CuCl-catalyzed cycloisomerization reaction of 1,2-allenyl carboxylic acids. A cost-effective synthesis of β-unsubstituted butenolides

Article abstract of DOI:10.1016/S0040-4020(00)01149-2

Fourteen examples of 2,3-butadienoic acids were synthesized according to the known procedures and their high-yielding CuCl-catalyzed cycloisomerization reaction to afford butenolides were described.

Full text of DOI:10.1016/S0040-4020(00)01149-2

TETRAHEDRON
Pergamon
Tetrahedron 57 (2001) 1585±1588
CuCl-Catalyzed cycloisomerization reaction of 1,2-allenyl
carboxylic acids. A cost-effective synthesis of b-unsubstituted
butenolides
p
Shengming Ma, Zhanqian Yu and Shulin Wu
Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu,
Shanghai 200032, People's Republic of China
Received 30 August 2000; revised 29 November 2000; accepted 7 December 2000
AbstractÐFourteen examples of 2,3-butadienoic acids were synthesized according to the known procedures and their high-yielding CuCl-
catalyzed cycloisomerization reaction to afford butenolides were described. q 2001 Elsevier Science Ltd. All rights reserved.
Butenolides are a class of compounds of current interest due
to their occurrence in natural products and the potential
able yields (entries 5±8, Table 1); the best reaction condi-
tions were found to be using 4 mol% CuCl in CH OH under
re¯ux (entry 11, Table 1).
1
3
broad range of biological activities of natural and unnatural
2
products. During the course of our study on transition
metal-mediated cyclization of functionalized allenes, we
3
7
8
Compared to AgNO , CuCl is relatively cheap, thus,
3
1
have developed the Pd(0)/Ag -cocatalyzed coupling-cycli-
the current high-yielding CuCl-catalyzed cyclization
reaction of 1,2-allenyl carboxylic acids merits close
attention and some of the typical results are summarized
in Table 2.
zation reaction of organic halides with 1,2-alleny4l
carboxylic acids to afford b-substituted butenolides.
Furthermore, among our efforts devoted to the development
of new methodology for the synthesis of b-halobuteno-
lides, a CuX -mediated cyclization reaction of 1,2-allenyl
5
From Table 2, it should be pointed out that the yields
for the reactions range from good to excellent; the
yields starting from 4-aryl substituted 2,3-dienoic acids
are usually higher than those starting from 4-alkyl
substituted 2,3-dienoic acids (compare entries 1±6, 11,
14 with 7±10, 12, 13, Table 2). The reaction is fairly
2
6
carboxylic acids has been established. Based on the obser-
vation that the in situ-generated CuCl mediated the forma-
tion of the by-products, i.e. b-unsubstituted butenolides, we
6
have developed the CuCl-catalyzed cycloisomerization
reaction of 1,2-allenyl carboxylic acids, which provides an
alternative cost-effective route for the synthesis of b-unsub-
stituted butenolides.
1
3
general, i.e. R can be aryl or alkyl while R can be H,
alkyl, benzyl, or allyl. With an allyl group as the R
3
substituent, further elaboration of the CvC bond in the
2d-type products would provide ef®cient access to other
butenolide derivatives (entries 4, 13, and 14, Table 2).
With fully substituted 2,3-allenoic acid 1k, the reaction
also afforded 2k in 93% yield (entry 11, Table 2).
The CuCl-mediated cyclization of 2-methyl-4-phenyl-
2
,3-butadienoic acid was chosen as a starting point to
optimize the reaction conditions and the results are
summarized in Table 1.
From Table 1, it is obvious that this cycloisomerization
reaction in aqueous acetone can be catalytic in CuCl
In conclusion, we have developed an ef®cient methodology
for the synthesis of b-unsubstituted butenolides from 2,3-
allenoic acids. Compared to known methodologies the
(
compare entry 1 with entries 2 and 3, Table 1). The corre-
sponding catalytic reaction in acetone also afforded 2a in a
higher yield (entry 4, Table 1); however, the reaction did not
occur in CH Cl (entry 10, Table 1). The reaction also
current catalyst has the advantage of being relatively
cheap (compared to AgNO ), and more abundant, low
9
3
molecular weight (compared to AgNO ), and high-yielding
3
2
2
7
(at least comparable to AgNO and much higher than that
occurred in aqueous MeCN or DMF to afford 2a in reason-
3
10
1
of the H -catalyzed reaction ), which makes this metho-
dology more attractive. Further studies on the synthetic
application to optically active butenolides and interesting
natural and unnatural products are being carried out in our
laboratory.
