A practical approach to stereodefined cyclopropyl-substituted heteroarenes using a Suzuki-type reaction

Article abstract of DOI:10.1039/b001051k

In the presence of Pd(PPh₃)₄, NaBr and KF · 2H₂O the cross-coupling reaction of heteroaryl triflates with trans-cyclopropylboronic acids proceeds readily to give pure trans-cyclopropyl heteroarenes in moderate to good yields. The X-ray crystallography of compound 3g and ¹H-NMR spectra of all products show that the configuration of the cyclopropyl group was retained during the reaction. Under the same reaction conditions, highly optically active cyclopropyl-substituted heteroarenes (up to 94% ee) were obtained by cross-coupling of heteroaryl/triflates with enantiomerically enriched cyclopropylboronic acids.

Full text of DOI:10.1039/b001051k

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A practical approach to stereodeÐned cyclopropyl-substituted
heteroarenes using a Suzuki-type reaction
Min-Liang Yao and Min-Zhi Deng*
L aboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry,
Academia Sinica, 354 FengL in L u, Shanghai 200032, China.
E-mail: dengmz=pub.sioc.ac.cn; Fax: ]86 21 6416 6128
Received (in Montpellier, France) 7th February 2000, Accepted 29th March 2000
In the presence of Pd(PPh ) , NaBr and KF É 2H O the cross-coupling reaction of heteroaryl triÑates with
3 4
2
trans-cyclopropylboronic acids proceeds readily to give pure trans-cyclopropyl heteroarenes in moderate to good
yields. The X-ray crystallography of compound 3g and 1H-NMR spectra of all products show that the
conÐguration of the cyclopropyl group was retained during the reaction. Under the same reaction conditions,
highly optically active cyclopropyl-substituted heteroarenes (up to 94% ee) were obtained by cross-coupling of
heteroary/triÑates with enantiomerically enriched cyclopropylboronic acids.
Cyclopropyl-substituted heteroaryl moieties are present in
many biologically active natural products1 and synthetic
drugs,2 so the stereodeÐned cyclopropyl unit is increasingly
being incorporated into molecules for their well-deÐned three-
dimensional structure.3 Although the introduction of simple
cyclopropyl groups into heteroarenes has been reported,4
general methods for introducing substituted and stereocon-
trolled cyclopropyl groups into heteroarenes are still scarce.
Recently, the preparation of heteroaryl derivatives using
palladium-catalyzed protocols has attracted the interest of
many chemists.5 Among these protocols, the Suzuki-type
reaction6 is appealing, since boronic acids (or esters) are ther-
mally stable and relatively unreactive to both air and water.
The success of cyclopropylboronic acids (or esters) in the
cross-coupling reaction7 demonstrated that they were useful
reagents to prepare cyclopropyl-substituted compounds. In
addition, enantiomerically enriched cyclopropylboronic acids
(or esters) can be readily prepared by several methods.8 So the
Suzuki-type reaction between cyclopropylboronic acids and
eletrophiles to prepare cyclopropyl-substituted heteroarenes is
considered to be quite practical. Furthermore, by changing
the substituent on the cyclopropylboronic acid component,
the fat-solubility of the products can be easily adjusted. This
modulation is always very important in pharmaceutical
research.
expected reaction did not take place (entries 1, 2). Even when
adding 1 equiv. NaBr, only a trace of desired product was
detected (entry 3). When the reaction was conducted in the
presence of a catalytic amount of Pd(PPh ) , using toluene as
3 4
the solvent and K PO É 3H O as the base (entry 4), the
3
4
2
anticipated 2-cyclopropyl-substituted pyridine product was
obtained in 43% yield. If 1 equiv. NaBr was added, the yield
increased to 74% (entry 5). But the yield decreased to 49%
when using 2 M KOH as the base (entry 6), this can be ration-
alized by the partial decomposition of 2-pyridyl triÑate by the
strong base.10 So we turned our attention towards the weakly
basic KF É 2H O and found that the yield improved to 77%
(entry 7). The addition of NaBr seemed to be necessary to
2
make the reaction proceed readily (compare entry 4 with 5
and entry 7 with 8). Presumably the bromide ion converts the
labile
cationic
palladium(II)
species
(A)
to
organopalladium(II)bromide (B), which favours the trans-
Table 1 Coupling of trans-pentylcyclopropylboronic acid with 2-
pyridyl triÑatea
Entry
1
Conditions
Yieldb/%
The coupling reaction of heteroaryl bromides with cyclo-
propylboronic acids has been reported by our group,9 con-
sidering that some heteroaryl bromides are difficult to
prepare, the study of the corresponding reaction of hetroaryl
triÑates to make up for this drawback is necessary. Herein we
wish to report the results.
