Heteroaromatic decarboxylative Claisen rearrangement reactions

Article abstract of DOI:10.1055/s-2005-918448

Furan-2-ylmethyl, thien-2-ylmethyl and pyrrol-2-ylmethyl tosylacetates undergo facile decarboxylative Claisen rearrangement upon exposure to N,O-bis(trimethylsilyl)acetamidepotassium acetate to yield the corresponding 2,3-disubstituted heteroaromatic prod

Full text of DOI:10.1055/s-2005-918448

PAPER
3279
Heteroaromatic Decarboxylative Claisen Rearrangement Reactions
Heteroaromatic
D
e
o
carboxylativ
n
e
Claisen
R
e
a
arrangemen
l
t
Reac
d
tions Craig,*a N. Paul King,b Jörg T. Kley,c David M. Mountforda
a
b
c
Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, UK
Fax +44(20)75945868; E-mail: d.craig@imperial.ac.uk
Neurology and GI Centre of Excellence for Drug Discovery, GlaxoSmithKline Research Limited, New Frontiers Science Park,
Third Avenue, Harlow, Essex CM19 5AW, UK
Department of Chemical Research, Boehringer Ingelheim Pharma GmbH & Co. KG, Birkendorfer Strasse 65,
88397 Biberach an der Riss, Germany
Received 14 September 2005
Dedicated with respect, admiration and affection to Professor Steven V. Ley, CBE, FRS, on the occasion of his 60th birthday
aromatic nucleus. This idea is not without precedent. For
example, when treated with diene 5, furfuryl alcohol 4 has
been reported to undergo Claisen rearrangement to give
aldehydes 7 and 8, presumably via transient intermediate
Abstract: Furan-2-ylmethyl, thien-2-ylmethyl and pyrrol-2-yl-
methyl tosylacetates undergo facile decarboxylative Claisen rear-
rangement upon exposure to N,O-bis(trimethylsilyl)acetamide–
potassium acetate to yield the corresponding 2,3-disubstituted het-
eroaromatic products in good yield. However, for 1-(thien-2-yl)ethyl 6.4 In addition, ethyl 2-thiopheneglycolate 9 underwent
tosylacetates and substrates derived from 3-(hydroxyalkyl)indoles
orthoester Claisen rearrangement to give 2,3-disubstituted
thiophene 10 (Scheme 2).5
rearomatisation does not occur.
Key words: heterocycles, heterogenous catalysis, pericyclic reac-
tions, sulfones, tautomerism
a
OH
CHO
O
4
5
Since its discovery in 1912, the Claisen rearrangement1
has found extensive use in synthetic organic chemistry
due to the highly stereoselective nature of the [3,3]-sigma-
tropic process. Modern variants of this rearrangement2
have ensured the continued use of this powerful C–C bond
forming tool in organic synthesis.
+
6
O
OEt
ref. 4
CHO
+
CHO
O
O
7 (21%)
8 (2%)
Previous studies from this laboratory have demonstrated
that tosylacetic esters 1 undergo Claisen rearrangement
with concomitant decarboxylation when treated with N,O-
bis(trimethylsilyl)acetamide (BSA) and sub-stoichiomet-
ric amounts of potassium acetate. These reactions provide
sulfones 3 in excellent yields for a range of structurally di-
verse substrates (Scheme 1).3 The use of microwave irra-
diation was found to greatly reduce the reaction time and
allow the reactions to be run in the absence of solvent,
without significant erosion of yield.
CO2Et
b
OH
S
S
CO2Et
CO2Et
10
9
ref. 5
OR'
OEt
O
O
S
S
CO2Et
CO2Et
O
O
Ts
R
Ts
R
Scheme 2 Reagents and conditions: a) Hg(OAc)2, NaOAc, 100 °C,
18 h; b) MeC(OEt)3, Me(CH2)4CO2H, reflux, 12 h then 185 °C, 6 h,
61%.