Keywords: butenolides; 1,2-allenyl carboxylic acids; CuCl-catalyzed
cycloisomerization; cyclization.
p
Corresponding author. Tel.: 186-21-6416-3300; fax: 186-21-6416-
6
128; e-mail: masm@pub.sioc.ac.cn
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)01149-2

1
586
S. Ma et al. / Tetrahedron 57 (2001) 1585±1588
Table 1. CuCl-mediated cyclization of 2-methyl-4-phenyl-2,3-butadienoic acid
Entry
Solvent
CuCl (mol%)
Temperature (8C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
MeCOMe/H
MeCOMe/H
MeCOMe/H
MeCOMe
2
2
2
O (3:2)
O (3:2)
O (3:1)
200
12
22
4
16
11
6
7
6
8
4
65±70
65±70
65±70
40
70±80
70±80
65±70
65±70
70
4.5
1
1
27
17.5
0.5
1
1
3.5
5
53
81
65
83
41
64
74
75
90
0
MeCN/H
2
O (3:2)
DMF/H
DMF/H
DMF/H
EtOH
2
2
2
O (3:2)
O (2:1)
O (5:1)
1
0
1
CH Cl
2
2
55
Re¯ux
1
MeOH
2
95
1
. Experimental
.1. Starting materials
,3-Undecadienoic acid (1l) was prepared according to a
1.2. General procedure
1
Under N , a solution of 1 (0.5 mmol) and CuCl (0.02 mmol)
2
in methanol (5 mL) was stirred under re¯ux for 2 h. After
evaporation, the residues were puri®ed via ¯ash chromato-
graphy on silica gel with petroleum ether/ethyl acetate
(10:1) as the eluent to afford the b-unsubstituted buteno-
lides. All of the solid products were recrystallized from
ethyl acetate and petroleum ether.
2
1
1
published procedure via the reaction of CO with the
2
corresponding 1,2-allenic lithium salt, which in turn, was
prepared from the treatment of the corresponding 1,2-allene
with n-BuLi. The other allenoic acids (1a±1k and 1m,n)
1
2
were prepared according to the known method by treat-
ment of the acid chlorides with ethyl 2-(triphenylphoranyl-
idene) propionate and subsequent hydrolysis of the 2,3-
allenoic esters with 1.5 equiv. NaOH. CuCl (reagent
grade) was puri®ed according to the known procedure.13
1
4
1.2.1. 3-Methyl-5-phenyl-2(5H)-furanone (2a). Starting
from 1a (84 mg, 0.48 mmol) to afford 80 mg (95%) of 2a;
colorless oil; H NMR (300 MHz, CDCl ) d 7.49±7.28 (m,
1
3
5H), 7.18 (bs, 1H), 5.92 (bs, 1H), 2.04 (s, 3H); EIMS m/z
1
1
1
75 (M 11, 49), 174 (M , 69), 105 (100); IR (neat) 1748,
1
2
654, 1492 cm .
1
a
Table 2. CuCl-catalyzed cyclization of 2,3-alkadienoic acids
1.2.2. 5-Phenyl-3-propyl-2(5H)-furanone (2b). Starting
from 1b (102 mg, 0.5 mmol) to afford 99 mg (97%) of 2b;
1
colorless oil; H NMR (300 MHz, CDCl ) d 7.40±7.22 (m,
3
5
H), 7.09 (d, Jˆ1.6 Hz, 1H), 5.88 (d, Jˆ1.6 Hz, 1H), 2.34
(
t, Jˆ5.2 Hz, 2H), 1.7±1.55 (m, 2H), 0.98 (t, Jˆ7.3 Hz,
1
1
3H); EIMS m/z 203 (M 11, 30), 202 (M , 32), 105
1
100); IR (neat) 1756, 1454 cm . HRMS (EI) Calcd for
2
(
C H O 202.0994. Found 202.0978.
