Ethyl vinyl ether as a synthetic equivalent of acetylene in a DABCO-catalyzed microwave-assisted Diels-Alder-elimination reaction sequence starting from 2H-pyran-2-ones

Article abstract of DOI:10.1055/s-0028-1083515

We present a study of the Diels-Alder reaction between various electron-deficient 2H-pyran-2-ones and ethyl vinyl ether. This microwave-accelerated sequence of a cycloaddition followed by a retro-Diels-Alder reaction (the elimination of CO₂) and a second elimination step of EtOH yields substituted aniline derivatives. The reaction sequence is greatly accelerated by the application of DABCO as a suitable base.

Full text of DOI:10.1055/s-0028-1083515

LETTER
2613
Ethyl Vinyl Ether as a Synthetic Equivalent of Acetylene in a DABCO-
Catalyzed Microwave-Assisted Diels–Alder–Elimination Reaction Sequence
Starting from 2H-Pyran-2-ones
Ethyl
Vinyl Ether
r
as
a
Syn
i
thetic
E
š
quivalent
t
of
A
ce
o
tylene f Kranjc,* Marijan Kočevar
Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva 5, 1000 Ljubljana, Slovenia
Fax +386(1)2419220; E-mail: kristof.kranjc@fkkt.uni-lj.si
Received 23 June 2008
Dedication to Professor Branko Stanovnik, University of Ljubljana, Slovenia, on the occasion of his 70th birthday.
suited to the reaction with electron-poor 2H-pyran-2-
ones.
Abstract: We present a study of the Diels–Alder reaction between
various electron-deficient 2H-pyran-2-ones and ethyl vinyl ether.
This microwave-accelerated sequence of a cycloaddition followed
by a retro-Diels–Alder reaction (the elimination of CO2) and a sec-
ond elimination step of EtOH yields substituted aniline derivatives.
The reaction sequence is greatly accelerated by the application of
DABCO as a suitable base.
There are some previous reports5 on the use of ethyl vinyl
ether as a dienophile with different cyclic dienes, includ-
ing derivatives of 1,2,4,5-tetrazines and 2H-pyran-2-ones.
However, in all the cases described so far, it seems that the
reaction times when 2H-pyran-2-ones were applied were
rather long (up to 10 days),5c that the yields were relative-
ly low, the reactions were not comprehensively investigat-
ed, and that no attempt was made to use catalysts. Ethyl
vinyl ether, with its low boiling point (33 °C), can present
a certain problem with the classical (reflux) conditions;
therefore, the reactions described in the literature so far
often used the ‘sealed vessel’ protocol. To avoid any prob-
lems in connection with this and also to conduct the reac-
tions under defined conditions, we decided to use focused
microwave-irradiation conditions6 in a closed vessel, a
well-established and useful way of carrying out reactions.
Key words: Diels–Alder reactions, arenes, catalysis with bases,
anilines, microwave reactions
Acetylene is not a very suitable dienophile for Diels–Al-
der reactions.1 First of all, it has no substituents that could
confer some polarization on its multiple bond and, sec-
ondly, under standard conditions it is a gaseous com-
pound, thus greatly limiting its possible usefulness. It is
well known that for Diels–Alder reactions the partners
should have complementary electronic properties (one
molecule should be electron rich, the other electron defi-
cient). Regardless of the problems described, an acetylene
fragment might often be a desirable part for inclusion in a
carbon scaffold when building larger molecules contain-
ing aniline or related aromatic moieties. Therefore, it is of
interest to find suitably masked alternative reagents that
contain the appropriate substituent(s), so conferring polar-
ization on the multiple bond of acetylene and, at the same
time, possessing a certain proclivity for the elimination of
these groups in the final step(s).