Ts
R
O
a
OTMS
3
1
2
We wished to investigate whether our mild decarboxyla-
tive Claisen rearrangement (dCr) reaction conditions
would bring about a similar rearrangement in heteroaro-
matic substrates. Esters 11, 13 and 17 were synthesised by
esterification of the corresponding (hydroxymethyl)arene
with tosylacetic acid using DCC and sub-stoichiometric
amounts of DMAP.6 Alkylation of 13 with sodium hy-
dride and allyl bromide yielded ester 15 in good yield
(72%). Under the previously established dCr reaction con-
ditions, good yields of the corresponding rearranged, de-
carboxylated sulfones were isolated (Table 1). The
transformation of the more highly branched substrate 15
Scheme 1 Reagents and conditions: a) BSA, KOAc, PhMe, 110 °C,
15 h, 75–98% or BSA, KOAc, PhMe or no solvent, 150 °C, micro-
wave, 3 min, 67–92%.
We became interested in applying this chemistry to sub-
strates derived from benzylic-type alcohols, in which the
rearrangement process would result in disruption of the
SYNTHESIS 2005, No. 19, pp 3279–3282
x
x.
x
x
.
2
0
0
5
Advanced online publication: 27.10.2005
DOI: 10.1055/s-2005-918448; Art ID: C07405SS
© Georg Thieme Verlag Stuttgart · New York

3280
D. Craig et al.
PAPER
was reproducibly less efficient than those of the other sub-
strates. Application of microwave irradiation resulted in
comparable yields of rearranged products 12, 14 and 18,
but not 16. Microwave heating in the absence of solvent
caused complete decomposition in the case of the furan
and thiophene substrates. However, a good yield of 18
(70%) was obtained from the pyrrolic substrate 17 under
the solvent-free conditions. The use of sub-stoichiometric
amounts of BSA in the rearrangement of esters 13 and 17
gave lower yields (47% and 48%, respectively) of the cor-
responding sulfones.
O
O
O
O
O
S
Ts
Ts
O
19
21
20
O
O
O
O
Ts
Ts
Figure 1
the weakening effect of the arene on the benzylic C–O
bond in the 3-substituted versus the 2-substituted isomers.
Table 1 dCr of Heteroaromatic Tosylacetatesa
Entry Substrate
Product
Yield (%)b
Next, we were interested in assessing the scope of the dCr
reaction with respect to the extent of substitution in the 2-
substituted substrates. The thiophene-containing ester 23
was readily synthesised in two steps from thiophene-2-
carboxaldehyde 22. To our surprise, exposure of ester 23
to the stoichiometric dCr resulted in the rearranged, non-
aromatic product 25 as a single geometric isomer in 58%
yield. Although we have not yet assigned unequivocally
double bond geometry, in the isomer shown, allylic 1,3-
strain appears to be minimised. Under the sub-stoichio-
metric conditions employed previously 25 was again ob-
tained (53%). These observations prompted a re-
examination of the dCr reaction of the more simple
thiophene-containing substrate 13, which revealed that
traces of the non-aromatic product 24 had indeed been
formed together with the major product sulfone 14
(Scheme 3). We speculate that the increased extent of for-
mation of non-aromatic product in the dCr reaction of the
more highly substituted substrate 23 is a consequence of
(i) the more highly substituted nature of the exocyclic alk-
ene linkage in 25 and (ii) the increase in steric buttressing
which would ensue in the non-observed 2-ethyl-substitut-
ed aromatic product.
A
B
Ts
Ts
Ts
O
O
O
O
1
2
3
63
75
22
–
O
O
O
O
O
O
S
S
12
11
Ts
Ts
Ts
75
58
S
13
14
Ts
Ts
S
15
16
4
67
77
N
N
Ts
Ts
The final part of this study was devoted to extending the
range of substrates to include indoles. According to the re-
activity differences observed for the monocyclic het-
eroarene-containing substrates, we anticipated that
tosylacetate substrates derived from 3-(hydroxyalkyl)in-
17
18
a All reactions were carried out with BSA (1.0 equiv) and KOAc (0.1
equiv).
b Reaction conditions: A: toluene, 110 °C, 15 h; B: toluene, micro-
wave, 150 °C, 3 min.
Ts
We next investigated heteroaromatic substrates substitut-
ed at the 3-position. Esters 19, 20 and 21 (Figure 1) were
synthesised from the corresponding alcohols in excellent
yield using the DCC–DMAP procedure described previ-
ously.
