O-Protected Lactaldehydes

Applying the general model to lactaldehydes, the alkoxy group is assumed to be the large group. Consequently, attack of the nucleophile from the less hindered side results in the predominant formation of an awrt -diol. 

If the organometallic reagent is capable of chelation, the second model becomes operative. This model, sometimes called Cram s cyclic model [147] involves the assistance of a che- 

---

The dibromoalkene S-40 can be prepared from S-ethyl lactate by introduction of the MEM (methoxyethoxymethyl) protecting group, reduction to the O-protected lactaldehyde and Corey-Fuchs carbonyl olefination (Scheme 19). The l -enantiomer of 40 is available analogously from f -isobutyl lactate and serves as the reagent in the enantiomeric series. The lithium carbenoid S-41 is generated from S-40 by treatment with n-butyllithium in diethyl ether and reacted with aliphatic and aromatic aldehydes in tetrahydrofuran. High diastereoselectivities are reached, as shown in Scheme 19 . ... 

Various racemic titanium-homoenolate reagents have been successfully applied to O-protected (R)- and (5)-lactaldehydes (X = (5 )-2-(iV,iV-dibenzylamino)al-... 

Aminoacid (o-threonine, L-aspartic) and hydroxyacid (L-tartaric, L-lactic) derivatives were repeatedly employed in the past as chiral substrates in the synthesis of daunosamine. More recently, protected L-lactaldehyde was used for the same purpose. Stereospecific synthesis of intermediate diol 95 was achieved with help of the enzyme (R)-hydroxynitrile lyase, which... 

---