Strecker/allylic alkylation reaction

More recently, Liao and coworkers developed an interesting tandem organocatalytic Strecker-allylic alkylation (S AA) reaction for the construction of highly functionalized a-aminonitriles 68 bearing an A-substituted quaternary carbon center. The scope and generality of this novel tandem SAA reaction are represented in Scheme 10.22 [61],... 

These a-aminonitriles were converted into a-methylene-y-butyrolactams 69 separately through cyclization of adducts 68 in the presence of a catalytic amount of DBU. However, a more complex application of this strategy was illustrated through a Strecker-allylic alkylation-cyclization (SAAC) reaction in a one-pot procedure to afford a-methylene-y-butyrolactam 69a as depicted in Scheme 10.23. 

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. 

Chiral amines have been transformed into chiral imines RCH=NG, which are usually in equilibrium with the tautomeric enamines. These enamines undergo asymmetric alkylations, and the best results are often obtained with ethers 1.58 or with valine derivatives 1.59 (R = i-Pr, R = tert-Bu) [169, 173,253] in the presence of bases. Enamines, lithioenamines and zinc enamines derived from imines are very potent Michael donors that often participate in highly stereoselective reactions [161, 162, 169, 173, 254, 257, 260, 262, 267], Chiral imines can suffer very selective addition reactions of organomagnesium reagents [139, 253, 254] and allyl-metals [154, 258]. They also suffer stereoselective Ti-catalyzed silylcyanation [268], Strecker reaction [266], and [2+2] or [4+2] cydoadditions [131, 256, 263], When the reaction produces an imine product, the chiral auxiliary is recovered after acidic hydrolysis. However, when an amine is obtained as the product, as is often the case from phenethylamine derivatives, the chiral residue is cleaved by hy-drogenolysis. In such cases, the chiral amine is not, strictly speaking, a chiral auxiliary. But these processes will be discussed anyway because of their importance in asymmetric synthesis. 

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