Chelation Control in the Allylation of a- and 5-Alkoxy Aldehydes

Use of y-alkoxy-substituted allylic stannane reagents with a-benzyloxy aldehydes provides convenient access to 1,2,3-triol subunits (Equation 8) [92]. Keck reported the chelation-controlled formation of 113 as a single dia-stereomer and suggested that this product is the result of the intermediacy of transition state structure 112, analogous to 100. 

Mikami has documented an example of chelation control in an aldehyde addition reaction in which 116 is isolated as the major diastereomer (dr=97 3, Equation 9) [93]. The relative configuration of the product has been suggested to result from the intermediacy of a transition state analogous to 101 because of considerable steric demands of the 2-methyl substituent in silane 115. 

An X-ray structure by Reetz showed the geometry around the ether oxygen to be almost planar [5] 

Yamamoto has reported the use of an intramolecular diastereoselective allylation reaction in a synthesis of hemibrevetoxin (Equation 10) [94]. Treatment of 124 with BF, effects diastereoselective C-C bond formation to afford the key oxepane ring in 125 in 94% yield. 

9 Diastereoselective Aliyiations with Chiral Silanes and Stannanes 

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