1
3
14
2
b
Product 2 Yield (%) of 2
Entry
1
1
2
3
1.2.3. 3-Benzyl-5-phenyl-2(5H)-furanone (2c). Starting
from 1c (125 mg, 0.5 mmol) to afford 113 mg (90%) of
2c; colorless platelet crystals mp 65±668C; H NMR
R
R
R
1
1
2
3
4
5
6
7
8
9
1
1
1
1
1
Ph
Ph
Ph
Ph
H
CH
n-C
PhCH
3
(1a)
2a
95
97
90
96
92
96
87
85
77
85
93
81
91
92
H
H
H
H
H
H
H
H
H
3
H
7
(1b) 2b
(1c) 2c
2d
2e
(1f) 2f
2g
(1h) 2h
(1i) 2i
(
300 MHz, CDCl ) d 7.40±7.18 (m, 10H), 6.91 (d,
3
2
Jˆ1.3 Hz, 1H), 5.87 (d, Jˆ1.3 Hz, 1H), 3.66 (s, 2H);
Allyl (1d)
CH (1e)
n-C
CH
PhCH
n-C
1
EIMS m/z 250 (M , 10), 205 (100); IR (KBr) 1740, 1644,
1-naphthyl
1-naphthyl
3
2
1
3
H
7
1492, 1452 cm . Anal. Calcd for C H O : C 81.58, H
17 14 2
n-C
n-C
CH
n-C
Ph
3
H
H
7
7
3
(1g)
5
.64. Found: C 81.79, H 5.90.
3
2
3
3
H
7
1.2.4. 3-Allyl-5-phenyl-2(5H)-furanone (2d). Starting
from 1d (106 mg, 0.53 mmol) to afford 102 mg (96%) of
0
1
2
3
4
6
H
13
CH
CH
3
(1j)
(1k)
H (1l)
Allyl (1m)
Allyl (1n)
2j
2k
2l
2m
2n
C
H
H
H
2
H
5
3
1
2d; colorless oil; H NMR (300 MHz, CDCl ) d 7.45±7.22
n-C
n-C
7
H
H
15
7
3
3
(
(
m, 5H), 7.15±7.11 (m, 1H), 5.82±6.00 (m, 2H), 5.26±5.13
1-naphthyl
1
m, 2H), 3.11 (d, Jˆ5.4 Hz, 2H); EIMS m/z 221 (M 11,
1
a
70), 220 (M , 58), 105 (100); IR (neat) 1758, 1636, 1492,
1452, 1428 cm . HRMS (EI) Calcd for C H O
13 12 2
The reactions were carried out using 1 (0.5 mmol), CuCl (4 mol%) in
MeOH (5 mL).
Isolated yield.
2
1
b
2
00.0834. Found 200.0833.

S. Ma et al. / Tetrahedron 57 (2001) 1585±1588
1587
1.2.5. 3-Methyl-5-naphthyl-2(5H)-furanone (2e). Starting
from 1e (112 mg, 0.5 mmol) to afford 103 mg (92%) of 2e;
(d, Jˆ5.6 Hz, 1H), 5.04 (t, Jˆ6.4 Hz, 1H), 1.82±1.59 (m,
2H), 1.5±1.19 (m, 10H), 0.88 (t, Jˆ4.0 Hz, 3H); EIMS m/z
183 (M 11, 4), 182 (M , 7), 111 (100); IR (neat) 1747,
1
1
colorless needle crystals, mp 91±928C; H NMR (300 MHz,
1
1
2
CDCl ) d 8.06 (d, Jˆ8.0 Hz, 1H), 7.96±7.82 (m, 2H), 7.68±
1599, 1465 cm .
3
7.50 (m, 2H), 7.50±7.38 (m, 3H), 6.65 (s, 1H), 2.02 (s, 3H);
EIMS m/z 224 (M , 100); IR (KBr) 1744, 1650, 1598, 1506,
1
1.2.13. 3-Propenyl-5-propyl-2(5H)-furanone (2m). Start-
ing from 1m (86 mg, 0.52 mmol) to afford 78 mg (91%) of
2
1
438 cm . Anal. Calcd for C H O : C 80.34, H 5.39.
1
Found: C 80.23, H 5.45.
