Transmetalations of Chiral Oxygenated Allylic Stannanes

12 Transmetalations of Chiral Oxygenated Allylic Stannanes 10.12.1 SnCU 

These reactions proceed by initial Se2 addition of SnCLt to the allylic stannane to yield an intermediate trichlorostannane (Fig. 13). This trichlorostannane derives stability by coordination with the adjacent benzyloxy oxygen. Aldehyde addition proceeds through a cyclic transition state in which the allyhc substituent adopts an axial orientation to minimize interactions with the tin substituents and to maintain effective coordination with the adjacent benzyloxy oxygen. As a result the (Z)-homoallylic alcohol adduct is highly favored. 

Additions of the presumed /3-oxygenated allylic trichlorostannane to a-methyl, a-benzyloxy and /3-benzyloxy aldehydes are characterized by high reagent-controlled diastereoselectivity (Eq. 48) [70]. In the several examples examined aldehyde facial attack is little influenced by the resident chirality of the aldehyde. The result is particularly striking with the a-methyl aldehyde where the syn, syn adduct is the product of Felkin-Ahn addition and the anti, syn adduct is the anh-Felkin-Ahn or chelation-controlled adduct. 

Additions to a-oxygenated aldehydes are characterized by more characteristic matching and mismatching with the chlorostannane reagent. The mismatched (R) enantiomers afford roughly 2 1 mixtures of diastereomers favoring the syn, syn adducts (Eq. 49). Additions of the same chlorostannane to the matched (5) aldehydes are highly diastereoselective. Only the anti, syn adducts are produced in these reactions (Eq. 50). 

The presence of an oxygen substituent at the -position of the allylic tributylstan-nane enables stabilization of the intermediate trichlorostannane Se2 substitution product as a result of participation in a five-membered chelate [71], The ensuing addition reaction can then proceed by way of a bicyclic transition state, as before, in which the allylic substituent adopts an axial orientation (Fig. 14). The products of these additions contain a (Z) double bond with an anti relationship between the alcohol and the group. 

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