The O-protected polyhydroxy ketones 9-14 and 39, 42 add σ-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 ...
