COMMUNICATIONS
Diego, 1992, chap. 3; c) ™The Pharmacology and Therapeutic Aspects
of Colchicine∫: C. Le Hello in The Alkaloids, Vol. 53 (Ed.: G. A.
Cordell), Academic Press, San Diego, 2000, chap. 5.
Aromatic Boranes with Planar-Tetracoordinate
À
Boron Atoms and Very Short B B Distances**
[2] a) P. Dustin, Microtubules, 2nd ed., Springer, Berlin, 1984; b) S. Guha,
B. Bhattacharyya, Curr. Sci. 1997, 73, 351.
Carsten Pr‰sang, Matthias Hofmann,
Gertraud Geiseler, Werner Massa, and Armin Berndt*
[3] a) Q. Shi, K. Chen, A. Brossi, P. Verdier-Pinard, E. Hamel, A. T.
Mcphail, A. Tropsha, K.-H. Lee, J. Org. Chem. 1998, 63, 4018; b) Q.
Shi, K. Chen, S. L. Morris-Natschke, K.-H. Lee, Curr. Pharm. Des.
1998, 4, 219; c) S.-X. Zhang, J. Feng, S.-C. Kuo, A. Brossi, E. Hamel,
A. Tropsha, K.-H. Lee, J. Med. Chem. 2000, 43, 167.
Dedicated to Professor Anton Meller
on the occasion of his 70th birthday
[4] Syntheses of 1 and advanced intermediates: a) J. Schreiber, W.
Leimgruber, M. Pesaro, P. Schudel, A. Eschenmoser, Helv. Chim.
Acta 1961, 44, 540; b) E. E. van Tamelen, T. A. Spencer, Jr., D. S.
Allen, Jr., R. L. Ovis, Tetrahedron 1961, 14, 8; c) T. Nakamura, Chem.
Pharm. Bull. 1962, 10, 299; d) R. B. Woodward, Harvey Lect. 1963, 59,
31; e) A. I. Scott, F. McCapra, R. L. Buchanan, A. C. Day, D. W.
Young, Tetrahedron 1965, 21, 3605; f) J. Martel, E. Toromanoff, C.
Huynh, J. Org. Chem. 1965, 30, 1752; g) S.-i. Kaneko, M. Matsui,
Agric. Biol. Chem. 1968, 32, 995; h) E. Kotani, F. Miyazaki, S. Tobinga,
J. Chem. Soc. Chem. Commun. 1974, 300; i) D. A. Evans, S. P. Tanis,
D. J. Hart, J. Am. Chem. Soc. 1981, 103, 5813; j) D. L. Boger, C. E.
Brotherton, J. Am. Chem. Soc. 1986, 108, 6713; k) M. G. Banwell,
Pure Appl. Chem. 1996, 68, 539; l) J. C. Lee, J. K. Cha, Tetrahedron
2000, 10175.
Boranes with planar-tetracoordinate boron atoms have
[1 3]
until now only been studied by computations.
The
prototype of such compounds is anion 1 (Scheme 1), whose
planar-tetracoordinate boron atom forms a three-center, two-
electron (3c2e) s bond as well as a 3c2e p bond with the two
other boron atoms.[2] Thus 1 is one of the simplest double
[5] L. F. Tietze, U. Beifuss, Angew. Chem. 1993, 105, 137; Angew. Chem.
Int. Ed. Engl. 1993, 32, 131.
[6] S. L. Schreiber, Science 2000, 287, 1964.
[7] The arrows in structure formula of 2 denote strategic bonds: E. J.
Corey, X.-M. Cheng, The Logic of Chemical Synthesis, Wiley, New
York, 1989.
[8] Earlier attemps to employ such a cycloaddition strategy for the
synthesis of colchicine were not successful (as the further conversion
of the cycloaddition products failed): a) A. Lupi, M. Patamia, F.
Arcamone, Gazz. Chim. Ital. 1990, 120, 277; b) B. J. McBride, M. E.
Garst, Tetrahedron 1993, 49, 2839; see also: c) P. G. Sammes, Gazz.
Chim. Ital. 1986, 116, 109.
[9] a) C. Pl¸g, W. Friedrichsen, Tetrahedron Lett. 1992, 33, 7509; b) C.