1,4-Dioxane, K PO É 3H O, Pd(PPh ) ,
100 ¡C
0
3
4
2
3 4
2
3
THF, K PO É 3H O, Pd(PPh ) , 70 ¡C
3 4
THF, K PO É 3H O, NaBr, Pd(PPh ) ,
3 4
0
3
4
2
Trace
3
4
2
70 ¡C
4
5
Toluene, K PO É 3H O, Pd(PPh ) , 100 ¡C
3 4
Toluene, K PO É 3H O, NaBr, Pd(PPh ) ,
3 4
43
74
3
4
2
3
4
2
100 ¡C
6
7
Toluene, 2 M KOH, NaBr, Pd(PPh ) ,
100 ¡C
49
77
3 4
Toluene, KF É 2H O, NaBr, Pd(PPh ) ,
2
3 4
Results and discussion
100 ¡C
8
9
Toluene, KF É 2H O, Pd(PPh ) , 100 ¡C
3 4
Toluene, KF É 2H O; NaBr, Pd(PPh ) Cl ,
3 2
51
0
2
Initially, we examined the cross-coupling reaction of trans-2-
pentylcyclopropylboronic acid with 2-pyridyl triÑate to
develop the optimum reaction conditions. As shown in Table
1, the solvent and base used a†ected the reaction greatly. Con-
sidering that ethereal solvents were frequently used in the
cross-coupling reactions of triÑates with organoboron com-
pounds, dioxane and THF were initially tested. However, the
2
2
100 ¡C
a Reactions were carried out for 22 h using 0.03 mmol of catalyst,
2-pyridyl triÑate (1.0 mmol) and trans-2-pentylcyclopropylboronic
acid (1.1 mmol) in 4 ml solvent. b Isolated yield based on 2-pyridyl
triÑate.
DOI: 10.1039/b001051k
New J. Chem., 2000, 24, 425È428
425
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2000

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metallation reaction between (B) and cyclopropylboronic
acids.
tion failed (entry 9). Thus, the nonpolar solvent toluene seems
to be more e†ective than ethereal solvents and the weakly
basic KF É 2H O is superior to the strong base KOH; the
addition of NaBr improved the conversion.
After having established the optimized coupling reaction
conditions, the scope of the reaction was evaluated by investi-
gating the coupling of various heteroaryl triÑates and cyclo-
Pd0Ln
Br~
2
ArOTf ÈÈÈ ArPdIIOTf ÈÈÈ ArPdIIBr
(A)
(B).
Considering that Pd(PPh ) Cl is air-stable and storable, we
3 2
tried to use Pd(PPh ) Cl instead of Pd(PPh ) , but the reac-
3 2 3 4
2
2
Table 2 Cross-coupling reactions of heteroaryl triÑates with trans-2-substituted cyclopropylboronic acidsa
Cyclopropylboronic
acid (1)
Heteroaryl
triÑate (2)
Product
(3)
Yieldb/
%
Entry
1
77
73
71
2
3
4c
5d
73
72
6
7
8
79
85
67
9
75
67
10e
11f
12
71
73
72
13
a Reactions were carried out at 100 ¡C using 0.03 mmol of Pd(PPh ) , cyclopropylboronic acid (1.1 mmol), heteroaryl triÑate (1.0 mmol), 3.3
3 4
equiv. KF É 2H O and 1 equiv. NaBr in toluene (4 ml). b Isolated yield based on heteroaryl triÑate. c ee: 89%, determined by HPLC (Chiralcel
2
OJ), [a]20 \ [302.4 (c: 0.81 in CHCl ). d ee: 93%, determined by HPLC (Chiralcel OJ) [a]20 \ ]311.5 (c: 0.29 in CHCl ). e ee: 89%, deter-
D
3
D
3
mined by HPLC (Chiralcel OD), [a]20 \ [700.1 (c: 0.58 in CHCl ). f ee: 94%, determined by HPLC (Chiralcel OD), [a]20 \ ]710.4 (c: 0.56 in
D
3
D
CHCl ).
