Diastereoselectivity allyl organometallics

The allene synthesis strategy outlined in equation 21 can be rendered asymmetric if non-racemic sulfoxides such as 47a are employed26. The diastereoselective step is an equilibration of the sp3-allylic organometallic species 53 before the -elimination step (equation 23). 

Vara Prasad and Rich [35] examined the diastereoselectivity of addition of allylic organometallics 19 to N,N-diprotected a-amino aldehydes 4 (Scheme 8). 

We have seen in Section 1.1.3 that reactions of many allyl organometallics and chiral C=X electrophiles proceed with only modest levels of relative diastereoselection. Significant improvement in dia-stereoselectivity is possible, however, by using double asymmetric synthesis, that is, by using the highly enantioselective allyl metal reagents described in Section 1.1.4 rather than the less diastereoface-selec-tive achiral allyl metal compounds discussed in Section 1.1.3. Double asymmetric synthesis is also... 

Table 19 Diastereoselectivity in the Addition of Allyl Organometallic Reagents (84) to 8-(-)-PhenylmenthylW-methoxyiminoacetate(83 Equation 19) ...

The addition of zinc organometallics to in situ generated oxenium ions by the reaction of mixed acetals with TMSOTf " allows a highly stereoselective preparation of protected a r/-l,3-diols of type 223 (Scheme 66). The addition of TMSOTf triggers also the allylic substitution of glycal derivatives, providing the substitution products of type 224 with excellent regio- and diastereoselectivity. 

Amino aldehydes provide useful starting materials for the sequence of Scheme 11.3 because they can easily be prepared from a-amino acids in enantiomerically pure forms and are, therefore, useful chiral building blocks.18 The addition of organometallic allylic reagents (including Zn, used in the reaction described) to such compounds can produce homoallylic alcohols 13 with a high anh-diastereoselectivity. 

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