Diastereoselectivity homoaldol reaction

Diastereoselective homoaldol reaction. The allylaluminum reagent (E)-2, obtained by deprotonation of (E)-l followed by transmetallation, undergoes a highly diastereoselective reaction with an aldehyde (or a ketone) to give E)-anti-3. In contrast, (Z)-2 reacts to give (E)-JV -3. ... 

Titanated allyl carbamates are reported to react regio- and stereospecifically with aldehydes [52]. An elegant and synthetically useful method based on diastereoselective and en-antioselective homoaldol reactions has been developed (Scheme 13.25) [53]. 

Scheme 13.25. Diastereoselective and enantioselective homoaldol reactions using titanated allyl carbamates.

A similar result is obtained in the homoaldol reaction of enantiomeric TBS-lactaldehydes with the titanated ( )-2-alkenyl carbamate 745 [213]. Reaction of 689 with 745 furnishes 747 with 96 4 diastereoselectivity. Likewise, the ccorresponding reaction of 741 with 745 provides 748 with 70 30 diastereoselectivity. As in the previous case, the newly formed... 

To expand of the scope of electrophiles in the lithiation-substitution reaction, aldehydes were employed and a highly diastereoselective homoaldol methodology was developed [104]. After generation of lithiated 141 under the standard conditions, transmetallation with EtjAlCl or TiCl(Oi-Pr)3, and substitution with various aldehydes provided the homoaldol products with both high diastereo-and enantioselectivities (Scheme 48). 

The reaction of a linear aliphatic-substituted homoaldol illustrates an interesting feature of this kinetic resolution (Scheme 14). As an additional acetal stereocenter is created in the transacetalization reaction, the high catalyst control of its formation results in ahighly diastereoselective reaction [26, 35, 36]. Thus, even in cases where the enantiodifferentiation of the starting material is not very pronounced, the less reactive enantiomer is converted into the minor frans-diastereomer (Scheme 14). 

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