Sn II Halide-Catalyzed Reactions

Cyclopentanoids are prepared by stereocontrolled synthesis with a 1.2-dicarbonyl component and 1,2-dianion synthon derived from the treatment of 3-halo-2-[(trimethylsilyl)-methyljpropenes (X = Br, I) with SnF2 in THF at 25 °C (Eq. 1) [12]. Several 1 -dike-tones (R, R = Me, Et, Ph, Pr, C1(CH2)4, -(CH2 4- 0 °C for R = R = Ph) afford the cis-diols. 

Snp2 seems unique for this annulation. After generation of the allylstannane, the Sn serves as a Lewis acid catalyst and provides internal chelation. The fluoride counter ion is an ideal nucleophile for the activation of the allylsilane. The high stereoselectivity (25 1 to 75 1) is attributed to chelation (Eq. 2) [12]. 

Cyclohexanediols are prepared in approximately 20-60 % yields (diastereomeric ratios from 4 1 to 50 1) by a [3 + 3] annulation using a,/3-epoxy aldehydes (R = alkyl, H) as the dielectrophilic partner (Eq. 3) [13]. The high stereoselectivity and the lack of appreciable [3 -h 2] annulation are attributed to internal chelation and internal frans-diaxial epoxide opening (Eq. 4) [13]. 

Aldehydes are efficiently converted to /3-diketo esters in 50-90 % yield by addition of ethyl diazoacetate in the presence of SnCl2 (Eq. 5). Although the reaction can be effected by a variety of Lewis acids, SnCl2, BF3, and GeCl2 are the most effective [14]. 1,3-Diketones can be prepared in 42-90 % yield by SnCl2-catalyzed reaction of a-diazo ketones with aldehydes (Eq. 6) [15]. 

Silyl enol ethers react with bromomethyl methyl ether in the presence of a catalytic amount of SnBrg to yield a-bromomethyl ketones (Eq. 9) [18]. Other tin halides such as SnFg and SnClg can be used successfully in the reaction. 

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