15 12 2
2m (eluent: petroleum ether/ethyl acetateˆ20:1); colorless
1
oil; H NMR (300 MHz, CDCl ) d 7.02 (s, 1H), 5.93±5.79
3
1.2.6. 5-Naphthyl-3-propyl-2(5H)-furanone (2f). Starting
from 1f (127 mg, 0.5 mmol) to afford 122 mg (96%) of 2f;
(m, 1H), 5.2±5.19 (m, 2H), 4.91 (m, 2H), 3.0 (d, Jˆ5.8 Hz,
2H), 1.75±1.38 (m, 4H), 0.94 (t, Jˆ7.3 Hz, 3H); EIMS m/z
167 (M 11, 3), 166 (M , 7), 95 (100); IR (neat) 1747,
1
1
colorless crystal mp 84±858C; H NMR (300 MHz, CDCl )
1
1
3
2
d 8.08 (d, Jˆ7.8 Hz, 1H), 7.9±7.8 (m, 2H), 7.6±7.5 (m,
1636, 1465, 1430 cm . HRMS (EI) Calcd for C10
166.0994. Found 166.0957.
H O
14 2
2
H), 7.45±7.3 (m, 3H), 6.66 (d, Jˆ1.6 Hz, 1H), 2.36 (t,
Jˆ7.1 Hz, 2H), 1.7±1.56 (m, 2H), 0.99 (t, Jˆ7.4 Hz, 3H);
1
EIMS m/z 252 (M , 5), 177 (100); IR (KBr) 1744, 1650,
1.2.14. 5-Naphthyl-3-propenyl-2(5H)-furanone (2n).
Starting from 1n (125 mg, 0.5 mmol) to afford 115 mg
(92% yield) of 2n (eluent: petroleum ether/ethyl ace-
2
1
1
8
596, 1540, 1453, 1436 cm . Anal. Calcd for C H O : C
17 16 2
0.93, H 6.39. Found: C 80.54, H 6.18.
.2.7. 3-Methyl-5-propyl-2(5H)-furanone (2g). Starting
from 1g (84 mg, 0.6 mmol) to afford 73 mg (87%) of 2g;
1
tateˆ20:1); colorless platelet crystals, mp 71±728C; H
1
5
1
NMR (300 MHz, CDCl
7.76 (m, 2H), 7.6±7.5 (m, 2H), 7.48±7.3 (m, 3H), 6.67 (s,
1H), 6.0±5.8 (m, 1H), 5.25±5.10 (m, 2H), 3.12 (d,
3
) d 8.05 (d, Jˆ8.3 Hz, 1H), 7.98±
1
colorless oil; H NMR (300 MHz, CDCl ) d 7.04 (bs, 1H),
3
1
4
.92±4.86 (m, 1H), 1.92 (s, 3H), 1.75±1.4 (m, 4H), 0.97 (t,
Jˆ6.7 Hz, 2H); EIMS m/z 250 (M , 100); IR (neat) 1758,
1
21
1635, 1597, 1507 cm , HRMS (EI) Calcd for C17
250.0994. Found 250.1029.
Jˆ7.5 Hz, 3H); EIMS m/z 141 (M 11, 100); IR (neat)
H
14
O
2
2
748, 1654, 1464 cm .
1
1
1.2.8. 3-Benzyl-5-propyl-2(5H)-furanone (2h). Starting
from 1h (110 mg, 0.51 mmol) to afford 93 mg (85%) of
Acknowledgements
1
h; colorless oil; H NMR (300 MHz, CDCl ) d 7.36±
.19 (m, 5H), 6.79±6.82 (m, 1H), 4.95±4.85 (m, 1H), 3.57
2
3
7
We are grateful to the Chinese Academy of Sciences and the
Major State Basic Research Development Program (Grant
No. G2000077500) for ®nancial support. Shengming Ma is
the recipient of a Special Starting Grant for Outstanding
Young Chemist (29525202) issued by the National Natural
Science Foundation of China and 1999 Qiu Shi Award for
Young Chinese Scienti®c Workers issued by Hong Kong
Qiu Shi Foundation of Science and Technology. Shulin
Wu thanks the Postdoctoral Foundation of Shanghai for
®nancial support.
(
s, 2H), 1.71±1.51 (m, 2H), 1.5±1.35 (m, 2H), 0.93 (t,
1
1
Jˆ2.9 Hz, 3H); EIMS m/z 217 (M 11, 33), 216 (M ,
2
1
2), 129 (100); IR (neat) 1748, 1600, 1492, 1452 cm .
