Aldehydes, a,p-dialkoxy

In the Held of chiral electrophiles, diastereoselective additions of enolsilanes to chiral a-fluoro-a-methyl-P-alkoxy aldehydes, a-methyl aldehydes, a-alkoxy aldehydes, a,p-dialkoxy aldehydes and a-methyl-P-alkoxy aldehydes were reported to proceed with good stereocontrol following Felkin-Anh or chelation models (c/. Section 2.4.4.1). Very good selectivities were reported in the addition of enolsilanes to chiral imines, particularly those derived from carbohydrates (Scheme 17 and 18).i. 6... 

Usefiil levels of stereoselectivity were obtained in intermolecular addition reactions of C(3)-sub-stituted allylsilanes to chiral aldehydes. Lewis acids that are citable of chelating to heteroatoms have been used to direct the stereochemical course of allylsilane additions to a-alkoxy and a,p-dialkoxy carbonyl compounds. The allylation of a-benzyloxy iddehyde (94) in the presence of TiG4 and SnOt furnished products with high levels of syn stereoselection (syn-9. In contrast, under nonchelation-controlled reaction conditions (BF3-OEt2) allyltrimethylsilane reacted to form predonunantly the anti-1,2-diol product (anti-95), as shown in Scheme 45. 

Additions to a,p-dialkoxy aldehydes (protected glyceraldehydes) are complicated by the possibility of chelation involving either the a or p alkoxy group. With 2,3-0-isopropylideneglyceraldehyde (62 equations 21 and 22) and Znh as catalyst, a preponderance of the p-chelated complex (63) was obtained with consequent formation of the C-3,C-4 anti compounds as major isomers (Table 13, entries 1-3 Table 14,... 

A new class of C -symmetric 3,3 -dialkoxy-2,2 -bipyrrolidines (e.g., 50) have been designed and developed by Zhang and his co-workers. The catalysts were used in asymmetric organocatalytic Diels-Alder reactions of a,P-unsaturated aldehydes 28 and dienes. Scheme 3.17 [30]. 

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