2396
M. K. Reilly, S. D. Rychnovsky
LETTER
(9) Molander, G. A.; Sandrock, D. L. Curr. Opin. Drug
equiv), and DABO complex (1.0 mmol, 2.0 equiv). A PFT crimp-
on septum cap was then placed on the reaction vial, and the system
was evacuated on high vacuum and backfilled with argon twice.
THF (3.0 mL) and H2O (0.6 mmol) were added via syringe followed
by 3-bromothiophene (0.047 mL, 0.5 mmol, 1.0 equiv). The vial
was then placed in a pre-heated heating block at 60 °C. After 6 h,
the vial was removed from the heating block and allowed to cool to
r.t. The reaction mixture was filtered through a pad of Celite, rinsing
with CH2Cl2. The resulting orange/brown solution was concentrat-
ed by rotary evaporation to give a viscous liquid. Purification by
chromatography on silica gel using hexanes–EtOAc (80:1) as the
eluant provided the product as a mixture with tert-butyl 1H-pyrrole-
1-carboxylate, resulting from protodeboronation of the boronic acid
in solution. The impurity was removed by subjecting the clear, col-
orless liquid to high vacuum to provide tert-butyl 2-(thiophen-3-yl)-
1H-pyrrole-1-carboxylate (21) as a clear, colorless oil (81 mg, 0.32
mmol, 65% yield). The sample was subjected to GC-MS, and the
chromatogram contained one peak. Doubling in the 1H NMR spec-
trum was attributed to rotamers: 1H NMR (500 MHz, CDCl3): d =
7.34 (m, 1 H), 7.28–2.24 (m, 2 H), 7.10 (d, J = 4.0 Hz, 1 H), 6.22–
6.18 (m, 2 H), 1.43 (s, 9 H). 13C NMR (125 MHz, CDCl3): d = 149.4,
134.4, 129.9, 129.5, 124.1, 123.0, 122.5, 114.8, 110.6, 83.7, 28.0.
IR (thin film): 3151, 3109, 2981, 2935, 2873, 1747 cm–1. HRMS
(ES/MeOH): m/z calcd for C13H15NO2SNa [M + Na]+: 272.0721;
found: 272.0722.
Discovery Dev. 2009, 12, 811.
(10) Mancilla, T.; Contreras, R.; Wrackmeyer, B. J. Organomet.
Chem. 1986, 307, 1.
(11) Gillis, E. P.; Burke, M. D. J. Am. Chem. Soc. 2007, 129,
6716.
(12) Knapp, D. M.; Gillis, E. P.; Burke, M. D. J. Am. Chem. Soc.
2009, 131, 6961.
(13) Alessi, M.; Larkin, A. L.; Ogilvie, K. A.; Green, L. A.; Lai,
S.; Lopez, S.; Snieckus, V. J. Org. Chem. 2007, 72, 1588.
(14) Bouillon, A.; Lancelot, J.-C.; Santos, J. S.; Callot, V.; Bovy,
P. R.; Rault, S. Tetrahedron 2003, 59, 10043.
(15) Hodgson, P. B.; Salingue, F. H. Tetrahedron Lett. 2004, 45,
685.
(16) Gros, P.; Doudouh, A.; Fort, Y. Tetrahedron Lett. 2004, 45,
6239.
(17) Davies, A. G.; Roberts, B. P.; Ramsay, W. J. Chem. Soc. B
1967, 17.
(18) Letsinger, R. L.; Skoog, I. J. Am. Chem. Soc. 1955, 77, 2491.
(19) Crystallographic data: Rettig, S. J.; Trotter, J. Can. J. Chem.
1975, 53, 1393.
(20) MIDA is over 100 times more expensive. KHF2 is similarly
priced, but 3.0 equiv are required for the synthesis of
heterocyclic potassium trifluoroborates. Pinacol is about 10
times more expensive than DEA. Prices were obtained from
the Sigma-Aldrich online catalogue.
(21) 2-Methoxyphenylboronic acid and 2-furyl DABO boronate
7 were mixed at r.t. in THF for 3 h to give an equilibrium
mixture of both boronic acids and their DABO counterparts.
A similar product mixture was observed beginning with 2-
methoxyphenyl DABO boronate 4 and 2-furylboronic acid.
(22) Caron, S.; Hawkins, J. M. J. Org. Chem. 1998, 63, 2054.
(23) Bonin, H.; Leuma-Yona, R.; Marchiori, B.; Demonchaux,
P.; Gras, E. Tetrahedron Lett. 2011, 52, 1132.
Supporting Information for this article is available online at
Acknowledgment
Support was provided by the National Institute of General Medicine
(NIGMS GM-43854) and the University of California, Irvine, USA.
(24) Excess diethanolamine can be removed by triturating the
crude solid in EtOAc and filtering. DABO complexes could
not be characterized by HRMS, and unidentifiable boron-
containing masses were observed by electron ionization.
Elemental analysis was attempted, but the carbon percentage
was consistently below the theoretical value by about 10.
(25) No decomposition of 2-furyl DABO boronate 7 was
observed by NMR analysis after 72 d of storage in open vials
at r.t. The average temperature and humidity were 24 °C and
65%, respectively, during this period. In contrast, the
commercial 2-furylboronic acid completely decomposed in
less than one week under these conditions. We recommend
storing DABO boronates at r.t. in a closed vial.
(26) Inoue, M.; Frontier, A. J.; Danishefsky, S. J. Angew. Chem.
Int. Ed. 2000, 39, 761.
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Synlett 2011, No. 16, 2392–2396 © Thieme Stuttgart · New York