Notes and references
1 For reviews, see: (a) J. G. Smith, Synthesis, 1984, 629; (b) X. E. Hu,
Tetrahedron, 2004, 60, 2701.
2 (a) K. Surendra, N. S. Krishnaveni, Y. V. D. Nageswar and K. R. Rao,
J. Org. Chem., 2003, 68, 4994; for catalytic asymmetric versions, see: (b)
J. M. Ready and E. N. Jacobsen, J. Am. Chem. Soc., 1999, 121, 6086; (c)
S. Matsunaga, J. Das, J. Roels, E. M. Vogl, N. Yamamoto, T. Iida,
K. Yamaguchi and M. Shibasaki, J. Am. Chem. Soc., 2000, 122, 2252.
3 P. A. Procopiou, A. C. Brodie, M. J. Deal, D. F. Hayman and
G. M. Smith, J. Chem. Soc., Perkin Trans 1, 1996, 2249.
4 H.-J. Altenbach, in Comprehensive Organic Synthesis, ed. B. M. Trost
and I. Fleming, Pergamon Press, New York, 1991, vol. 6, ch. 4.5,
pp. 847.
Fig. 2 Plausible mechanism for the observed syn-stereoselectivity.
5 K. Fagnou and M. Lautens, Org. Lett., 2000, 2, 2319 and references
cited therein. Using this procedure phenol gives lower yields and
catechol proves to be ineffective.
6 B. M. Trost and A. Tenaglia, Tetrahedron Lett., 1988, 29, 2931.
7 For the use of a palladium- and boron-cocatalyzed asymmetric reaction
of vinyl epoxides with alcohols, see: B. M. Trost, E. J. McEachern and
F. D. Toste, J. Am. Chem. Soc., 1998, 120, 12702.
8 W. Gerrard, M. F. Lappert and B. A. Mountfield, J. Chem. Soc., 1959,
1529. In general, the preparation of a triarylborate can be simply
accomplished by acid–base reaction of BH3 with a phenol followed by
evaporation of the volatiles under a vacuum.
9 Typical Procedure as follows: aryl borate 1a (1.2 mmol) or 1b (2.0 mmol)
was added at rt to a solution of 1.0 mmol of epoxide or aziridine in the
appropriate solvent (1.0 mL) (see Table 1) with stirring. The reaction
was followed by TLC and was quenched with 5% aqueous HCl (2.0 mL)
or 5% aqueous NaOH after the times indicated in Table 1. The solution
was diluted with Et2O or CH2Cl2 (30 mL) and washed with brine
(3.0 ml) for reactions carried out with borate 1b and with 5% NaOH for
reactions performed with 1a. Evaporation of the dried organic solution
afforded a crude reaction mixture which was subjected to flash
chromatography to give the pure compounds 2–12.
10 However, DMF was found to be an unsuitable solvent for most
epoxides because it gave extensive formation of the corresponding trans
diols, possibly due to borate decomposition.
11 B. Rickborn, in Comprehensive Organic Synthesis, ed. B. M. Trost,
Pergamon Press, New York, 1991, vol. 3, ch 3.3, pp 733 and references
cited therein.
the new trans phenoxy(butoxy) N-Cbz amine 10 with
moderate yields (entry 9). On the other hand, a completely syn-
stereo- and regioselective ring opening occurred when the
N-tosyl aziridine derived from indene was allowed to react with
borate 1a (entry 10), and similar results were obtained with
borate 1b (see ESI{). A good syn-stereoselectivity was also
observed when the Cbz-aziridine of b-methyl styrene was
employed, although in this case the regioselectivity in the benzylic
position was not complete (entry 11). Unfortunately, all
N-protected vinyl aziridines examined gave a low yield of mixtures
of products.
The fact that an adjacent unsaturation is mandatory to obtain a
high level of syn-stereoselectivity is a clear indication that the
reaction is under electronic control. Probably in the initial step the
coordination of the boron to the heteroatom X (X 5 O, N-PG) is
followed by a lengthening of the C–X bond, with an advanced
formation of a carbocationic species, as depicted in Fig. 2. The
internal delivery of the phenoxide anion,16 initially tethered to the
boron atom, occurs preferentially in an entropically favored
intramolecular way from the same side of X. Accordingly,
whenever the development of a stabilized carbocation is not
possible, as for examples with simple aliphatic aziridines and
epoxides, a complete anti-stereoselective ring-opening reaction is
observed.
12 Mioskowski and co-workers reported the total synthesis of (2)-mur-
icatacin, using as the key step a regio- and anti-stereoselective ring-
opening of a substituted vinyl epoxide with 3,4-dimethoxybenzyl alcohol
under the catalysis of BF3?Et2O, see: C. Baylon, G. Prestat, M.-P. Heck
and C. Mioskowski, Tetrahedron Lett., 2000, 41, 3833; for the use of
BF3?Et2O with simple epoxides in combination with alcohols, see also
A. Brandes, U. Eggert and H. M. R. Hoffmann, Synlett, 1994, 745.
13 For an example, see: M. Massacret, P. Lhoste, R. Lakhmiri, T. Parella
and D. Sinou, Eur. J. Org. Chem., 1999, 2665 and pertinent references
therein.
We gratefully acknowledge funding by Ministero dell’
Istruzione, dell’Universita` e della Ricerca (M.I.U.R. Rome,
P.R.I.N. 2002 and P.R.I.N. 2004) and by the University of
Pisa.
14 K. Kato, K. Furuta and H. Yamamoto, Synlett, 1992, 565.
15 B. A. B. Prasad, R. Sanghi and V. K. Singh, Tetrahedron Lett., 2002, 58,
7355.
Mauro Pineschi,* Ferruccio Bertolini, Robert M. Haak, Paolo Crotti
and Franco Macchia
Dipartimento di Chimica Bioorganica e Biofarmacia, Universita` di Pisa,
Via Bonanno 33, 56126, Pisa, Italy. E-mail: pineschi@farm.unipi.it;
Fax: +390502219660
16 The little or no reactivity of trialkyl borates, with vinyl and aryl epoxides
respectively, might be ascribed to the fact that the B–OAr bond is
reasonably more polarized than the corresponding B–OR bond, due to
the greater stabilization of a phenoxide anion with respect to an alkoxide
anion.
1428 | Chem. Commun., 2005, 1426–1428
This journal is ß The Royal Society of Chemistry 2005