In the literature2 there are some reports on the use of dif-
ferent synthetic equivalents of acetylene, like phenyl vinyl
sulfoxide,2b–2d fluoroalkyl vinyl sulfoxides,2e phenyl vinyl
sulfide,2f ethynyl para-tolyl sulfone,2g and 1-benzenesul-
fonyl-2-trimethylsilylacetylene.2h However, we thought
that ethyl vinyl ether might also be a molecule that would
satisfy the above-mentioned conditions and act as a
masked acetylene in the Diels–Alder reaction with 2H-py-
ran-2-ones. Based on literature examples3 and our previ-
ous research4 on Diels–Alder reactions with 2H-pyran-2-
ones, we were well aware that the electron-rich double
bond of ethyl vinyl ether will preclude its reaction with
electron-rich 2H-pyran-2-ones, although it should be well
We envisaged the reaction sequence starting with a Diels–
Alder cycloaddition of ethyl vinyl ether (2) on 3-acylami-
no-2H-pyran-2-ones 17 substituted with suitable electron-
withdrawing groups. In the first step an oxabicy-
clo[2.2.2]octene system 3 would be produced. If the reac-
tion conditions applied would be appropriate,
intermediate 3 would spontaneously eliminate a molecule
of CO2 in a retro-Diels–Alder reaction. This step is well
known (also from our previous research)4a,c,d and should
be facilitated by heating. The cyclohexadiene system 4
thus produced would probably not be stable and the elim-
ination of a molecule of EtOH would take place, yielding
the desired substituted aniline derivative 5 (Scheme 1).
Preliminary research has shown that the reaction between
2H-pyran-2-one
1a
(R1 = Ph,
R2 = CO2Me,
R3 = CH2CO2Me) and ethyl vinyl ether (2) can be carried
OEt
O
O
R3
R3
R2
O
O
R2
2
– CO2
OEt
MW
DABCO
NHCOR1
3
NHCOR1
1
OEt
R3
R2
R3
R2
– EtOH
NHCOR1
NHCOR1
5
SYNLETT 2008, No. 17, pp 2613–2616
x
x
.x
x
.2
0
0
8
4
Advanced online publication: 01.10.2008
DOI: 10.1055/s-0028-1083515; Art ID: G22308ST
Scheme 1
© Georg Thieme Verlag Stuttgart · New York

2614
K. Kranjc, M. Kočevar
LETTER
out in a solution of DMF under microwave-irradiation between the sterically hindered bases; however, classical
conditions (at 160 °C) without using any catalyst, yielding bases like pyridine and Et3N show appreciably reduced
the aniline derivative 5a (Table 1, entry 1) with complete catalytic activity.
conversion. However, using water or MeCN as the solvent
We have also found that water might be a suitable reaction
at lower temperature (120 °C) without a catalyst did not
medium (Table 1, entries 9 and 16), as the conversions ob-
yield any product 5a (Table 1, entries 2 and 3). These data
served in water are comparable or even higher than with
show that a high temperature is indeed needed for this
the same catalysts in MeCN. However, the products ob-
transformation to proceed toward 5a. But the rather harsh
tained in water contained a larger amount of decomposed
conditions in DMF also cause some decomposition of the
material (according to 1H NMR analyses of the crude re-
starting compounds and thus decrease the purity of the
action mixtures) and pure products cannot be isolated as
product and its yield.
easily as from MeCN reaction mixtures. In contrast to our
previous research,4c,d,9 this reaction cannot be carried out
Table 1 Effect of Different Reaction Conditions and Bases on the
Transformation between 2H-Pyran-2-one 1a and Ethyl Vinyl Ether
under solvent-free conditions, as it would not be possible
to heat the reaction mixture to an adequate temperature
without an excessive increase in the pressure (above 20
bar) as a consequence of the low boiling point of ethyl vi-
nyl ether.
(2) Yielding 5a
Entry
1
Base (mol%)
Time (min) Conversion (%)
a
c
e
–
–
–
120b
120d
120d
90d
100
0
1,4-Diazabicyclo[2.2.2]octane and DBU were found to be
the most appropriate bases. However, with DBU in certain
cases the isolation presented some problems, as its use
tended to lead to more decomposition products than when
DABCO was applied. Therefore, as the standard proce-
dure, we decided to apply 10 mol% of DABCO as the cat-
alyst and MeCN as the solvent. The reactions were carried
out under microwave irradiation at 120 °C, yielding the
desired substituted aniline derivatives 5a–g with good
yields (64–85%, Table 2).10
2
3
0
4
Pyridine (10)e
Et3N (10)e
38
71
72
100
100
91
71
87
100
80
82
95
93
5
90d
6
DABCO (10)e
DABCO (20)e
DABCO (10)e
DABCO (10)c
DBU (5)e
90d
7
90d
8
120d
90d
The use of acid catalysts in the reaction between 1a and
ethyl vinyl ether (2) did not prove to be advantageous;
moreover, with the use of PTSA the dienophile 2 is instan-
taneously decomposed (already at room temperature),
whereas ZrCl4 causes decomposition during the reaction.
