Stereoselective reactions allylations, allyltrimethylsilane

The addition of an allyl metal to a-amino aldehydes has been used by Vara Prasad and Rich (Scheme 12)J23l After the addition step, the allyl group is oxidized to a carboxylic acid and lactonized. Then, the a-carbon of the lactone is alkylated stereoselectively. These investigators also systematically examined the addition reaction to determine its diastereo-selectivity. The highest diastereoselectivity was obtained when allyltrimethylsilane was used in the presence of tin(IV) chloride. An increase in the steric bulk of the protecting group and of the side chain also resulted in a better diastereoselection. Alternatively, Taddei and co-workers 24-26 used a 2-(halomethyl)allylsilane and the side chains were introduced by nucleophilic substitution of the halogen (Scheme 13). 

Aldehydes, ketones, and acetals react with allyltrimethylsilane in the presence of a catalytic amount of BiX3 (X = C1, Br, OTf) to give homoallyl alcohols or homoallyl alkyl ethers (Equation (52)).91-93 The BiX3-catalyzed allylation of aldehydes and sequential intramolecular etherification of the resulting homoallylic silyl ethers are involved in the stereoselective synthesis of polysubstituted tetrahydropyrans (Equation (53)).94,95 Similarly, these Lewis acids catalyze the cyanation of aldehydes and ketones with cyanotrimethylsilane. When a chiral bismuth(m) catalyst is used in the cyanation, cyanohydrines are obtained in up to 72% ee (Equation (54)). a-Aminonitriles are prepared directly from aldehydes, amines, and cyanotrimethysilane by the BiCl3-catalyzed Strecker-type reaction. 

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. 

Iodine transfer addition to allyltrimethylsilane provides a more environmentally friendly alternative to allyltributylstannane. In these allylations, which are exemplified in Scheme 6, the initially formed I-transfer product undergoes spontaneous loss of TMSI to generate the observed allylation product. Guindon has shown that allylation of the Lewis acid complexes of ) -alkoxy esters in this manner can lead to products with high anti stereoselectivity [22]. It is also believed that the presence of Lewis acids enhances the electrophilicity of the radical. Allylations of this type can also prove successful when Br-transfcr or PhSe-transfer reactions are employed. 

MBH adducts smoothly undergo allylic nucleophilic substitution (5n2 ) with allyltrimethylsilane in the presence of BF3 OEt2 under mild reaction conditions to afford 1,5-diene derivatives in good yields with high stereoselectivity... 

Stereoselective Allylation Reaction. The enantioselective allylation of aldehydes with allyltrimethylsilane is now possible with good control. One example is the use of a chiral acyloxy borane (CAB) catalyst. Besides that, chiral Ti(OiPr)2X2-BINOL and TiFa-BINOL catalysts are effective for the enantioselective allylation of glyoxylates and aldehydes, respectively (eq 35). Chiral homoallylamines can be prepared from the reaction of imines and allyltrimethylsilane using chiral tt-allylpalladium complexes with TBAF (eq 36) or an (5)-Tol-BINAP-CuPFe system. Free radical allyl transfers from allyltrimethylsilane provides another method for this enantioselective C-C bond formation. Promoted by chiral Lewis acids, both... 

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