Pl¸g, W. Friedrichsen, J. Chem. Soc. Perkin Trans. 1 1996, 1035; c) C.
Pl¸g, T. Debaerdemaeker, W. Friedrichsen, J. Prakt. Chem. 1997, 339,
205; see also: d) J. E. Baldwin, A. V. W. Mayweg, K. Neumann, G. J.
Prithard, Org. Lett. 1999, 1, 1933.
Scheme 1. Borane prototypes 1 3, 5u, and 6u with planar-tetracoordinate
boron atoms. Solid lines symbolize 2c2e bonds, triangles drawn with dashed
lines 3c2e s bonds, circles 3c2e p bonds, and ellipsoids 4c2e p bonds. In the
distorted tetrahedral 4 the four connecting lines between the boron atoms
are drawn to indicate the spatial relationships. In 4 there are ten electrons
available to connect the boron atoms. In contrast, in the isomer 3 as well as
in 5u and 6u there are only six electrons each.
[10] Compound 2 with X OH, Y H has been converted into colchicine
(1) under inversion of the configuration at C7; see ref. [4k].
À
[11] It can be anticipated that the benzylic C O bond in compounds of
type 3 can be selectively cleaved via a cationic intermediate stabilized
by both the adjacent double bond and the electron-rich arene group;
compare: P. Chiu, M. Lautens, Top. Curr. Chem. 1997, 190, 1; see also
ref. [9].
aromatics.[4] Upon protonation the classical B B bond in 1 is
À
[12] a) A. Padwa, G. E. Fryxell, L. Zhi, J. Am. Chem. Soc. 1990, 112, 3100;
b) A. Padwa, S. F. Hornbuckle, G. E. Fryxell, Z. J. Zhang, J. Org.
Chem. 1992, 57, 5747; c) A. Padwa, M. D. Weingarten, Chem. Rev.
1996, 96, 223; d) M. P. Doyle, M. A. McKervey, T. Ye, Modern
Catalytic Methods for Organic Synthesis with Diazo Compounds,
Wiley, New York, 1998.
[13] a) S. Nahm, S. M. Weinreb, Tetrahedron Lett. 1981, 22, 3815; for a
review, see: b) M. P. Sibi, Org. Prep. Proced. Int. 1993, 25, 15.
[14] E. A. Larson, R. A. Raphael, J. Chem. Soc. Perkin Trans. 1 1982, 521.
[15] L. Boymond, M. Rottl‰nder, G. Cahiez, P. Knochel, Angew. Chem.
1998, 110, 1801; Angew. Chem. Int. Ed. 1998, 37, 1701.
transformed into a 3c2e B-H-B bond in 2. However, the
double aromatic electronic structure of 1 is retained in 2. In
tetraborane(6) 3[5] the hydrogen bond in 2 is replaced by a
planar BH2 bridge.[6, 7] Known tetraboranes(6) are derivatives
of the distorted tetrahedral isomer 4, which according to
computations at the MP2/6-31G* level is 9.2 kcalmolÀ1 lower
in energy than 3.[5] Here we present 5a and 6a (Scheme 2), the
first derivatives of the prototypes 5u and 6u; both represent
[16] T. Ye, M. A. McKervey, Tetrahedron 1992, 48, 8007.
[*] Prof. Dr. A. Berndt, C. Pr‰sang, G. Geiseler, Prof. Dr. W. Massa
Fachbereich Chemie
[17] Products of this kind had been observed before in (related)
intermolecular reactions when unactivated dipolarophiles were em-
ployed[12a] but also in attempts to prepare ring systems with two fused
seven-membered rings by intramolecular cycloaddition.[12b]
[18] For crystal data of rac-17a see Supporting information. CCDC-177454
contains the supplementary crystallographic data for this paper. These
retrieving.html (or from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge CB21EZ, UK; fax: (44)1223-336-033;
or deposit@ccdc.cam.ac.uk).
Philipps-Universit‰t Marburg
Hans-Meerwein-Strasse, 35032 Marburg (Germany)
Fax : (49)6421-282-8917
Dr. M. Hofmann
Anorganisch-Chemisches Institut
Ruprecht-Karls-Universit‰t Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
1526
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