9
10
11
12
13
14
15
16
90d
DBU (10)e
90d
DBU (10)e
120d
90d
Finally, we also decided to investigate the Diels–Alder re-
action between ethyl vinyl ether and an electron-rich 2H-
pyran-2-one 1h containing a 4-methoxyphenyl substituent
DBN (10)e
DMAP (10)e
DMAP (10)e
DMAP (10)c
90d
Table 2 Microwave-Assisted Base-Catalyzed Transformation of
2H-Pyran-2-ones 1 into Aniline Derivatives 5
120d
90d
Entry Starting 1
Time Product
(min)a (yield, %)b
a DMF as solvent.
R1
Ph
Ph
Ph
Ph
Ph
Ph
Me
Ph
Ph
R2
R3
b MW irradiation, temperature set to 160 °C.
c Distilled H2O as solvent.
1
2
3
4
5
6
7
8
9
CO2Me
CO2Et
CO2Me
CO2Et
Ac
CH2CO2Me 1a
CH2CO2Et 1b
100
100
90
5a (73)
5b (70)
5c (68)
5d (64)
5e11 (85)
5f (82)
5g11,12 (78)
–
d MW irradiation, temperature set to 120 °C.
e MeCN as solvent.
Me
Me
Me
Me
Me
Me
Me
1c
1d
1e
1f
Therefore, we decided to embark on a search for an appro-
priate catalyst that would make a lower reaction tempera-
ture possible (around 120 °C). First, we employed
pyridine (10 mol%) as a standard base catalyst for the re-
action between 1a and ethyl vinyl ether (2) in MeCN as
the solvent under microwave irradiation conditions (at
120 °C, Table 1, entry 4). However, the 1H NMR analysis
of the crude mixture obtained after 90 minutes of irradia-
tion showed a very low conversion (only around 38%).
We decided to test some other bases, for example Et3N,
DMAP, DABCO, DBU, and DBN.8 The results show
(Table 1, entries 5–16) that there are no great differences
90
60
Bz
60
Ac
1g
1h
1i
60
PMP
H
120
120
–
a At 120 °C in MeCN with DABCO (10 mol%).
b Isolated yield.
Synlett 2008, No. 17, 2613–2616 © Thieme Stuttgart · New York

LETTER
Ethyl Vinyl Ether as a Synthetic Equivalent of Acetylene
2615
(4) (a) Kranjc, K.; Leban, I.; Polanc, S.; Kočevar, M.
(Table 2, entry 8) or an electronically relatively unper-
turbed 6-methyl derivative 1i (Table 2, entry 9). However,
neither of these combinations proved to be reactive, due to
the noncomplementary electronic nature of both partners
and consequently no product of the type 5 was detected.
Heterocycles 2002, 58, 183. (b) Kranjc, K.; Kočevar, M.
New J. Chem. 2005, 29, 1027. (c) Kranjc, K.; Kočevar, M.;
Iosif, F.; Coman, S. M.; Parvulescu, V. I.; Genin, E.; Genêt,
J.-P.; Michelet, V. Synlett 2006, 1075. (d) Kranjc, K.;
Kočevar, M. Bull. Chem. Soc. Jpn. 2007, 80, 2001.
(e) Kranjc, K.; Kočevar, M. Tetrahedron 2008, 64, 45.
(5) (a) Jung, M. E.; Hagenah, J. A. J. Org. Chem. 1987, 52,
1889. (b) Jung, M. E.; Hagenah, J. A. Heterocycles 1987, 25,
117. (c) Balázs, L.; Kádas, I.; Tőke, L. Tetrahedron Lett.
2000, 41, 7583. (d) Sauer, J.; Mielert, A.; Lang, D.; Peter, D.
Chem. Ber. 1965, 98, 1435. (e) Hamasaki, A.; Ducray, R.;
Boger, D. L. J. Org. Chem. 2006, 71, 185.
