Chemistry Letters 2000
39
The cleavage of the auxiliary is quantitative, very easy,
occurs in a completely exocyclic way and does not require a
specific treatment. The oxazolone 3, separated during the
oxidative workup, is recycled.
References and Notes
l
a) D. J. Ager, I. Prakash, and D. R. Schaad, Aldrichimica
Acta, 30 (1), 3 (1997), b) A. Sudo, and K. Saigo,
Tetrahedron: Asym., 7, 2939 (1996), c) D. A. Evans, A.
M. Ratz, B. E. Huff, and G. S. Sheppard, Tetrahedron
Lett., 35, 7171 (1994), d) D. A. Evans, J. R. Gage, J. L.
Leighton, and A. S. Kim, J. Org. Chem., 57, 1961 (1992).
A. Cipiciani, F. Fringuelli, V. Mancini, O. Piermatti, F.
Pizzo, and R. Ruzziconi, J. Org. Chem., 62, 3744 (1997).
A. Cipiciani, F. Fringuelli, V. Mancini, O. Piermatti, A.
M. Scappini, and R. Ruzziconi, Tetrahedron, 53, 11853
(1997).
The enantioselectivity of the reaction can be explained by
considering that the the double bond of boron enolates 66 reacts
preferentially with its si-face on the re-face of benzaldehyde
(Scheme 3) via the closed transition state B as proposed by
Evans et al.7 The addition of the re-face of enolate on the si-
face of benzaldehyde is obstructed by steric interaction of the
phenyl ring with the C7-C8 ethylene bridge of the [2.2]paracy-
clophane moiety and the transition state A is less preferred than
B. The strong steric interactions of the phenyl ring with the
ethylene bridge of the [2.2]paracyclophane moiety and Bu-Ph
1,3-diaxial interactions are an obstacle to the formation of anti
aldols.
The (+)-(R)-[2.2]paracyclophano[4,5-d]-oxazol-2(3H)-one
(3) is a promising chiral auxiliary for many asymmetric reac-
tions via the corresponding chiral imide enolate. Both enan-
tiomers of 3 are available and the removal of the auxiliary is
easy and quantitative. Studies on synthetic applications are in
progress.
2
3
4
5
The configuration of 5 was proven by comparison with
authentic specimens.5
a) F. Fringuelli, O. Piermatti, and F. Pizzo, J. Org. Chem.,
60, 7006 (1995), b) F. Fringuelli, O. Piermatti, and F.
Pizzo, Synthesis, 1207 (1996).
6
7
The boron enolate 6 (R1 = Me) has been assumed to have
Z-configuration because this is the stereochemistry of
boron enolates of 5-substituted acyloxazolidinones.1 The
C7-C8 ethylene bridge of [2.2]paracyclophane moiety
favours this configuration.
a) D. A. Evans, Aldrichimica Acta, 15, 318 (1982), b) K.
Hayashi, Y. Hamada, and T. Shioiri, Tetrahedron Lett., 32,
7287 (1991), c) K. Iseki, S. Oishi, and Y. Kobayashi,
Tetrahedron, 52, 71 (1996).
The Ministero dell'Università e della Ricerca Scientifica e
Tecnologica (MURST), the Consiglio Nazionale delle Ricerche
(CNR) and the Università degli Studi di Perugia are thanked
for financial support.