3
426
New J. Chem., 2000, 24, 425È428

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In conclusion, we have for the Ðrst time studied the cross-
coupling reaction of heteroaryl triÑates13 with cyclo-
propylboronic acids. This reaction provides a novel and
convenient method to prepare stereodeÐned cyclopropyl-
substituted heteroarenes. The biological activities of these
compounds are being studied.
Scheme 1
Experimental
propylboronic acids. The results are summarized in Table 2,
where it can be seen that the yields were moderate to good.
The 1H-NMR spectra and HPLC of all products showed
clearly that they were pure trans-isomers. The conÐguration of
the trans-cyclopropane was further determined by the 1H-1H
NOESY spectrum of 3a (Scheme 1), the most downÐeld cyclo-
propane proton Hd can be assigned easily for its NOE e†ect
with the proton He on the pyridyl group. In addition there
were strong NOE e†ects between Hb and Hd; Hb and Hc; Ha
and Hc, but no NOE e†ects between Hd and Ha; Hd and Hc;
Ha and Hb. These observations suggested that 3a was pure
2-trans-cyclopropyl-substituted pyridine.
All reactions were performed under an argon atmosphere. The
aryl triÑates were prepared according to the literature pro-
cedure.13 Melting points are uncorrected. 1H-NMR and
13C-NMR spectra were recorded on a Bruker AMX-300 (300
MHz), using CDCl as the solvent with TMS as an internal
3
standard. MS spectra were obtained with an HP5989A
spectrometer using EI ionization at 70 eV. Elemental analyses
were conducted using a FossÈHeracus Vario EL instrument.
IR spectra were recorded on a Shimadzu IR-440 infrared
spectrometer. Optical rotations were measured using
a
PerkinÈElmer 241 MM polarimeter with a thermally jacketed
10 cm cell at 20 ¡C. The ee values were determined by chiral
HPLC on Chiralcel OD and Chiralcel OJ columns.
To further conÐrm this deduction an X-ray crystallographic
study of compound 3g was performed. A crystal of 3g suitable
for an X-ray analysis was obtained by crystallization in Et O.
2
General procedure for the coupling reaction
All measurements were made on a Rigaku AFC7R di†ractom-
eter with graphite monochromated Mo-Ka radiation and a 12
kW rotating anode generator. The crystal structure analysis of
Heteroaryl triÑate (1 mmol), cyclopropylboronic acid (1.1
mmol), Pd(PPh ) (35 mg, 0.03 mmol), KF É 2 H O (310 mg,
3 4
2
product 3g gave: empirical formula C
H
N, formula weight
3.3 mmol) and NaBr (102 mg, 1 mmol) were placed in a Ñask
under an argon atmosphere, then degassed toluene (4 ml) was
added; the reaction mixture was stirred at 100 ¡C and moni-
tored by TLC. After the reaction was complete, the reaction
mixture was cooled to room temperature and 10 ml of water
was added. The mixture was then extracted twice with pet-
roleum (bp 60È90 ¡C) (2 ] 15 ml). The combined organic layer
18 15
245.33, crystal system triclinic, space group P1 (no. 2), Z \ 4,
a \ 12.064(3), b \ 12.797(2), c \ 9.794(3) A, a \ 102.99(2),
b \ 97.17(2), c \ 68.70(2)¡, u \ 1371.1(6) AŽ 3, k \ 0.69 mm~1,
T \ 20 ^ 1 ¡C, measured reÑections 3470, independent reÑec-
tions 3265 (R \ 0.040), Ðnal R \ 0.041, R \ 0.045. The
m
w
X-ray crystallographic structure (Fig. 1) of 3g shows clearly
that the conÐguration of the cyclopropane is trans, meaning
that the conÐguration of the cyclopropyl group was retained
during the reaction.
was washed with brine (3 ] 10 ml) and dried over MgSO .
Removal of petroleum in vacuo, followed by silica gel chroma-
4
tography [15È25% diethyl ether in petroleum (bp 60È90 ¡C)],
gave the corresponding cyclopropyl-substituted heteroarenes
3aÈi.
