CH3), 41.27 (d, JPC 124.0 Hz, P@C), 50.70 (d, JPC 13.9 Hz, CH),
3
58.56, 61.45, and 62.51 (3OCH2), 63.00 (d, JPC 3.8 Hz, quaternary
carbon) 116.82 (CN), 126.03 (d, JPC 93.5 Hz, Cipso), 128.64 (d, JPC
12.4 Hz, Cmeta), 132.34 (Cpara), 134.23 (d, JPC 8.2 Hz, Cortho), 164.47
1
3
2
2
(C@O, amide), 169.99 (C@O, ester), 170.45 (d, JPC 5.5 Hz, C@O,
2
ester), 172.02 (d, JPC 12.3 Hz, C@O, ester); (3b-E): dC 13.53, 13.61,
1
and 14.14 (3CH3), 22.77(CH3), 40.09 (d, JPC 125.3 Hz, P@C), 49.71
(d, 2JPC 13.4 Hz, CH), 58.67, 61.58, and 62.95 (3OCH2), 61.36 (d, 3JPC
1
6.0 Hz, quaternary carbon), 116.96 (C„N), 126.51 (d, JPC 93.1 Hz,
3
2
C
ipso), 128.69 (d, JPC 12.5 Hz, Cmeta), 132.49 (Cpara), 132.94 (d, JPC
8.1 Hz, Cortho), 165.03(C@O, amide), 169.94 (C@O, ester), 170.00 (d,
3
2JPC 5.0 Hz, C@O, ester), 171.95 (d, JPC 12.1 Hz, C@O, ester); 31P
NMR (202.5 MHz, CDCl3): dP 25.68, 26.28; MS (EI), m/z (%): 602
(1), 434 (4), 262 (8), 184 (16), 152 (24), 77 (88), 43 (100). Anal. Calcd
for C33H35N2O7P (602.63): C, 65.77; H, 5.85; N, 4.65. Found: C,
65.82; H, 5.89; N, 4.69.
Compound 3c: White powder, mp 120–123 °C, yield 80%; IR (KBr)
(mmax, cmÀ1): 3437 (NH), 2356 (CN), 1776, 1732 (C@O), 1689 (C@C);
1H NMR (500.1 MHz, CDCl3): dH 0.88 (9H, s, CMe3), 1.01 (3H, t,
3JHH 7.1 Hz, CH3), 1.46 (9H, s, CMe3), 2.08 (3H, s, CH3), 3.13 (1H, d,
3JPH 18.8 Hz, CH), 3.94–4.06 (2H, m, OCH2), 7.23–7.92 (15H, m,
arom), 9.95 (1H, s, NH); 13C NMR(125.8 MHz, CDCl3): dC 13.65
13. (a) Langlois, N.; Radom, M.-O. Tetrahedron Lett. 1998, 39, 857–860;
(b) Langlois, N.; Calvez, O.; Radom, M.-O. Tetrahedron Lett. 1997,
38, 8037–8040; (c) Luker, T.; Koot, W.-J.; Hiemstra, H.; Speckamp,
W. N. J. Org. Chem. 1998, 63, 220–221.
14. (a) Mattern, R.-H. Tetrahedron Lett. 1996, 37, 291–294; (b) Woo,
C.-K.; Jones, K. Tetrahedron Lett. 1991, 32, 6949–6952; (c) Zhang, J.;
Blazecka, P. G.; Davidson, J. G. Org. Lett. 2003, 5, 553–556; (d)
Chatani, C.; Kamitani, A.; Murai, S. J. Org. Chem. 2002, 67, 7014–
7018.
15. Yavari, I.; Asghari, S.; Samadi-Zadeh, M. M. Iran. J. Chem. Chem.
Eng. 1998, 17, 38–41.
16. Yavari, I.; Asghari, S.; Esmaili, A. A. J. Chem. Res. 1999, 234–235.
17. Asghari, S.; Khabbazi-Habibi, A. J. Phosphorus, Sulfur, Silicon 2005,
180, 2451–2456.
1
(CH3), 23.15 (CH3), 28.08 and 28.12 (2CMe3), 41.01 (d, JPC
2
124.7 Hz, P@C), 51.67 (d, JPC 14.1 Hz, CH), 62.30 (OCH2), 63.02
3
(d, JPC 3.6 Hz, quaternary carbon), 78.45 and 81.92 (2OCMe3),
1
3
116.80 (CN), 125.8 (d, JPC 93.5 Hz, Cipso), 128.42 (d, JPC 12.3 Hz,
2
Cmeta), 132.13 (Cpara), 134.77 (d, JPC 8.5 Hz, Cortho), 164.49 (C@O,
2
amide), 169.78 (d, JPC 6.0 Hz, C@O ester), 169.84 (C@O, ester),
171.77 (d, 3JPC 11.82 Hz, C@O ester); 31P NMR (202.5 MHz, CDCl3):
dP 25.85; MS (EI), m/z (%): 658 (1), 332 (4), 295 (34), 262 (5), 105 (4),
57 (48), 44 (100). Anal. Calcd for C37H43N2O7P (658.73): C, 67.46; H,
6.58; N, 4.25. Found: C, 67.51; H, 6.53; N, 4.21.
