Additions of Achiral and Racemic Oxygenated Allylic Stannanes to Aldehydes

9 Additions of Achiral and Racemic Oxygenated Allylic Stannanes to Aldehydes 

Both a- and y-oxygenated allylic stannanes add to aldehydes under thermal or Lewis-acid-promoted conditions. These reagents are less reactive and more acid-labile than their non-oxygenated counterparts. Consequently, the best results are obtained with relatively reactive aldehydes. Strong Lewis acids cannot be used because they tend to cause decomposition of the stannanes. Initial studies employed thermal conditions to effect the additions. Thus, the frans-a-OMOM crotylstannane, prepared from crotonaldehyde by addition of BuaSnLi and etherification of the alcohol adduct, afforded the anti-(Z) adduct upon treatment with benzaldehyde under reflux in toluene (Eq. 28) [46]. 

A series of aldehydes was examined with this stannane (Table 30). These reactions were conducted neat with a twofold excess of stannane at 100-140 °C. The anti, (Z) adducts were always formed exclusively via a proposed chair transition state in which the OMOM substituent adopts an axial orientation to avoid interactions with the adjacent stannane substituents, as previously seen with alkylstannanes (Eq. 9). Unbranched aliphatic aldehydes gave the adducts in low yield because of a competing aldol self-conden-sation reaction. Pivaldehyde also proved unsatisfactory. 

Hydrostannation of methoxyallene affords a mixture of cis- and trans-y-methoxy allylic stannanes, among other products [48]. These stannanes can be separated by chromatography. Treatment with aldehydes in the presence of BF3 OEta affords the s yn-l,2-diol methyl ethers as major products (Table 31). Formation of the syn adduct from both the cis and trans allylic stannanes is consistent with an acyclic transition state (see Fig. 1). 

It was subsequently shown that free-radical hydrostannation of methoxyallene affords at least five products [49], Subsequent chromatography on silica gel enables elimination of those isomers with adjacent MeO and SnBu3 substituents leaving only the y-methoxy allylic stannanes in ca 25% yield for each. A more efficient preparation of these stannanes employs Pd(0)-catalyzed hydrostannation (Table 32). 

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