(6) (a) Microwaves in Organic Synthesis; Loupy, A., Ed.;
Wiley-VCH: Weinheim, 2002. (b) Hayes, B. L. Microwave
Synthesis: Chemistry at the Speed of Light; CEM Publishing:
Matthews NC, 2002. (c) Lidström, P.; Tierney, J.; Wathey,
B.; Westman, J. Tetrahedron 2001, 57, 9225.
The data presented might hint at the possibility that the
base facilitates the elimination steps, whereas it is not in-
volved in the first cycloaddition step. This is further cor-
roborated by the data obtained in the reaction between 1d
and ethyl vinyl ether (2), where after 90 minutes of micro-
wave irradiation at 120 °C in MeCN a mixture of starting
1d and product of type 3 (67% conversion) is obtained,
whereas the product 5d is not detected. However, the
same reaction carried out in DMF at 160 °C yields only
5d. Therefore, it seems that a high temperature (160 °C
vs. 120 °C), more polar solvents (DMF vs. MeCN), or the
use of base catalysts (like DABCO) are necessary for the
reaction to proceed toward the products 5.
(d) Polshettiwar, V.; Varma, R. S. Acc. Chem. Res. 2008, 41,
629. (e) Kappe, C. O. Angew. Chem. Int. Ed. 2004, 43,
6250. (f) de la Hoz, A.; Díaz-Ortiz, A.; Moreno, A. Chem.
Soc. Rev. 2005, 34, 164.
In conclusion, we have presented a concise and useful
way of incorporating an acetylene fragment via the Diels–
Alder reaction of ethyl vinyl ether with electron-deficient
2H-pyran-2-ones, yielding aromatic aniline systems 5.
This relatively general reaction might also be used for oth-
er related transformations, leading to more complex prod-
ucts. Further work will be needed to elucidate the role of
the base in this transformation and to try to prepare CO2-
containing products of type 3.
(7) For the synthesis of starting compounds 1, see: (a) Kepe,
V.; Kočevar, M.; Polanc, S.; Verček, B.; Tišler, M.
Tetrahedron 1990, 46, 2081. (b) Vraničar, L.; Polanc, S.;
Kočevar, M. Tetrahedron 1999, 55, 271. (c) Požgan, F.;
Krejan, M.; Polanc, S.; Kočevar, M. Heterocycles 2006, 69,
123. (d) Kepe, V.; Kočevar, M.; Petrič, A.; Polanc, S.;
Verček, B. Heterocycles 1992, 33, 843. (e) Kepe, V.;
Kočevar, M.; Polanc, S. J. Heterocycl. Chem. 1996, 33,
1707. (f) Požgan, F.; Kranjc, K.; Kepe, V.; Polanc, S.;
Kočevar, M. Arkivoc 2007, (viii), 97.
(8) Baidya, M.; Kobayashi, S.; Brotzel, F.; Schmidhammer, U.;
Riedle, E.; Mayr, H. Angew. Chem. Int. Ed. 2007, 46, 6176.
(9) Hren, J.; Kranjc, K.; Polanc, S.; Kočevar, M. Heterocycles
2007, 72, 399.
Acknowledgment
We thank the Ministry of Higher Education, Science, and Techno-
logy of the Republic of Slovenia and the Slovenian Research Agen-
cy for financial support (P1-0230-0103 and J1-6693-0103). Dr. B.
Kralj and Dr. D. Žigon (Center for Mass Spectroscopy, ‘Jožef Ste-
fan’ Institute, Ljubljana, Slovenia) are gratefully acknowledged for
the mass measurements.
(10) General Procedure
A mixture of the starting 2H-pyran-2-one 1 (1 mmol), ethyl
vinyl ether (2, 721.1 mg, 10 mmol), and DABCO (11.2 mg,
0.1 mol) in MeCN (2 mL) was irradiated in the focused
microwave equipment (CEM Discover) for the time
specified (Table 1). The final temperature was set to 120 °C,
the power to 120 W, and the ramp time to 5 min. Thereafter,
the reaction mixture was cooled, the volatile components
were removed in vacuo, the remaining solid was treated with
mixture of EtOH and H2O (10:1), and cooled. The
precipitated product 5 was filtered off and washed with
EtOH–H2O (10:1).
References and Notes
(1) Vijaya, R.; Dinadayalane, T. C.; Sastry, G. N. J. Mol. Struct.
(Theochem) 2002, 589-590, 291.