18. General procedure for the preparation of compound 3. To a magnet-
ically stirred solution of 0.52 g of triphenylphosphine (2 mmol) and
0.34 g of ethyl acetamidocyanoacetate (2 mmol) in 5 ml of dry CH2Cl2
was added, dropwise, a mixture of 0.28 g of dimethyl acetylenedi-
carboxylate (2 mmol) in 3 ml of dry CH2Cl2 at À5 °C over 10 min.
The reaction mixture was then allowed to warm to room temperature
and stirred for 24 h. The solvent was removed under reduced pressure
and the residue was purified by silica gel (Merck Silica Gel 60, 70–
Preparation of 2-ethyl-3-methyl 1-acetyl-2-cyano-5-oxo-2,5-dihydro-
1H-pyrrole-2,3- dicarboxylate 4a. Compound 3a was refluxed in
toluene for 96 h. The solvent was removed under reduced pressure
and the residue was purified by silica gel (Merck, Silica Gel, 230–
400 mesh) column chromatography using hexane/ethyl acetate (3:1)
as eluent. The solvent was removed under reduced pressure and 4a

S. Asghari et al. / Tetrahedron Letters 49 (2008) 1824–1827
1827
was obtained as a yellow oil, yield (35%); IR (KBr) (mmax, cmÀ1): 2371
(CN), 1787, 1725 (C@O); 1H NMR (500.1 MHz, CDCl3): dH 1.36
180 (72), 152 (81), 134 (30), 57 (10), 43 (100). Anal. Calcd for
13H14N2O6 (294.27): C, 53.06; H, 4.80; N, 9.52. Found: C, 53.09; H,
C
3
(3H, t, JHH 7.3 Hz, CH3), 2.54 (3H, s, CH3), 3.82 (3H, s, OCH3),
4.85; N, 9.58.Compound 4c: Yellow oil, yield 45% IR (KBr) (mmax,
4.14–4.23 (2H, m, OCH2), 6.95 (1H, s, CH); 13C NMR (125.8 MHz,
CDCl3): dC 13.73 (CH3), 24.43 (CH3), 52.47 (OCH3) 64.36 (OCH2),
82.30 (quaternary carbon), 112.29 (CN), 134.64 and 146.54 (olefinic
carbons), 158.75 and 162.87 (2C@O, imide), 164.76 and 168.42
(2C@O, ester); MS (EI), m/z (%): 280 (2), 265 (3), 223 (2), 181(2.5),
151 (2.4), 59 (2), 43 (5). Anal. Calcd for C12H12N2O6 (280.24): C,
51.43; H, 4.32; N, 9.99. Found: C, 51.49; H, 4.36; N, 10.06.
cmÀ1): 2370 (CN), 1785, 1725 (C@O); 1H NMR (500.1 MHz, CDCl3):
3
dH 1.35 (3H, t, JHH 7.1 Hz, CH3), 1.54 (9H, s, CMe3), 2.55 (3H, s,
2
3
CH3), 4.29 (1H, ABX3, dq, JHH 10.6 Hz, JHH 7.1 Hz, OCH), 4.40
2
3
(1H, ABX3, dq, JHH 10.6 Hz, JHH 7.1 Hz, OCH), 6.92(1H, s, CH);
13C NMR (125.8 MHz, CDCl3): dC 13.75 (CH3), 24.33 (CH3), 27.83
(CMe3), 64.97 (OCH2), 86 (OCMe3), 111.65 (CN), 82.60 (quaternary
carbon), 134.17 and 146.78 (olefinic carbons), 157.62 and 161.71
(2C@O, imide), 165.21 and 168.43 (2C@O ester); MS (EI), m/z (%),
322 (2), 239 (8), 194 (90), 180 (98), 152 (98), 134 (48), 57 (71), 43 (100).
Anal. Calcd for C15H18N2O6 (322.32): C, 55.90; H, 5.63; N, 8.69.
Found: C, 55.94; H, 5.59; N, 8.67.
Compound 4b:Yellow oil, yield 45%, IR (KBr) (mmax, cmÀ1): 2375
(CN), 1781, 1730 (C@O); 1H NMR (500.1 MHz, CDCl3): dH 1.36
(3H, t, 3JHH 7.1 Hz, CH3), 1.37 (3H, t, 3JHH 7.1 Hz, CH3), 2.57 (3H, s,
CH3), 4.33–4.41 (4H, m, 2OCH2), 6.98 (1H, s, CH); 13C NMR
(125.8 MHz, CDCl3); dC 13.75 and 13.89 (2CH3), 24.32 (CH3), 63.34
and 65.10 (2OCH2), 82.40 (quaternary carbon), 111.51 (CN), 134.64
and 145.24 (olefinic carbons), 158.77 and 161.67 (2C@O, imide),
165.04 and 168.43 (2C@O, ester); MS (EI), m/z (%): 294 (3), 249 (4),
19. Corbridge, D. E. C. Phosphorus an Outline of its Chemistry,
Biochemistry and Technology, 5th ed.; Elsevier: Amsterdam, 1995.
20. Becker, K. B. Tetrahedron 1980, 36, 1717–1745.
21. Zebiral, E. Synthesis 1974, 775–797.