Specific and chemoselective multi-α-arylation reaction of benzoylformic acid with or without decarbonylation in P2O5-MsOH and related acidic media

Article abstract of DOI:10.1021/jo990524l

In P₂O₅-MsOH, or related acidic media, benzoylformic acid (1) undergoes three types of di- or mono-α-arylation reactions with or without decarbonylation ((1) decarbonylative α,α-diarylation, yielding triarylmethanols 6, (2) decarbonylative α-monoarylation, giving benzophenone derivatives 7, and (3) α,α-diarylation without decarbonylation, affording diarylated carboxylic acids 5) and one simple decarbonylation, without arylation, to form benzoic acid (8), instead of the conventional Friedel- Crafts acylation type reaction. The product ratios are governed by the capability of the acidic medium to form mixed anhydrides with carboxylic acids and the ability of the arenes to accept electrophiles.

Full text of DOI:10.1021/jo990524l

J . Org. Chem. 2000, 65, 941-944
941
Sp ecific a n d Ch em oselective Mu lti-r-a r yla tion Rea ction of
Ben zoylfor m ic Acid w ith or w ith ou t Deca r bon yla tion in
P 2O5-MsOH a n d Rela ted Acid ic Med ia
Noriyuki Yonezawa,* Tetsuo Hino,† Kazuhisa Matsuda,‡ Toshiyuki Matsuki,‡
Daisuke Narushima, Masato Kobayashi, and Tomiki Ikeda†
Department of Material Systems Engineering, Tokyo University of Agriculture and Technology,
Koganei, Tokyo 184-8588, J apan, Research Laboratory of Resources Utilization,
Tokyo Institute of Technology, Nagatsuda, Midori-ku, Yokohama 226-8503, J apan, and
Department of Chemistry, Gunma University, Kiryu, Gunma 376-8515, J apan
Received March 24, 1999
In P2O5-MsOH, or related acidic media, benzoylformic acid (1) undergoes three types of di- or
mono-R-arylation reactions with or without decarbonylation ((1) decarbonylative R,R-diarylation,
yielding triarylmethanols 6, (2) decarbonylative R-monoarylation, giving benzophenone derivatives
7, and (3) R,R-diarylation without decarbonylation, affording diarylated carboxylic acids 5) and
one simple decarbonylation, without arylation, to form benzoic acid (8), instead of the conventional
Friedel-Crafts acylation type reaction. The product ratios are governed by the capability of the
acidic medium to form mixed anhydrides with carboxylic acids and the ability of the arenes to
accept electrophiles.
The Friedel-Crafts reaction is widely used for the
synthesis of aromatic compounds.1 Various mediators,
other than conventional Lewis acids, such as metal
trifluoromethanesulfonates for catalytic Friedel-Crafts
acylation reaction2,3 and heterogeneous mediators (inor-
ganic-solid-supported reagents or solid acids) for Friedel-
Crafts reaction2,4 have recently been reported. Electro-
philic aromatic substitution reactions in acidic solvents
such as TfOH (trifluoromethanesulfonic acid),5,6 PPA
(polyphosphoric acid), and P2O5-MsOH (phosphorus
pentoxide-methanesulfonic acid mixture)7 have also
been widely investigated.
P2O5-MsOH is an excellent direct condensing reagent
for solubilizing various organic compounds.8,9 It has
generally been explained that in P2O5-MsOH or PPA,
mixed carboxylic-phosphoric anhydrides should act as
acyl cation equivalents in an electrophilic aromatic
substitution reaction. We have recently communicated
that P2O5-MsOH mediated specific R,R-diarylation and
R-monoarylation reactions of pyruvic acid, with or with-
out decarbonylation, compete with Lewis acid mediated
simple ketone formation with pyruvoyl chloride.10 We
subsequently studied the reactions of other R-keto acids.
In contrast, when benzoylformic acid (1) was allowed to
react with arenes 2a -i in several acidic media, various
decarbonylated/nondecarbonylated products 5-10 were
obtained, with all of the arylations occurring at the R-keto
carbon. Neither of the conventional Friedel-Crafts acyl-
ation type products, R-diketones 3 or triarylmethyl aryl
ketones 4, were obtained (Scheme 1).