(2) (a) Boyall, D.; López, F.; Sasaki, H.; Frantz, D.; Carreira,
E. M. Org. Lett. 2000, 2, 4233. (b) Paquette, L. A.; Moerck,
R. E.; Harirchian, B.; Magnus, P. D. J. Am. Chem. Soc. 1978,
100, 1597. (c) Jackson, P. M.; Moody, C. J. Tetrahedron
1992, 48, 7447. (d) Lin, Y. S.; Chang, S. Y.; Yang, M. S.;
Rao, C. P.; Peddinti, R. K.; Tsai, Y. F.; Liao, C. C. J. Org.
Chem. 2004, 69, 447. (e) Moïse, J.; Goumont, R.; Magnier,
E.; Wakselman, C. Synthesis 2004, 2297. (f) Pearson, W.
H.; Lee, I. Y.; Mi, Y.; Stoy, P. J. Org. Chem. 2004, 69, 9109.
(g) Davis, A. P.; Whitham, G. H. J. Chem. Soc., Chem.
Commun. 1980, 639. (h) Williams, R. V.; Chauhan, K.;
Gadgil, V. R. J. Chem. Soc., Chem. Commun. 1994, 1739.
(3) (a) Afarinkia, K.; Vinader, V.; Nelson, T. D.; Posner, G. H.
Tetrahedron 1992, 48, 9111. (b) Woodard, B. T.; Posner,
G. H. In Advances in Cycloaddition, Vol. 5; Harmata, M.,
Ed.; JAI Press: Greenwich, 1999, 47–83. (c) Tolmachova,
N. A.; Gerus, I. I.; Vdovenko, S. I.; Essers, M.; Fröhlich, R.;
Haufe, G. Eur. J. Org. Chem. 2006, 4704.
Selected Data of the Products
Methyl 5-(Benzoylamino)-2-[(methoxycarbonyl)-
methyl]benzoate (5a)
Mp 169–171 °C (MeOH). IR (KBr): 3339, 1737, 1720,
1653, 1590, 1526, 1435, 1419 cm–1. 1H NMR (300 MHz,
CDCl3): d = 3.71 (s, 3 H, Me), 3.83 (s, 3 H, Me), 3.98 (s, 2
H, CH2), 7.21 (d, 1 H, J = 8.4 Hz, 3-H), 7.52 (m, 3 H, Ph),
7.86 (m, 3 H, Ph, 4-H), 8.10 (br s, 1 H, NH), 8.18 (d, 1 H,
J = 2.1 Hz, 6-H). 13C NMR (75.5 MHz, DMSO-d6):
d = 39.1, 51.4, 51.9, 122.0, 123.8, 127.6, 128.4, 129.4,
130.9, 131.7, 132.8, 134.5, 138.3, 165.6, 166.7, 171.5. MS
(EI): m/z (%) = 327 (5) [M+], 105 (100). Anal. Calcd for
C18H17NO5: C, 66.05; H, 5.23; N, 4.28. Found: C, 66.32; H,
5.17; N, 4.26.
Ethyl 5-(Benzoylamino)-2-[(ethoxycarbonyl)-
methyl]benzoate (5b)
Mp 170.5–171.5 °C (MeOH). IR (KBr): 3298, 1736, 1714,
1651, 1588, 1523, 1420 cm–1. 1H NMR (300 MHz, CDCl3):
d = 1.25 (t, 3 H, J = 7.1 Hz, CH2CH3), 1.37 (t, 3 H, J = 7.1
Synlett 2008, No. 17, 2613–2616 © Thieme Stuttgart · New York

2616
K. Kranjc, M. Kočevar
LETTER
Hz, CH2CH3), 3.99 (s, 2 H, CH2), 4.15 (q, 2 H, J = 7.1 Hz,
CH2CH3), 4.32 (q, 2 H, J = 7.1 Hz, CH2CH3), 7.24 (d, 1 H,
J = 8.9 Hz, 3-H), 7.53 (m, 3 H, Ph), 7.88 (m, 2 H, Ph), 7.95
(dd, 1 H, J1 = 8.9 Hz, J2 = 2.4 Hz, 4-H), 7.98 (br s, 1 H, NH),
8.10 (d, 1 H, J = 2.4 Hz, 6-H).
