J. Org. Chem., 62 (12), 3938 -3943 jo961542v S0022-3263(96)01542-3

Copyright © 1997 by the American Chemical Society

Diastereoselective Grignard Additions to O-Protected Polyhydroxylated Ketones: A Reaction Controlled by Groundstate Conformation?

Johann Mulzer* and Catarina Pietschmann

Institut für Organische Chemie der Universität Wien, Waehringer Strasse 38, A-1090 Wien, Austria

Jürgen Buschmann and Peter Luger

Institut für Kristallographie der Freien Universität Berlin, Takustrasse 3, D-14195 Berlin, Germany

Received: August 7, 1996

Abstract:

The O-protected polyhydroxy ketones 9-14 and 39, 42 add -type Grignard reagents with >90:10 stereoselectivity to give the 3,4-syn-adducts 17-22 and 43, 45, respectively, as the major diastereomers (Tables 1 and 2). The stereoselectivity is interpreted in terms of early transition states which are very close to the groundstate conformations shown in Figure 6 and 7. These demonstrate that the "top face" of the carbonyl group is much less shielded than the "bottom" face. Complexation phenomena are of minor importance. It is also shown that the classical transition state models (Felkin-Anh or chelate Cram) are not applicable to polyoxygenated ketones.