C O M M U N I C A T I O N S
Scheme 6. Carboalkoxylation of a Benzoate
Scheme 2. PtCl2-Catalyzed Indole Synthesis
Scheme 3. Heterocycle Synthesis by Oxidative Insertion9,10
intramolecular etherification,12 affording the polycyclic skeleton 11
of the pterocarpane family of phytoalexins.13 Furthermore, pre-
liminary experiments indicate that PtCl2-catalyzed carboalkoxylation
reactions are not limited to phenolic substrates. The successful
cyclization of the benzoic acid ester 12 to isochromene-1-one 13
illustrates this point (Scheme 6). In any case, it is important to
note that none of the acetalic substrates described in this paper is
amenable to heteroannulation using Pd(PPh3)49,10 because they are
unable to generate a catalytically competent species by oxidative
addition. Therefore, the PtCl2-catalyzed protocol is not only more
convenient but also significantly broader in scope and constitutes
a useful supplement to existing methodology for the preparation
of physiologically relevant heterocycles.14
Scheme 4. Proposed Reaction Mechanism
Acknowledgment. Financial support by the MPG and the Fonds
der Chemischen Industrie is gratefully acknowledged. We thank
Umicore, Hanau, for a gift of noble metal salts.
Supporting Information Available: Experimental part and spec-
troscopic data of all new compounds. This material is available free of
Table 3. PtCl2-Catalyzed Benzofuran Synthesis by Intramolecular
Carboalkoxylation (MOM/BOM/SEM Shift Reactions)
References
(1) (a) Fu¨rstner, A.; Szillat, H.; Gabor, B.; Mynott, R. J. Am. Chem. Soc.
1998, 120, 8305. (b) Mamane, V.; Gress, T.; Krause, H.; Fu¨rstner, A. J.
Am. Chem. Soc. 2004, 126, 8654. (c) Fu¨rstner, A.; Hannen, P. Chem.
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(2) (a) Fu¨rstner, A.; Szillat, H.; Stelzer, F. J. Am. Chem. Soc. 2000, 122,
6785. (b) Fu¨rstner, A.; Stelzer, F.; Szillat, H. J. Am. Chem. Soc. 2001,
123, 11863.
(3) For other carboalkoxylations catalyzed by PtCl2, see: Nakamura, I.;
Bajracharya, G. B.; Wu, H.; Oishi, K.; Mizushima, Y.; Gridnev, I. D.;
Yamamoto, Y. J. Am. Chem. Soc. 2004, 126, 15423 and references cited
therein.
(4) Reviews: (a) Aubert, C.; Buisine, O.; Malacria, M. Chem. ReV. 2002,
102, 813. (b) Me´ndez, M.; Echavarren, A. M. Eur. J. Org. Chem. 2002,
15. (c) Lloyd-Jones, G. C. Org. Biomol. Chem. 2003, 1, 215. (d) Me´ndez,
M.; Mamane, V.; Fu¨rstner, A. Chemtracts 2003, 16, 397. (e) Pioneering
study: Chatani, N.; Furukawa, N.; Sakurai, H.; Murai, S. Organometallics
1996, 15, 901.
(5) (a) Leading reference: Arcadi, A.; Cacchi, S.; Del Rosario, M.; Fabrizi,
G.; Marinelli, F. J. Org. Chem. 1996, 61, 9280 and references cited therein.
(b) Review: Larock, R. C. J. Organomet. Chem. 1999, 576, 111.
(6) (a) Fu¨rstner, A.; Davies, P. W.; Gress, T. J. Am. Chem. Soc. 2005, 127,
8244. (b) For another noble metal catalyzed rearrangement under CO
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Entry
R2
R3
t (h)
Yield
1
2
3
4
5
6
7
8
9
-C5H11
-CH2CH2Ph
Me
Me
Bn
2
91%
84%
78%
81%
84%
95%
62%
74%
84%
94%
0.5
0.5
0.5
2
0.5
2
2
0.5
2
CH2CH2SiMe3
cyclopropyl
-C6H5
m-MeO-C6H4-
m-F3C-C6H4-
Me
Me
Me
Me
Bn
Bn
10
o-(i-PrO)-C6H4-
Scheme 5. Preparation of the Pterocarpane Skeletona
(7) PtCl2 is by far the most effective catalyst among the metal salts screened;
cf. Supporting Information.
(8) (a) For related N f C acyl migrations, see: Shimada, T.; Nakamura, I.;
Yamamoto, Y. J. Am. Chem. Soc. 2004, 126, 10546. (b) See also: Arcadi,
A.; Bianchi, G.; Marinelli, F. Synthesis 2004, 610.
(9) (a) Cacchi, S.; Fabrizi, G.; Moro, L. Synlett 1998, 741. (b) Cacchi, S.;
Fabrizi, G.; Moro, L. Tetrahedron Lett. 1998, 39, 5101. (c) Battistuzzi,
G.; Cacchi, S.; Fabrizi, G. Eur. J. Org. Chem. 2002, 2671.
(10) (a) Monteiro, N.; Balme, G. Synlett 1998, 746. (b) Balme, G.; Bouyssi,
D.; Lomberget, T.; Monteiro, N. Synthesis 2003, 2115.
(11) Although we assume that the allyl fragment remains bound to the transition
metal template, further investigation of this point is ongoing.
(12) Kuwabe, S.; Torraca, K. E.; Buchwald, S. L. J. Am. Chem. Soc. 2001,
123, 12202.
(13) Representative examples: (a) Engler, T. A.; Reddy, J. P.; Combrink, K.
D.; Vander Velde, D. J. Org. Chem. 1990, 55, 1248. (b) Miki, Y.;
Kobayashi, S.; Ogawa, N.; Hachiken, H. Synlett 1994, 1001.
(14) After completion of this study, we became aware of closely related results.
Nakamura, I.; Mizushima, Y.; Yamamoto, Y. J. Am. Chem. Soc. 2005,
127, 15022.
a Conditions: (a) PtCl2 (5 mol %), CO (1 atm), toluene, 80 °C, 3 h,
75%; (b) (i) HF, pyridine; (ii) Pd(OAc)2 cat., Cs2CO3, (tBu)2P(2-biphenyl).
(Table 3). Analogous transfer of the BOM group (entries 3, 9, and
10) opens further options for subsequent manipulation of the
protected hydroxymethyl substituent at C-3 of the heterocycle, either
by oxidative (CAN) or hydrogenolytic cleavage of the benzyloxy
entity. Along the same lines, SEM-ethers are valuable substituents
susceptible to selective cleavage with fluoride after O f C transfer
(entry 4). The application in Scheme 5 illustrates this point. Since
the aryl bromide group of substrate 9 is compatible with the PtCl2-
catalyzed cyclization, the resulting product 10 allows for subsequent
JA055659P
9
J. AM. CHEM. SOC. VOL. 127, NO. 43, 2005 15025