When using an optically active cyclopropylboronic acid8c
as the reactant, the coupling products were also the pure trans
isomers, judging from their 1H-NMR spectra. Furthermore,
the coupling products of aryl triÑate/heteroaryl triÑate with
the same optically active cyclopropylboronic acid not only
have the same ee value but also have the same optical
activity11 (Scheme 2). Therefore, the absolute conÐguration of
the cyclopropyl group is conÐrmed to be retained during the
coupling reaction, as in the results obtained in previous
studies.8c,12
3a. Liquid; IR l /cm~1: 2919; 1596; 1568; 1475; 1455;
max
772; 743. 1H-NMR d : 8.40 (d, 1H, J \ 4.5); 7.48È7.54 (m,
H
1H); 7.08 (d, 1H, J \ 8.0 Hz); 6.98È7.03 (m, 1H); 1.76 (ddd,
1H); 1.27È1.45 (m, 9H, 4 ] CH ] H); 1.18È1.20 (m, 1H);
2
0.78È0.90 (m, 4H, CH ] H). 13C-NMR d: 163.0; 149.2;
3
135.7; 121.1; 120.0; 34.0; 31.7; 29.0; 25.0; 24.4; 22.7; 17.0;
14.1. MS(EI) m/z: 190 (100); 132 (92.45); 119 (62.12); 118
(54.74); 117 (39.85); 133 (27.30); 146 (17.23); 130 (16.18). Anal.
calcd. for C
C%, 82.26, H%, 10.27, N%, 7.58.
H
N: C%, 82.48, H%, 10.11, N%, 7.40. Found:
13 19
3b. Liquid; IR l /cm~1: 2921; 1597; 1569; 1478; 1457;
max
771; 742. 1H-NMR d : 8.40 (d, 1H, J \ 4.5); 7.47È7.54 (m,
H
1H); 7.08 (d, 1H, J \ 8.0 Hz); 6.99È7.04 (m, 1H); 1.76 (ddd,
1H); 1.26È1.43 (m, 7H, 3 ] CH ] H); 1.18È1.20 (m, 1H);
2
0.78È0.89 (m, 4H, CH ] H). MS(EI) m/z: 132 (100); 118
3
(64.55); 119 (56.76); 117 (48.26); 93 (33.52); 105 (29.30); 133
(25.29); 79 (23.80). Anal. calcd. for C
9.78, N%, 7.99. Found: C%, 81.92, H%, 9.98, N%, 7.98.
H
N: C%, 82.22, H%,
12 17
Fig. 1 Molecular structure of product 3g.
3c. Liquid; IR l /cm~1: 3001; 1597; 1567; 1478; 1444;
max
743; 699. 1H-NMR d : 8.51 (d, 1H, J \ 4.5 Hz); 7.53È7.58 (m,
H
1H); 7.05È7.43 (m, 7H); 2.57 (ddd, 1H); 2.31 (ddd, 1H); 1.80
(ddd, 1H); 1.49 (ddd, 1H). MS(EI) m/z: 194 (100); 195 (80.40);
93 (33.30); 196 (21.64); 193 (20.74); 118 (18.85); 180 (16.76);
167 (12.76). Anal. calcd. for C
N%, 7.17. Found: C%, 85.67, H%, 6.95, N%, 7.31.
H
N: C%, 86.12, H%, 6.71,
14 13
3d. Liquid; IR l /cm~1: 2933; 1625; 1608; 1508; 1433;
max
827; 756. 1H-NMR d : 7.96È8.00 (m, 2H); 7.74 (d, 1H,
H
Scheme 2
J \ 8.1); 7.62È7.67 (m, 1H); 7.40È7.45 (m, 1H); 7.12 (d, 1H,
New J. Chem., 2000, 24, 425È428
427

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J \ 8.5 Hz); 1.99 (ddd, 1H); 1.26È1.53 (m, 8H, 3 ] CH ] H
m/z: 154 (100); 168 (95.86); 167 (89.15); 182 (53.80); 169
(43.94); 155 (33.69); 253 (18.12); 252 (17.19). Anal. calcd. for
2
] H); 0.82È0.95 (m, 4H, CH ] H). MS(EI) m/z: 168 (100);
3
182 (99.44); 169 (90.88); 167 (87.16); 183 (36.62); 180 (28.70);
C
H
N: C%, 85.32, H%, 9.15, N%, 5.53. Found: C%,
18 23
143 (28.39). Anal. calcd. for C
N%, 6.22. Found: C%, 85.58, H%, 8.94, N%, 6.01.