* To whom correspondence should be addressed. Tokyo University
of Agriculture and Technology. Tel: +81-42-388-7053. Fax: +81-42-
381-7979. E-mail: yonezawa@cc.tuat.ac.jp
† Tokyo Institute of Technology.
‡ Gunma University.
(1) Olah, G. A. Friedel-Crafts and Related Reaction; Wiley-Inter-
science: New York, 1964; Vol. 3. Olah, G. A.; Molna´r, AÄ . Hydrocarbon
Chemistry; Wiley-Interscience: New York, 1995; p 141.
(2) For a review, see: Yonezawa, N.; Hino, T.; Ikeda, T. Recent Res.
Dev. Synth. Org. Chem. 1998, 1, 213 and reference cited therein.
(3) For example: (a) Isumi, J .; Mukaiyama, T. Chem. Lett. 1996,
739. (b) Suzuki, K.; Kitagawa, H.; Mukaiyama, T. Bull. Chem. Soc.
J pn. 1993, 66, 3729. (c) Hachiya, I.; Moriwaki, M.; Kobayashi, S.
Tetrahedron Lett. 1995, 36, 409. (d) Mukaiyama, T.; Ohno, T.;
Nishimura, T.; Suda, S.; Kobayashi, S. Chem. Lett. 1991, 1059. (e)
Desmurs, J . R.; Labrouille`re, M.; Roux, C. Le; Gaspard, H.; Laporterie,
A.; Dubac, J . Tetrahedron Lett. 1997, 38, 8871.
(4) For example: (a) Kodomari, M.; Suzuki, Y.; Yoshida, K. Chem.
Commun. 1997, 1567. (b) Clark, J . H.; Kybett, A. P.; Macquarrie, D.
J . H.; Barlow, S. J .; Landon, P. J . Chem. Soc., Chem. Commun. 1985,
1353. (c) Barlow, S. J .; Clark, J . H.; Darby, M. R.; Kybett, A. P.; Landon,
P.; Martin, K. J . Chem. Res. Synop. 1991, 74. (d) Bastock, T. W.; Clark,
J . H.; Landon, P.; Martin, K. J . Chem. Res. Synop. 1994, 104. (e)
Chiche, B.; Finiels, A.; Gauthier, C.; Geneste, P. J . Org. Chem. 1986,
51, 212. (f) Wang, Q. L.; Ma, Y.; J i, X.; Yan, H.; Qiu, Q. J . Chem. Soc.,
Chem. Commun. 1995, 2307. (g) Ranu, B. C.; Ghosk, K.; J ana, U. J .
Org. Chem. 1996, 61, 9546. (h) Miller, J . M.; Wails, D.; Hartman, J .
S.; Schebesh, K.; Belelie, J . L. Can. J . Chem. 1998, 76, 382.
(5) TfOH is defined as a superacidic medium by Olah. See: (a) Olah,
G. A.; Prakash, G. K. S.; Sommer, J . Science 1979, 206, 13. (b) Olah,
G. A.; Prakash, G. K. S.; Sommer, J . Superacids; Wiley: New York,
1985. (c) Saito, S.; Ohwada, T.; Shudo, K. J . Am. Chem. Soc. 1995,
117, 11081. (d) Saito, S.; Ohwada, T.; Shudo, K. J . Org. Chem. 1996,
61, 8089. (e) Olah, G. A.; Rasul, G.; York, C.; Prakash, G. K. S. J . Am.
Chem. Soc. 1995, 117, 11211.
In this paper, we describe three types of specific
R-arylation and simple decarbonylation reactions of ben-
zoylformic acid (1) that show distinguishable chemose-
lectivity as compared to the expected Friedel-Crafts and
related reactions. Table 1 shows the results of the various
(7) P2O5-MsOH is a convenient alternative to PPA because of its
reduced viscosity and circumstantially better solvent power. See:
Eaton, P. E.; Carson, G. R.; Lee, J . T. J . Org. Chem. 1973, 38, 4071.
(8) (a) Ueda, M. Synlett 1992, 605. (b) Yonezawa, N.; Namie, T.;
Ikezaki, T.; Hino, T.; Nakamura, H.; Tokita, Y.; Katakai, R. React.
Funct. Polym. 1996, 30, 261. (c) So, Y. H.; Heeschen, J . P. J . Org. Chem.
1997, 62, 3552.