O
a, b
O
87%
CHO
S
S
22
23
c (58%) or
d (53%)
In stark contrast to substrates 11, 13, 15 and 17, when 19–
21 were subjected to the stoichiometric dCr conditions no
Claisen rearrangement occurred. In the case of the furan
derivatives 19 and 21 only starting material was observed
with a trace of decomposition product even after pro-
longed (30 h) reaction periods. With the thiophene deriv-
ative 20, extensive substrate decomposition occurred. We
interpret these differences in reactivity in terms of the
greater electron density located at the 2-position relative
to the 3-position, which results also in an attenuation of
Ts
Ts
single
isomer
S
S
24
25
Scheme 3 Reagents and conditions: a) MeMgCl, THF, 0 °C, 2 h; b)
TsCH2CO2H, DCC, DMAP, CH2Cl2, 15 h; c) BSA (1.0 equiv), KOAc
(0.1 equiv), PhMe, 110 °C, 18 h); d) BSA (0.1 equiv), KOAc (0.1
equiv), PhMe, 110 °C, 18 h.
Synthesis 2005, No. 19, 3279–3282 © Thieme Stuttgart · New York

PAPER
Heteroaromatic Decarboxylative Claisen Rearrangement Reactions
Conventional Heating; Typical Procedure
3281
doles, but not the 2-substituted isomers, would be effective
in this chemistry because of the greater electron density at
the indole 3-position. This supposition was borne out in
practice: while tosylacetate 30 was inert to the dCr condi-
tions, substrate 26 synthesised from indole-3-carbalde-
hyde as depicted in Scheme 4 underwent moderately
efficient rearrangement–decarboxylation using the sub-
stoichiometric conditions to give the methylene-substitut-
ed indoline 28. Furthermore, the homologous substrate 27
reacted similarly under both stoichiometric and sub-stoi-
chiometric conditions to give 29 as a single isomer; again
it has not been possible thus far to assign exocyclic alkene
geometry. Methyleneindolines similar to 28 have been
synthesised by Larock via palladium-catalysed heteroan-
nulation of allenes using substituted aryl halides.7,8 All at-
tempts to convert indoline 28 into the isomeric indole
under both acidic and basic conditions failed; the stability
of 28 was demonstrated by the high-yield recovery of it in
these fruitless efforts.
To a solution of ester 11 (200 mg, 0.68 mmol, 1 equiv) in toluene (6
mL) was added BSA (152 mL, 0.75 mmol, 1.1 equiv) and KOAc
(6.7 mg, 0.07 mmol, 0.1 equiv); the mixture was heated at 110 °C
for 15 h. Concentration under reduced pressure and chromatogra-
phy (EtOAc–petroleum ether, 1:4) gave furan 12 (108 mg, 63%) as
colourless crystals.
Microwave Heating; Typical Procedure
To a microwave vial was added KOAc (3.3 mg, 0.03 mmol, 0.1
equiv) followed by ester 11 (100 mg, 0.34 mmol, 1 equiv), BSA
(91.7 mL, 0.37 mmol, 1.1 equiv) and toluene (1 mL). The resulting
mixture was heated to 150 °C (250 W) for 3 min. Concentration un-
der reduced pressure and chromatography (EtOAc–petroleum
ether, 1:4) gave furan 12 (63.4 mg, 75%) as colourless crystals.
12
Rf 0.33 (EtOAc–petroleum ether, 1:4); colourless crystals; mp 72–
74 °C.
IR (film): 2924, 1597, 1517, 1446, 1403, 1316, 1303, 1290, 1231,
1188, 1144, 1086, 899, 816, 765, 733, 665, 640 cm–1.
1H NMR (270 MHz, CDCl3): d = 7.54 (2 H, d, J = 8.0 Hz, o-Ph),
7.24 (2 H, d, J = 8.0 Hz, m-Ph), 7.17 (1 H, d, J = 2.0 Hz, H-5), 6.14
(1 H, d, J = 2.0 Hz, H-4), 4.04 (s, 2 H, CH2), 2.39 (s, 3 H, CH3Ph),
1.82 (s, 3 H, CH3).
CHO
CHO
a
79%
13C NMR (67.5 MHz, CDCl3): d = 152.3, 144.8, 140.7, 135.0,
N
N
H
Ts
129.7, 128.6, 112.2, 107.4, 53.8, 21.7, 11.1.
b, c
d, c
58%
MS (CI): m/z = 268 [M + NH4]+, 205, 188, 155, 107, 98, 81.
87%
HRMS: m/z calcd for C13H18NSO3 [M + NH4]+, 268.1001; found,
268.1011.