H
N: C%, 85.29, H%, 8.50,
85.41, H%, 8.96, N%, 5.30.
16 19
3e. Liquid; IR l /cm~1: 2931; 1623; 1607; 1507; 1431;
max
Acknowledgement
826; 757. 1H-NMR d : 7.93È7.96 (m, 2H); 7.71 (d, 1H,
H
J \ 8.1); 7.59È7.65 (m, 1H); 7.37È7.42 (m, 1H); 7.11 (d, 1H,
We are grateful to the National Science Foundation of China
for Ðnancial support.
J \ 8.6 Hz); 1.96 (ddd, 1H); 1.29È1.50 (m, 10H, 4 ] CH ] H
2
] H); 0.85È0.94 (m, 4H, CH ] H). MS(EI) m/z: 168 (100);
3
169 (97.20); 182 (96.58); 167 (78.68); 240 (38.41); 183 (29.35);
143 (28.54); 180 (24.77). Anal. calcd. for C
H%, 8.84, N%, 5.85. Found: C%, 85.14, H%, 8.83, N%, 5.90.
H
N: C%, 85.31,
17 21
Notes and references
3f. Liquid; IR l /cm~1: 2927; 1621; 1605; 1505; 1434;
1
(a) J. D. White, T.-S. Kim and M. Nambu, J. Am. Chem. Soc.,
1997, 119, 103 and references cited therein; (b) A. D. Rodr•guez,
J.-G. Shi and S. D. Huang, J. Org. Chem., 1998, 63, 4425.
(a) D. W. Brooks, B. W. Horrom, K. E. Rodriques and Mazdiy-
asni (Abbott Laboratories), Eur. Pat. 436199, 1991; Chem. Abstr.,
1991, 115, 279483; (b) M. Suzuki, K. Tanikawa and K. Sakoda,
Heterocycles, 1999, 50, 479.
(a) H. N. C. Wong, M.-Y. Hon, C.-W. Tse, Y. C. Yip, J. Tanko
and T. Hudlicky, Chem. Rev., 1989, 89, 165; (b) J. R. Y. Salaun,
T op. Curr. Chem., 1988, 144, 1.
(a) A. B. Charrette and P. Chua, J. Org. Chem., 1998, 63, 908; (b)
J. Bivin, J. Pothier and S. Z. Zard, T etrahedron L ett., 1999, 40,
3701; (c) F. Freeman, T. Chen and J. B. Linden, Synthesis, 1997,
861.
(a) D. W. Gordon, Synlett, 1996, 893; (b) C. N. Johnson, G.
Stemp, N. Anand, S. C. Stephen and T. Gallagher, Synlett, 1998,
1025; (c) O. Lohse, P. Thevenin and E. Waldvogel, Synlett, 1999,
45; (d) I. Collins and J. L. Castro, T etrahedron L ett., 1999, 40,
4069.
(a) N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457; (b) A.
Suzuki, J. Organomet. Chem., 1999, 576, 147.
(a) X.-Z. Wang and M.-Z. Deng, J. Chem. Soc., Perkin. T rans. 1,
1996, 2663; (b) J. P. Hildebrand and S. P. Marsden, Synlett, 1996,
893; (c) A. B. Charrette and R. P. DeFreitasÈGil, T etrahedron
L ett., 1997, 38, 2809; (d) M.-L. Yao and M.-Z. Deng, Synthesis,
2000, in press.
(a) H. Imai and S. N. Mineta, J. Org. Chem., 1990, 55, 4986; (b) J.
Pietruszka and M. Widenmeyer, Synlett, 1997, 977; (c) S.-M.
Zhou, M.-Z. Deng, L.-J. Xia and M.-H. Tang, Angew. Chem., Int.
Ed.,, 1998, 37, 2845; (d) J. E. A. Luithle and J. Pietruszka, J. Org.
Chem., 1999, 64, 8287.
