Diastereoselective Grignard Additions to O-Protected Polyhydroxylated Ketones: A Reaction Controlled by Groundstate Conformation?
摘要:
The O-protected polyhydroxy ketones 9-14 and 39, 42 add sigma-type Grignard reagents with >90:10 stereoselectivity to give the 3,4-syn-adducts 17-28 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.
An Efficient Synthesis of Enantiopure 1-Alkoxy-1,2-propadienes from Propargyl Bromide
作者:Patrick Rochet、Jean-Michel Vatèle、Jacques Goré
DOI:10.1055/s-1994-25577
日期:——
A number of enantiopure functionalized alcohols have been O-allenylated by reaction of their corresponding sodium alcoholates with propargyl bromide followed by t-BuOK catalyzed isomerization of the resulting propargyl ethers, in good overall yields.