O
O
O
O
Anal. Calcd for C13H14O3S: C, 62.18; H, 5.45. Found: C, 62.38; H,
5.64.
Ts
Ts
N
N
Ts
Ts
26
27
14
e (28%)
or f (61%)
e (58%)
or f (60%)
Rf 0.21 (EtOAc–petroleum ether, 1:4); colourless crystals; mp 134–
136 °C.
IR (film): 3055, 2985, 1598, 1442, 1421, 1315, 1265, 1147, 1086,
895, 731 704, 634, 609 cm–1.
1H NMR (270 MHz, CDCl3): d = 7.49 (2 H, d, J = 8.0 Hz, o-Ph),
7.24 (2 H, d, J = 8.0 Hz, m-Ph), 6.98 (1 H, d, J = 5.0 Hz, H-5), 6.75
(1 H, d, J = 5.0 Hz, H-4), 4.24 (2 H, s, CH2), 2.41 (3 H, s, CH3Ph),
2.00 (3 H, s, CH3).
N
N
Ts
Ts
single
isomer
Ts
Ts
28
29
O
Ts
13C NMR (67.5 MHz, CDCl3): d = 144.8, 135.4, 129.7, 129.6,
129.5, 128.6, 121.9, 112.4, 55.9, 21.7, 12.7.
O
30: inert to dCr reaction
MS (CI): m/z = 284 [M + NH4]+, 128, 111.
HRMS: m/z calcd for C13H18NO2S2 [M + NH4]+, 284.0779; found,
N
Ts
Scheme 4 Reagents and conditions: a) NaH, TsCl, THF, 0 °C→r.t.,
2 h; b) LiAlH4, THF, r.t., 1.5 h; c) TsCH2CO2H, DCC, DMAP,
CH2Cl2, 15 h; d) MeMgCl, THF, 0 °C, 2 h; e) BSA (1.0 equiv), KOAc
(0.1 equiv), PhMe, 110 °C, 18 h; f) BSA (0.1 equiv), KOAc (0.1
equiv), PhMe, 110 °C, 18 h.
284.0789.
16
Rf 0.28 (EtOAc–petroleum ether, 1:4); pale yellow oil.
IR (film): 3080, 2922, 1641, 1597, 1441, 1311, 1290, 1209, 1146,
1086, 993, 920, 815, 705, 661 cm–1.
1H NMR (270 MHz, CDCl3): d = 7.78 (1 H, d, J = 8.0 Hz, o-Ph),
7.38 (2 H, d, J = 8.0 Hz, m-Ph), 7.17 (1 H, d, J = 8.0 Hz, o-Ph), 7.00
(2 H, s, H-4, H-5), 5.57–5.42 (1 H, m, CH=CH2), 5.09–4.92 (2 H,
m, CH=CH2), 4.13 (1 H, dd, J = 11.5, 4.0 Hz, CHOSO2), 3.21–3.09
(1 H, m, CHHCH=CH2), 2.87–2.73 (1 H, m, CHHCH=CH2), 2.38 (
3 H, s, CH3Ph), 1.81 (3 H, s, CH3).
13C NMR (67.5 MHz, CDCl3): d = 144.7, 133.1, 130.3, 130.1,
129.4, 129.0, 127.2, 122.0, 118.4, 106.5, 64.3, 32.0, 21.7, 12.5.
MS (CI): m/z = 324 [M + NH4]+, 307 [M + H]+, 301, 247, 205, 177,
167, 151.
In summary, we have demonstrated that a range of hetero-
cyclic substrates undergo dCr to yield 2,3-disubstituted
heteroarenes products in good yield. We have shown also
that through careful choice of protecting group and substi-
tution pattern, the dCr of heterocyclic substrates may be
used in the synthesis of non-aromatic products. It is hoped
that these readily accessible compounds will prove to be
valuable building blocks in target-orientated natural prod-
uct synthesis.
Synthesis 2005, No. 19, 3279–3282 © Thieme Stuttgart · New York

3282
D. Craig et al.
PAPER
HRMS: m/z calcd for C16H22NO2S2 [M + NH4]+, 324.1092; found,
MS (CI): m/z = 471 [M + NH4]+, 317, 302, 300, 188, 174, 144, 132,
324.1093.
108.
HRMS: m/z calcd for C24H27N2O4S2 [M + NH4]+, 471.1412; found,
471.1412.