H.-R. Ma, X.-H. Wang and M.-Z. Deng, Synth. Commun., 1999,
29, 2477.
max
828; 758. 1H-NMR d : 7.93È7.96 (m, 2H); 7.71 (d, 1H); 7.59È
H
7.65 (m, 1H); 7.37È7.42 (m, 1H); 7.11 (d, 1H); 1.97 (ddd, 1H);
2
1.25È1.50 (m, 12H, 5 ] CH ] H ] H); 0.85È0.94 (m, 4H,
2
CH ] H). MS(EI) m/z: 182 (100); 168 (93.46); 169 (93.25);
3
167 (67.94); 143 (37.96); 155 (24.75); 196 (22.23); 156 (21.12).
Anal. calcd. for C
Found: C%, 85.17, H%, 9.26, N%, 5.70.
H
N: C%, 85.32, H%, 9.15, N%, 5.53.
3
4
18 23
3g. White solid, mp: 70È72 ¡C; IR l /cm~1: 3007; 1618;
max
1600; 1560; 1503; 1496; 1196; 819; 757. 1H-NMR (400 MHz)
d : 7.96È8.00 (m, 2H); 7.64È7.73 (m, 2H); 7.42È7.44 (m, 1H);
H
7.17È7.30 (m, 6H); 2.65È2.69 (ddd, 1H,); 2.44È2.48 (ddd, 1H);
5
1.96È1.99 (ddd, 1H); 1.54È1.57 (ddd, 1H). MS(EI) m/z: 244
(100); 245 (65.49); 143 (21.66); 246 (12.04); 168 (9.99); 242
(9.71); 167 (7.86); 230 (7.18). Anal. calcd. for C
88.13, H%, 6.16, N%, 5.71. Found: C%, 87.90, H%, 6.13, N%,
H
N: C%,
18 15
6
7
5.74.
CCDC reference number 440/169. See http://www.rsc.org/
suppdata/nj/b0/b001501k/ for crystallographic Ðles in .cif
format.
3h. Liquid; IR l /cm~1: 2914; 1599; 1500; 1467; 1368;
max
823; 795. 1H-NMR d : 8.98 (d, 1H, J \ 4.1); 8.13 (d, 1H,
8
9
H
J \ 8.3); 7.60 (d, 1H, J \ 8.1); 7.38È7.46 (m, 2H); 7.15 (d, 1H,
J \ 7.2); 3.00 (ddd, 1H); 1.25È1.55 (m, 7H, 3 ] CH ] H);
2
0.96È1.04 (m, 2H, H ] H); 0.90 (t, 3H, J \ 7.2 Hz). MS(EI)
m/z: 154 (100); 168 (91.77); 167 (66.93); 182 (46.39); 169
(44.10); 226 (29.76); 155 (27.59); 225 (15.32). Anal. calcd. for
C
H
N: C%, 85.29, H%, 8.49, N%, 6.22. Found: C%,
10 V. Percec, J.-Y. Bae and D. H. Hill, J. Org. Chem., 1995, 60, 1060.
11 (1R,2R)-1,2-Diphenylcyclopropane: [a]20 \ [356.2 (c 0.78 in
16 19
85.23, H%, 8.54, N%, 6.39.
D
CHCl ), ee: 89%; 3c
: [a]20 \ [302.4 (c 0.81 in CHCl ),
3
(1R,2R)
D
3
ee: 89%; 3g
: [a]20 \ [700.1 (c 0.58 in CHCl ), ee: 89%.
3i. Liquid; IR l /cm~1: 2917; 1600; 1501; 1472; 1372;
(1R,2R)
D
3
max
12 (a) B. H. Ridgway and K. A. Woerpel, J. Org. Chem., 1998, 63,
458; (b) K. Matos and J. A. Soderquist, J. Org. Chem., 1998, 63,
461.
824; 797. 1H-NMR d : 8.97 (d, 1H, J \ 4.0); 8.11 (d, 1H,
H
J \ 8.2); 7.58 (d, 1H, J \ 8.2); 7.34È7.45 (m, 2H); 7.13 (d, 1H,
J \ 7.2); 3.01 (ddd, 1H); 1.25È1.55 (m, 11H, 5 ] CH ] H);
13 (a) P. J. Stang, M. Hanack and L. R. Subramanian, Synthesis,
2
0.96È1.04 (m, 2H, H ] H); 0.87 (t, 3H, J \ 6.8 Hz). MS(EI)
1982, 85; (b) K. Ritter, Synthesis, 1993, 735.
428
New J. Chem., 2000, 24, 425È428