Methyl 5-(Benzoylamino)-2-methylbenzoate (5c)
Mp 126–127 °C (EtOH–H2O). IR (KBr): 3270, 1732, 1720,
1649, 1582, 1528, 1498, 1428 cm–1. 1H NMR (300 MHz,
CDCl3): d = 2.57 (s, 3 H, Me), 3.86 (s, 3 H, Me), 7.22 (d,
1 H, J = 8.3 Hz, 3-H), 7.50 (m, 3 H, Ph), 7.81 (dd, 1 H,
J1 = 2.2 Hz, J2 = 8.3 Hz, 4-H), 7.86 (m, 2 H, Ph), 8.02 (br s,
1 H, NH), 8.08 (d, 1 H, J = 2.2 Hz, 6-H). 13C NMR (75.5
MHz, CDCl3): d = 21.1, 51.9, 122.3, 124.0, 127.0, 128.7,
129.8, 131.9, 132.3, 134.6, 135.7, 136.3, 165.8, 167.5. MS
(EI): m/z (%) = 269 (30) [M+], 105 (100). Anal. Calcd for
C16H15NO3: C, 71.36; H, 5.61; N, 5.20. Found: C, 71.30; H,
5.68; N, 5.22.
Ethyl 5-(Benzoylamino)-2-methylbenzoate (5d)
Mp 119–120.5 °C (EtOH–H2O). IR (KBr): 3296, 1726,
1648, 1583, 1530, 1448, 1404 cm–1. 1H NMR (300 MHz,
CDCl3): d = 1.38 (t, 3 H, J = 7.1 Hz, CH2CH3), 2.57 (s, 3 H,
Me), 4.35 (q, 2 H, J = 7.1 Hz, CH2CH3), 7.23 (d, 1 H, J =
8.1 Hz, 3-H), 7.50 (m, 3 H, Ph), 7.87 (m, 3 H, Ph, 4-H), 7.96
(br s, 1 H, NH), 8.02 (d, 1 H, J = 2.4 Hz, 6-H).
N-(3-Acetyl-4-methylphenyl)benzamide (5e)11
Mp 131–132 °C (EtOH–H2O); lit.11 mp 137–138 °C (EtOH).
IR (KBr): 3410, 1861, 1788, 1699, 1653, 1533, 1492 cm–1.
1H NMR (300 MHz, CDCl3): d = 2.49 (s, 3 H, Me), 2.57 (s,
3 H, Me), 7.21 (d, 1 H, J = 8.4 Hz, 5-H), 7.53 (m, 4 H, Ph,
6-H), 7.87 (m, 2 H, Ph), 8.05 (br s, 1 H, NH), 8.13 (d, 1 H,
J = 2.4 Hz, 2-H).
N-(3-Benzoyl-4-methylphenyl)benzamide (5f)
Mp <35 °C (EtOH–H2O). IR (neat): 1671, 1597, 1522, 1501
cm–1. 1H NMR (300 MHz, CDCl3): d = 2.28 (s, 3 H, Me),
7.28 (d, 1 H, J = 8.4 Hz, 5-H), 7.52 (m, 7 H, 2 × Ph, 2-H),
7.77 (dd, 1 H, J1 = 2.4 Hz, J2 = 8.4 Hz, 6-H), 7.83 (m, 4 H,
Ph), 7.94 (br s, 1 H, NH).
N-(3-Acetyl-4-methylphenyl)acetamide (5g)11,12
Mp 86–88 °C (EtOH–H2O); lit.11 mp 94–95 °C (EtOH); lit.12
mp 124 °C (EtOH). IR (KBr): 3331, 1686, 1668, 1585, 1536,
1497, 1453 cm–1. 1H NMR (300 MHz, CDCl3): d = 2.19 (s,
3 H, Me), 2.47 (s, 3 H, Me), 2.57 (s, 3 H, Me), 7.16 (br s,
1 H, NH), 7.18 (d, 1 H, J = 8.2 Hz, 5-H), 7.40 (dd, 1 H,
J1 = 2.0 Hz, J2 = 8.2 Hz, 6-H), 7.97 (d, 1 H, J = 2.0 Hz, 2-H).
(11) Thomsen, A. D.; Lund, H. Acta Chem. Scand. 1969, 23,
2930.
(12) Desai, G.; Desai, K. K.; Desai, C. M. J. Indian Chem. Soc.
1987, 64, 370.
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