2
HRMS (EI) Calcd for C H O 216.1146. Found 216.1183.
1
4
16
2
1
.2.9. 5-Methyl-3-propyl-2(5H)-furanone (2i). Starting
6
1
from 1i (81 mg, 0.58 mmol) to afford 63 mg (77%) of 2i;
1
colorless oil; H NMR (300 MHz, CDCl ) d 6.99 (d,
3
Jˆ1.2 Hz, 1H), 4.99 (dq, Jˆ1.2 and 6.9 Hz, 1H), 2.23 (t,
Jˆ7.3 Hz, 2H), 1.65±1.51 (m, 2H), 1.39 (d, Jˆ6.9 Hz, 3H),
1
0
(
.94 (t, Jˆ7.5 Hz, 3H); EIMS m/z 141 (M 11, 17), 140
1
21
M , 48), 97 (100); IR (neat) 1746, 1650, 1454 cm .
References
1
.2.10. 5-Hexyl-3-methyl-2(5H)-furanone (2j). Starting
7
1
from 1j (100 mg, 0.55 mmol) to afford 85 mg (85%) of 2j;
1. Larock, R. D.; Rie¯ing, B.; Fellows, C. A. J. Org. Chem. 1978,
43, 131 (and the references cited). For some of the most recent
examples, see: Chia, Y.; Chang, F.; Wu, Y. Tetrahedron Lett.
1999, 40, 7513. Takahashi, S.; Maeda, K.; Hirota, S.; Nakata,
T. Org. Lett. 1999, 1, 2025. Siddiqui, B. S.; Afshan, F.;
Ghiasuddin; Faizi, S.; Naqvi, S. N.-H.; Tariq, R. M.
J. Chem. Soc., Perkin Trans. 1 1999, 2367. Cortez, D. A. G.;
Fermandes, J. B.; Vieria, P. C.; das, M. F.; Da Silva, G. F.;
Ferreira, A. G.; Cass, Q. B.; Pirani, J. R. Phytochemistry 1998,
49, 2493. Ostuka, H.; Kotani, K.; Bando, M.; Kido, M.;
Takeda, Y. Chem. Pharm. Bull. 1998, 46, 1180. Ishikawa,
T.; Nishigaya, K.; Uchikoshi, H.; Chen, I. J. Nat. Prod.
1998, 64, 534.
1
colorless oil; H NMR (300 MHz, CDCl ) d 7.04 (bs, 1H),
3
4
1
.95±4.85 (m, 1H), 1.92 (s, 3H), 1.76±1.58 (m, 2H), 1.50±
.24 (m, 8H), 0.88 (t, Jˆ3.0 Hz, 3H); EIMS m/z 183
2
464 cm .
1
1
(
M 11, 3), 182 (M , 3), 43 (100); IR (neat) 1752,
1
1
10c
.2.11. 5-Ethyl-3-methyl-5-phenyl-2(5H)-furanone (2k).
1
Starting from 1k (106 mg, 0.52 mmol) to afford 99 mg
(
colorless oil; H NMR (300 MHz, CDCl ) d 7.61±7.22 (m,
93%) of 2k (eluent: petroleum ether/ethyl acetateˆ20:1);
1
3
6
H), 2.21±2.06 (m, 1H), 2.04±1.89 (m, 4H), 0.86 (t,
1
Jˆ5.6 Hz, 3H); EIMS m/z 202 (M , 3), 173 (100); IR
2. Brima, T. S. US 4,968,817, 1990; Chem. Abstr. 1991, 114,
185246y. Tanabe, A., Jpn. Kokai. Tokyo. Koho JP.
2
neat) 1758, 1660, 1494, 1448 cm .
