Polysubstituted homoallylic alcohols, synthesis

Recently, a new multicomponent condensation strategy for the stereocontrolled synthesis of polysubstituted tetrahydropyran derivatives was re-published by the Marko group, employing an ene reaction combined with an intramolecular Sakurai cyclization (IMSC) (Scheme 1.14) [14]. The initial step is an Et2AlCl-promoted ene reaction between allylsilane 1-50 and an aldehyde to afford the (Z)-homoallylic alcohol 1-51, with good control of the geometry of the double bond. Subsequent Lewis acid-media ted condensation of 1-51 with another equivalent of an aldehyde provided the polysubstituted exo-methylene tetrahydropyran 1-53 stereoselectively and... 

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. 

The allylation and cyanation of aldehydes and ketones are mediated by BiCl3 and BiBr3 [174, 175]. When a chiral bismuth(lll) catalyst is used for cyanation, cyanohydrins are obtained in up to 72% ee (Scheme 14.85) [175]. The Bi(OTf)3-promoted intramolecular Sakurai cyclization of homoallylic alcohols is involved as a key step in the stereoselective synthesis of polysubstituted tetrahydropyrans (Scheme 14.86) [176]. In the presence of the BiCl3-xMl binary catalyst, allyltrimethylsilane [177] and silyl enolates [178] are acylated to give aUyl ketones and /l-dikelories, respectively. 

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