(9) We recently reported R,R-diarylation reaction of R-alkoxyalkanoic
acids in P2O5-MsOH: (a) Yonezawa, N.; Tokita, Y.; Hino, T.; Naka-
mura, H.; Katakai, R. J . Org. Chem. 1996, 61, 3551. (b) Yonezawa,
N.; Hino, T.; Tokita, Y.; Matsuda, K.; Ikeda, T. Tetrahedron 1997, 42,
14287. (c) Yonezawa, N.; Hino, T.; Ikeda, T. Recent Res. Dev. Synth.
Org. Chem. 1998, 1, 213. (d) Yonezawa, N.; Hino, T.; Shimizu, M.;
Matsuda, K.; Ikeda, T. J . Org. Chem. 1999, 64, 4179.
(6) Effengerger, F.; Epple, G. Angew. Chem., Int. Ed. Engl. 1972,
11, 299.
(10) Yonezawa, N.; Hino, T.; Kinuno, T.; Matsuki, T.; Ikeda, T.
Synth. Commun. 1999, 29, 1687.
10.1021/jo990524l CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/21/2000

942 J . Org. Chem., Vol. 65, No. 4, 2000
Yonezawa et al.
Sch em e 1
The reactions with anisole (2a ) were highly dependent
on the acidity of the media. The reactions in TFA, MsOH,
or TfOH at both room temperature and 60 °C produced
R,R-dianisylcarboxylic acid 5a (Table 1, runs 1-6) exclu-
sively. The reaction in PPA (room temperature, Table 1,
run 7) afforded acid 5a exclusively, but at 60 °C (Tabel
1, run 8) new components were formed (triarylmethanol
6a and phenone 7a ) with acid 5a . In P2O5-MsOH, (room
temperature, Table 1, runs 9 and 10) triarylmethanol 6a
was obtained exclusively.
In contrast, the product distribution with arenes 2b-i
in P2O5-MsOH, at various temperatures, was distinctly
different (Table 1, runs 11-25). The reactions with o- and
m-dimethoxybenzenes (2b,c) yielded triarylmethanols
6b,c (Table 1, runs 11-14). Phenylenebis(diarylmetha-
nol) 10 was also obtained in the reaction with arene 2c
(Table 1, runs 13-14). The treatment of p-dimethoxy-
benzene (2d ) with acid 1 (room temperature, Table 1, run
15) afforded triarylmethanol 6d , together with phenone
7d and lactone 9, but at 60 °C (Table 1, run 16)
triarylmethanol 6d was not formed. Although the reac-
tions with o-xylene (2e), p-xylene (2g), and toluene (2h )
(room temperature, Table 1, runs 17, 21, and 23) gave
phenones 7 together with triarylmethanols 6 and/or
benzoic acid (8), those at 60 °C (Table 1, runs 18, 22, 24)
yielded phenones 7 only. The reaction with m-xylene (2f)
exclusively afforded phenone 7f (Table 1, runs 19 and
20). The reaction with benzene (2i) or without arenes
gave solely acid 8 (Table 1, runs 25-26). The similar
reactions of acid chloride 1′ with anisole (2a ) yielded the
mixture of decarbonylated products (triarylmethanol 6a ,
benzophenone 7a , and/or benzoic acid (8)), even in TFA
or MsOH, and diarylated acid 5a was never obtained
(Table 1, runs 27-34).