18
Rf 0.14 (EtOAc–petroleum ether, 3:7); colourless crystals; mp 170–
172 °C.
29
IR (film): 3055, 2987, 2925, 1597, 1421, 1367, 1315, 1267, 1173,
1161, 1146, 1088, 1028, 897, 814, 750, 704, 688, 646 cm–1.
Rf 0.31 (EtOAc–petroleum ether, 3:7); colourless oil.
IR (film): 2958, 2924, 2863, 1596, 1457, 1399, 1357, 1320, 1302,
1168, 1141, 1087, 944, 813, 750, 737, 660 cm–1.
1H NMR (270 MHz, CDCl3): d = 7.62 (2 H, d, J = 8.5 Hz, o-Ph),
7.35 (2 H, d, J = 8.0 Hz, o-Ph), 7.31 (2 H, d, J = 8.5 Hz, m-Ph), 7.23
(1 H, d, J = 3.5 Hz, H-5), 7.02 (2 H, d, J = 8.0 Hz, m-Ph), 6.10 (1 H,
d, J = 3.5 Hz, H-4), 4.00 (2 H, s, CH2), 2.44 (3 H, s,
CH2OSO2PhCH3), 2.34 (3 H, s, NOSO2PhCH3), 1.73 (3 H, s, CH3).
13C NMR (67.5 MHz, CDCl3): d = 145.2, 144.6, 134.6, 130.4,
130.1, 129.5, 128.6, 127.2, 126.9, 121.5, 113.3, 54.6, 21.8, 21.7,
10.3.
MS (CI): m/z = 421 [M + NH4]+, 404 [M + H]+, 362, 267, 250, 189,
94.
HRMS: m/z calcd for C20H25N2O4S2 [M + NH4]+, 421.1256; found,
421.1257.
1H NMR (270 MHz, CDCl3): d = 7.76 (2 H, d, J = 8.0 Hz, o-Ph),
7.62 (1 H, d, J = 8.0 Hz, H-4), 7.45 (2 H, d, J = 8.0 Hz, o-Ph), 7.40
(1 H, d, J = 7.5 Hz, H-7), 7.31 (2 H, d, J = 8.0 Hz, m-Ph), 7.22–7.01
(2 H, m, H-5, H-6), 7.11 (2 H, d, J = 8.0 Hz, m-Ph), 5.96 (1 H, qd,
J = 7.0, 1.5 Hz, C=CHCH3), 5.04–4.96 (1 H, m, H-1), 3.83 (1 H, dd,
J = 14.5, 3.0 Hz, CHHTs), 3.63 (1 H, dd, J = 14.5, 8.0 Hz, CHHTs),
2.41 (3 H, s, CH2OSO2PhCH3), 2.28 (3 H, s, NOSO2PhCH3), 1.84
(3 H, dd, J = 7.5, 1.5 Hz, C=CHCH3).
13C NMR (67.5 MHz, CDCl3): d = 144.8, 144.5, 143.3, 137.6,
134.3, 133.6, 129.9, 129.8, 128.9, 128.1, 127.4, 125.1, 124.9, 122.3,
116, 62.7, 61.4, 21.8, 21.6, 14.3.
MS (CI): m/z = 485 [M + NH4]+, 391, 337, 314, 256, 236, 220, 188,
77.
Anal. Calcd for C20H21NO4S2: C, 59.48; H, 5.36; N, 3.54. Found: C,
59.53; H, 5.25; N, 3.47.
HRMS: m/z calcd for C25H29N2S2O4 [M + NH4]+, 485.1569; found,
25
485.1553.
Rf 0.42 (EtOAc–petroleum ether, 2:3); colourless oil.
IR (film): 3063, 2969, 2924, 1596, 1449, 1404, 1316, 1301, 1289,
1149, 1086, 1018, 848, 815, 766, 707, 696 cm–1.
Acknowledgment
We thank EPSRC/GlaxoSmithKline (supported DTA studentship to
D.M.M.) and Boehringer Ingelheim (postdoctoral secondment of
J.T.K. to Imperial College) for support of this research.