1
(
6
3,211,276 [88,211,276], 1988; Chem. Abstr., 1989, 110,
1
.2.12. 5-Heptyl-2(5H)-furanone (2l). Starting from 1l
8
1
(
94978q. Lee, G. C. M. Eur. Pat. Appl. EP. 372,940, 1990;
Chem. Abstr., 1990, 113, 191137j. Ducharme, Y.; Gauthier,
J. Y.; Prasit, P.; Leblanc, Y.; Wang, Z.; Leger, S.; Therien, M.
PCT Int. Appl. WO 95, 00,501, 1995; Chem. Abstr., 1996,
98 mg, 0.54 mmol) to afford 79 mg (81%) of 2l (eluent:
H
1
petroleum ether/ethyl acetateˆ20:1); colorless oil;
NMR (300 MHz, CDCl ) d 7.46 (d, Jˆ5.6 Hz, 1H), 6.10
3

1
588
S. Ma et al. / Tetrahedron 57 (2001) 1585±1588
24, 55954y. Lee Gary, C. M.; Garst, M.E. PCT Int. Appl. 10. The reported procedure with H SO has two disadvantages: a
1
WO. 91 16,055, 1991; Chem. Abstr., 1992, 116, 59197m.
2
4
2 4
large amount of H SO was used and the yields are moderate
3
. For the corresponding reaction of 2,3-dienols with organic
halides to afford oxiranes, see: Ma, S.; Zhao, S. J. Am.
Chem. Soc. 1999, 121, 7943. For the corresponding reaction
of 1,2-allenyl ketones with organic halides to afford furans,
see: Ma, S.; Zhang, Z. Chem. Commun. 2000, 117. Ma, S.; Li,
L. Org. Lett. 2000, 2, 941.
(60±65%). The control experiment showed that with 1a the
yield is rather poor (25%). For the corresponding references:
(a) Kresze, G.; Runge, W.; Ruch, E. Liebigs Ann. Chem. 1972,
756, 112. (b) Musierowicz, S.; Wroblewski, A. E. Tetrahedron
1978, 54, 461. (c) Kresze, G.; Kloimstein, L.; Runge, W.
Liebigs Ann. Chem. 1976, 979.
4. Ma, S.; Shi, Z. J. Org. Chem. 1998, 63, 6387. Ma, S.; Duan,
D.; Shi, Z. Org. Lett. 2000, 2, 1419.
5. (a) Ma, S.; Shi, Z.; Yu, Z. Tetrahedron Lett. 1999, 40, 2393.
11. Venkruijsse, H. D.; Brandsma, L. In Synthesis of Acetylenes,
Allenes and Cumulenes. A Laboratory Manual, Elsevier:
Amsterdam, The Netherlands, 1981; pp 33.
(
b) Ma, S.; Shi, Z.; Yu, Z. Tetrahedron 1999, 55, 12137.
. Ma, S.; Wu, S. J. Org. Chem. 1999, 64, 9314.
12. Bestmann, H. J.; Hartung, H. Chem. Ber. 1966, 99, 1198.
13. Perrin, D. D.; Armarego, L. F.; Perrin, D. R. In Puri®cation of
Laboratory Chemicals, 2nd, Pergamon Press: New York,
1980; pp 486.
6
7
. (a) Marshall, J. A.; Wolf, M. A.; Wallace, E. M. J. Org. Chem.
1997, 62, 367. (b) Marshall, J. A.; Bartley, G. S.; Wallace,
E. M. J. Org. Chem. 1996, 61, 5729.
14. Gharbi-Benarous, J.; Essayegh, M. M.; Dana, G. Can. J.
Chem. 1987, 65 (9), 2031.
8
. At comparable levels of purity, CuCl is in most cases cheaper.
Local market: CuCl (A. R.) US$ 5.50/500 g, AgNO
(
3
(A. R.)
(99%) US$ 35.00/10 g,
15. Henin, F.; Pete, J. P. Tetrahedron Lett. 1983, 24 (43), 4687.
16. Black, T. H.; Brown, G. A.; Smith, D. C.; Martinie, S. M.
Synth. Commun. 1995, 25 (4), 479.
US$ 15.17/100 g; ACROS: AgNO
CuCl (99%) US$ 29.4/25 g).
3
9
. Copper ranks twenty-second in abundance among all the
elements in the earth's crust and silver ranks 63rd. Sneed,
M. C.; Maynard, J. L.; Brasted, R. C.; Comprehensive Inor-
ganic Chemistry; New York, 1954; Vol. VII, p10 and p114.
17. Tsuboi, S.; Wada, H.; Muranaka, K.; Takeda, A. Bull. Chem.
Soc. Jpn 1987, 60 (8), 2917.
18. Chiacchio, U.; Piperno, A.; Resci®na, A.; Romeo, G.; Uccella,
N. Tetrahedron 1998, 54 (21), 5695.