The reactions of acid 1 with arenes in various acidic
media occur in four mechanistic categories (Scheme 1):
(1) R,R-diarylation without decarbonylation (acids 5 and
lactone 9), (2) decarbonylative R,R-diarylation (triaryl-
methanols 6 and phenylenebis(diarylmethanol) 10), (3)
decarbonylative R-monoarylation (substituted benzophe-
nones 7), and (4) decarbonylation without arylation
(benzoic acid (8)). The selectivity, whether or not decar-
bonylation proceeds, is anticipated to be governed by the
capacity of the acidic medium to form mixed anhydrides
(or equivalents) at the initial step.11,12
TFA, MsOH, and TfOH were too inert to attain a
significant concentration of the mixed anhydride with
acid 1, even at 60 °C. In those acidic media, only the R,R-
diarylations proceeded to give acids 5 (Table 1, runs
1-6).13 In PPA at room temperature, the simple R,R-
Ta ble 1. Rea ction s of Ben zoylfor m ic Acid (1) w ith Ar en e
(2) in Sever a l Acid ic Med ia a
isolated yield/%
rt, 24 h
60 °C, 2 h
ArH
(2)
acidic
medium
run
1, 2
3, 4
5, 6
7, 8
9, 10
11, 12 2b
13, 14 2c
15, 16 2d
17, 18 2e
19, 20 2f
21, 22 2g
23, 24 2h
5
6
7
8
5
6
7
8
2a
2a
2a
2a
2a
TFA
MsOH
TfOH
PPA
P2O5-MsOH
P2O5-MsOH
P2O5-MsOH
P2O5-MsOH
P2O5-MsOH
P2O5-MsOH
P2O5-MsOH
P2O5-MsOH
P2O5-MsOH
4
0
0
0
0
0
0
0
0
0
7
100
87
0
0
0
0
0
0
10
0
0
100
98
92
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
39 34
95
88
38b
0
0
0
0e
0
0
0
0
92
78
28c
0
0
0
0
0
0
0
0d 21
20
73 11
48
100
88
86
86
0
0
0
0
0
0
0
0
0
0
13
0
0
6
0
0
7
45
18
50
100
0
52 37
36 57
0
0
25
26
2i
0
0
90
99
none P2O5-MsOH
27,f 28f 2a
29,f 30f 2a
31,f 32f 2a
33,f 34f 2a
TFA
MsOH
PPA
P2O5-MsOH
13 53
20
33 26
44
0
0
0
0
5
36
40
45
14 55
(11) In comparison of runs 9/33 or 10/34 (Table 1), our interpretation
of the highly chemoselective reaction of acid 1 is that it should be
readily converted to a more labile intermediate (mixed anhydrides or
the equivalent) than should acid chloride 1′. In addition, the low
selectivity of the reaction of acid chloride 1′ is interpreted to indicate
the formation of the acyl cation and mixed anhydrides, which gives
the various decarbonylated products 6a -8a (Table 1, runs 27-34). On
the other hand, with regard to these results, one of the reviewers
proposed that the chloride ion is quite a strong nucleophile and would
be able to compete with the arene to trap the oxonium ion PhArCd
O+-[P] (which had very stable arenium-type resonance structures) to
afford phenone 7a after hydrolysis.
(12) In ref 9d, the P2O5-MsOH mediated specific decarbonylative
R,R-diarylation of 2-methoxypropanoic acid, giving 1,1-diarylethane
and 1,1-diarylethene, was concluded to proceed only after the formation
of carboxylic-phosphoric anhydride at the initial step.
(13) Because the resultant cation 13 from an R-monoarylated
intermediate is highly electrophilic, the second R-arylation is consid-
ered to proceed more readily. In contrast, the first R,R-arylation in
TFA is hardly even considered to proceed at 60 °C (Table 1, run 2).
0
4
45
54
60
0
0
0
a
Reaction conditions: for every 1 mmol portion of benzoylformic
acid (1), 2 mL of P2O5-MsOH and 2 mmol of arene were used.
b
Phenylenebis(diarylmethanol) 10 was also obtained in a 17%
yield. c Phenylenebis(diarylmethanol) 10 was also obtained in a
22% yield. Lactone 9 was also obtained in a 37% yield. Lactone
9 was also obtained in a 16% yield. f Phenylglyoxyl chloride (1′)
was used instead of benzoylformic acid (1).
d
e
reactions of acid 1 or phenylglyoxyl chloride (1′) with
arenes 2a -i in TFA (trifluoroacetic acid), MsOH, TfOH,
PPA, and P2O5-MsOH at room temperature for 24 h and
at 60 °C for 2 h.

Multi-R-arylation Reaction of Benzoylformic Acid
J . Org. Chem., Vol. 65, No. 4, 2000 943
Sch em e 2
diarylation progressed to exclusively yield acid 5, pre-
sumably because formation of mixed carboxylic-phos-
phoric anhydrides 11 would be very slow compared to
that at 60 °C (Table 1, run 7). P2O5-MsOH is assumed
to form mixed carboxylic-phosphoric (methanesulfonic)
anhydrides 11 spontaneously, even at room temperature.
