Stereoselective addition of chiral titanium enolates to 5-substituted five-membered oxocarbenium ions

The stereochemical outcome of the addition of chiral titanium enolates to γ-lactols derived from (S)-glutamic acid were examined. When 5-substituted γ-lactols were treated with chiral titanium enolates of N-acetyl-, N-propionyl-, N-bromoacetyl- and N-phenylacetyl oxazolidine-2- ones, only two diastereoisomers were obtained, revealing complete facial control by the chiral titanium enolate while the oxocarbenium ion facial discrimination was dependent on the nature of the enolate R3 substituent. After desilylation with aqueous HF/CH3CN, LiBH4 reduction allowed the efficient preparation of the corresponding trans-2,5-disubstituted tetrahydrofuran diols and recovery of the chiral auxiliary. (author)