Stereochemistry allenylsilanes

Allenylsilanes undergo intramolecular additions to appropriately positioned aldehydes, imines, conjugated esters and alkenes to afford various alkynylcyclopentane and cyclohexane derivatives (Eqs. 9.66-9.70) [66]. The reactions are promoted by SnCl4 or by thermolysis. The stereochemistry of these cyclization reactions is consistent with a concerted sigmatropic process as illustrated in Scheme 9.17. 

Irradiation of the enantiomerically enriched allenenone 42 afforded alkylididecy-clobutane 43 with high levels of chirality transfer. The silyl moiety of optically active allenylsilanes 44 and 47 functioned as a removable auxiliary to control the stereochemistry. Thus, the silyl-substituted photoadducts 45 and 48 underwent protode-silylation on treatment with TBAF to give the unsubstituted exo-mcth ylenccyclo-butanes 46 and 49, respectively [46]. 

Allenylsilanes 159 (Eq. 13.53) and 161 (Eq. 13.54) differ in the axial stereochemistry of the allene function. In each case, formation of the benzyl imine, followed either by treatment with tin(IV) chloride in benzene at room temperature or heating in toluene, leads to diastereomeric products 160 and 162 [63], Significantly, there is no crossover, pointing to a concerted (or fast, stepwise) process. Since the absolute stereochemistry of the allenylsilanes is easily controlled, the methodology is ideal for applications in total synthesis. Weinreb and co-workers have used the reaction for his synthesis of the marine natural product (-)-papuamine. 

A variety of aldehydes can function as heteroallenophiles in this [3 + 2] annelation (Figure 13 and Table 9). Reactions of the C(3)-substituted allenylsilanes (66) gave predominantly the ds-substituted dihydrofurans (71). The cis stereochemistry was anticipated, based on the well-documented stereochemical course of Lewis acid catalyzed additions of 3-substituted allylsilanes to aldehydes. ... 

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