It is thought that three kinds of decarbonylative
reactions of mixed anhydride 11 may proceed according
to the reactivity of each arene (Table 1, runs 9-25): (1)
One or more methoxy groups on the benzene ring may
give enough potential to promote decarbonylative R,R-
diarylation (Table 1, runs 9-16).14 (2) Considerably less-
activated arenes, such as xylenes (2e-g) and toluene
(2h ), may primarily undergo decarbonylative R-monoaryl-
ation (Table 1, runs 17-24). (3) The reactivity of benzene
(2i) may be too low to propagate R-arylation (Table 1,
run 25). The formation of acid 8 in the reactions with
arenes 2e,g-i shows the relatively weak electrophile
accepting ability of these arenes (Table 1, runs 17, 21,
23, and 25).
The decarbonylative R,R-diarylation triarylmethanols
6 are expected to be generated from acid 1 via decarbo-
nylation of acids 515,16 (Scheme 1, 1 f 5 f 6) or addition
of arenes to phenones 717 (1 f 7 f 6). However, neither
of them converted to triarylmethanols 6 in P2O5-MsOH
under the present conditions; the starting material was
recovered in each case. On the other hand, the decarbo-
nylative R-arylation phenones 7 are also anticipated to
be produced from acid 1 via direct condensation of
carboxylic acids and arenes (1 f 8 f 7).2 In P2O5-MsOH,
however, the yield of phenone 7h from the reaction of
acid 8 with toluene (2h ), under the same conditions (room
temperature, 24 h), was lower than that of acid 1 by 23%.
Accordingly, phenones 7 are considered to be mainly
formed via a distinctly different intermediate other than
acid 8 under such conditions.
On the basis of the above considerations, the plausible
reaction pathways for the decarbonylative reactions of
acid 1 are as follows (Scheme 2): (1) Mixed anhydride
11 forms from acid 1 and the phosphoric anhydride
moiety (O ) [P]) of P2O5-MsOH, or PPA, at the initial
step. (2) When the reactivity of the arenes (2) is higher
than that of the phosphoric anhydride moiety, the first
arylation to the R-carbon of the R-keto acids proceeds,
(14) One of the reviewers pointed out that the results showing that
arene 2c afforded only decarbonylative diarylation compounds 6c and
10, whereas arene 2d gave a mixture of two types of compounds, 6d
and 7d (Table 1, runs 13-16), indicated the lack of stabilization of
the arenium ion PhArCdO+-[P] derived from arene 2d . This is due
to the 1,4-substitution pattern and, consequently, its lack of selectivity
when reacting with the nucleophile in the last step (see Scheme 2).
On the other hand, we supposed that the lower reactivity of the arene
(2d ), than that of other dimethoxybenzene isomers, should give rise
to the lack of selectivity.
(15) Decarbonylation of R,R-trisubstituted carboxylic acids: (a)
Gillespie, R. J . J . Chem. Soc. 1950, 2997. (b) Burton, H.; Praill, P. F.
G. J . Chem. Soc. 1951, 726.
(16) Diarylation to the
R carbon of free R-keto acids in cold
concentrated sulfuric acid or in the presence of AlCl3: (a) March, J .
Advanced Organic Chemistry, Reaction, Mechanisms, and Structure,
3rd ed.; Wiley-Interscience: New York, 1985. (b) Wegmann, J .; Dahn,
H. Helv. Chim. Acta 1946, 29, 415.
(17) Addition of arenes to ketones or diketones in strong/superacid
media such as H2SO4 and TfOH: (a) Olah, G. A. Friedel-Crafts and
Related Reaction; Wiley-Interscience: New York, 1964, Vol. 2, pp 597-
640. (b)Yamazaki, T.; Saito, S.; Ohwada, T.; Shudo, K. Tetrahedron
Lett. 1995, 36, 5749.

944 J . Org. Chem., Vol. 65, No. 4, 2000
Yonezawa et al.
Typ ica l Rea ction P r oced u r e for Rea ction of Ben zoyl-
for m ic Acid (1) in P 2O5-MsOH: Rea ction of Acid 1 a n d
An isole (2a ). To an ice-cooled mixture of acid 1 (75 mg, 0.5
mmol) and anisole (2a , 108 mg, 1.0 mmol) was added P2O5-
MsOH (1 mL), under vigorous stirring. The mixture was
stirred at the prescribed temperature and duration and poured
into ice-water. The aqueous solution was extracted with
benzene (40 mL × 2).21 The combined organic layer was
washed with saturated aqueous NaCl solution, dried over
MgSO4 overnight, and concentrated under reduced pressure.