1H NMR (270 MHz, CDCl3): d = 7.78 (2 H, d, J = 8.5 Hz, o-Ph),
7.34 (2 H, d, J = 8.5 Hz, m-Ph), 6.23 (1 H, dt, J = 6.5, 1.5 Hz, H-5),
5.82 (1 H, dd, J = 6.5, 2.5 Hz, H-4), 5.48 (1 H, qdd, J = 6.5, 2.0, 1.0
Hz, C=CHCH3), 4.12–4.03 (1 H, m, H-3), 3.26 (1 H, dd, J = 14.0,
10.0 Hz, CHHTs), 3.14 (1 H, dd, J = 14.0, 3.5 Hz, CHHTs), 2.42 (3
H, s, CH3Ph), 1.60 (3 H, dd, J = 6.5, 2.0 Hz, C=CHCH3).
References
13C NMR (67.5 MHz, CDCl3): d = 145.1, 140.9, 136.6, 130.1,
(1) Claisen, L. Ber. Dtsch. Chem. Ges. 1912, 45, 3157.
(2) (a) Wick, A. E.; Felix, D.; Steen, K.; Eschenmoser, A. Helv.
Chim. Acta 1964, 47, 2425. (b) Johnson, W. S.;
128.0, 124.8, 124.3, 117.4, 63.4, 46.8, 21.8, 17.0.
MS (CI): m/z = 298 [M + NH4]+, 150, 124, 61.
Werthemann, L.; Bartlett, W. R.; Brocksom, T. J.; Li, T.-T.;
Faulkner, D. J.; Petersen, M. R. J. Am. Chem. Soc. 1970, 92,
741. (c) Ireland, R. E.; Mueller, R. H. J. Am. Chem. Soc.
1972, 94, 5897.
HRMS: m/z calcd for C14H20NO2S2 [M + NH4], 298.0935; found,
298.0930.
(3) (a) Bourgeois, D.; Craig, D.; King, N. P.; Mountford, D. M.
Angew. Chem. Int. Ed. 2005, 44, 618. See also: (b) Craig,
D.; Grellepois, F. Org. Lett. 2005, 7, 463. (c) Craig, D.;
Grellepois, F.; White, A. J. P. J. Org. Chem. 2005, 70, 6827.
(d) Bourgeois, D.; Craig, D.; Grellepois, F.; Mountford, D.
M.; Stewart, A. J. W. Tetrahedron 2005, 61, in press.
(4) Thomas, A. F.; Ozainne, M. J. Chem. Soc. C 1970, 220.
(5) Raucher, S.; Lui, A. S.-T.; Macdonald, J. E. J. Org. Chem.
1979, 44, 1885.
28
Rf 0.33 (EtOAc–petroleum ether, 2:3); colourless oil.
IR (film): 3062, 2957, 2925, 1598, 1493, 1461, 1359, 1318, 1267,
1170, 1140, 1088, 1045, 1020, 971, 814, 753, 737, 659 cm–1.
1H NMR (270 MHz, CDCl3): d = 7.83 (2 H, d, J = 8.0 Hz, o-Ph),
7.67 (1 H, d, J = 8.0 Hz, H-4), 7.47 (2 H, d, J = 8.0 Hz, o-Ph), 7.37
(2 H, d, J = 8.0 Hz, m-Ph), 7.34–7.01 (3 H, m, H-5, H-6, H-7), 7.14
(2 H, d, J = 8.0 Hz, m-Ph), 5.59–5.49 (2 H, m, C=CH2), 4.98 (1 H,
dd, J = 8.0, 2.0 Hz, CHHTs), 4.01 (1 H, dd, J = 14.5, 2.0 Hz,
CHHTs), 3.67 (1 H, dd, J = 14.5, 8.0 Hz, H-2), 2.46 (3 H, s,
CH2OSO2PhCH3), 2.32 (3 H, s, NOSO2PhCH3).
(6) Kazmaier, U.; Schneider, C. Synthesis 1998, 1321.
(7) Larock, R. C.; Berrios-Peña, N. G.; Fried, C. A. J. Org.
Chem. 1991, 56, 2615.
13C NMR (67.5 MHz, CDCl3): d = 145.0, 144.7, 142.8, 137.4,
133.0, 130.2, 130.0, 129.9, 129.2, 128.3, 127.5, 125.0, 121.1, 116.5,
106.9, 62.7, 60.3, 21.8, 21.7.
(8) Zenner, J. M.; Larock, R. C. J. Org. Chem. 1999, 64, 7312.
Synthesis 2005, No. 19, 3279–3282 © Thieme Stuttgart · New York