The mixture was chromatographed over silica gel column
chromatography (benzene/hexane 1/1).
giving R-monoarylated intermediate 12. (3) In accordance
with the electrophile accepting abilities of the arenes, the
replacement of the carboxy moiety on intermediate 12
by the arenes 2 (12 f 6′ f 6) and the phosphoryloxy
moiety (O ) [P]) (12 f 7′ f 7) takes place, resulting in
the evolution of carbon monoxide, thereby yielding the
decarbonylative di- and monoarylated products (6, 7),
respectively.18 Whether the arylation and the nucleophilic
attack of the phosphoryloxy moiety with decarbonylation
proceed in a concerted or a stepwise manner has not yet
been specified.19 (4) In the reaction of acid 1 with benzene
(2i), or without arenes, mixed anhydride 11 undertakes
nucleophilic attack of the phosphoryloxy moiety to yield
acid 8, instead of the R-arylation (1 f 11 f 8′ f 8). (5)
In the reactions with TFA, MsOH, and TfOH (or PPA at
room temperature), both the first and the second aryla-
tions proceed prior to the formation of mixed anhydrides
11 to yield acid 5 (1 f 13 f 5).
4,4′-Bis(m eth oxy)tr ityl a lcoh ol (6a ): IR (KBr) 3484,
2836, 1609, 1510, 830, 756, 702 cm-1; 1H NMR (CDCl3) δ 2.74
(1H, s), 3.80 (6H, s), 6.85 (4H, pseudo d of AA′BB′ pattern),
7.18 (4H, pseudo d of AA′BB′ pattern), 7.24-7.32 (5H, m); 13
C
NMR δ (CDCl3) 55.21, 81.49, 113.11, 127.15, 127.88, 127.90,
129.34, 139.52, 147.48, 158.65. Anal. Calcd for C21H20O3: C,
78.19; H, 6.36. Found: C, 77.87; H, 6.33.
The structures of compounds 6h ,22 7a ,23 7e-h ,23 and 823
were identical to that described in the literature. Compound
1′ was prepared according to the literature.24
Exp er im en ta l Section
Ack n ow led gm en t. The authors thank the Totoku
Toryo Co, Ltd. for financial support and also the
reviewers for their fruitful comments.
Gen er a l. The purification of reagents, when needed, was
performed according to the literature.20 NMR spectra were
recorded at 200 and 500 MHz (1H) and at 50 and 125 MHz
(13C) in CDCl3, using TMS as an internal reference. P2O5-
MsOH was prepared by stirring the 1:10 wt/wt mixture of P2O5
and MsOH at room temperature, according to the literature.7
Su p p or tin g In for m a tion Ava ila ble: The other synthesis
of methanol 6a , identification of the evolved carbon monoxide
gas, and spectral data for compounds 5a ,d , 6b-e, 9, and 10.
This material is available free of charge via the Internet at
htpp://pubs.acs.org.
(18) The predominant formation of phenones 7d -h in the reactions
with arenes 2d -h at 60 °C indicates that the relative nucleophilicity
of phosphoryloxy moiety (O-[P]) in the reaction system, after the first
arylation, is enhanced to a level able to overcome the second arylation
(Table 1, runs 16, 18, 20, 22, 24).
J O990524L
(19) Our consideration about the main reaction pathway is that a
stepwise manner, via cation intermediates formed by the elimination
of carbon monoxide, should somewhat implausible, because such
cations will be too inert to propagate electrophilic aromatic substitution
reactions. On the other hand, one of the reviewers insisted that the
stepwise manner was best one when arenium ions were involved.
(20) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: Oxford, 1988.
(21) For this extraction procedure, ether is not an effective solvent.
Aromatic solvents are more suitable. We have ascertained that toluene,
the solvent preferred over benzene, is also employable without causing
a large drop in efficiency.
(22) Hellwinkel, D.; Heidelberg, U. Chem. Ber. 1989, 2351.
(23) Pouchert, C. J .; Behnke, J . Aldrich Library of 13C and 1H FT-
NMR Spectra; Aldrich Chemical: Milwaukee, 1992.
(24) Ottenheijm, H. C. J .; de Man, J . H. M. Synthesis 1975, 163.