Z-crotyl boronate reacts with aldehydes to give syn alcohols via a chair transition state in which the aldehyde R group adopts a pseudoequatorial position to minimize steric repulsion. The short bonds to boron create a very tight transition state, which converts the two-dimensional stereochemistry of the reagent into the three-dimensional structure of the product. The low temperature is a testament to the reactivity of the crotyl boronates and also helps minimize any isomerization of the reagents while maximizing the effect of the energy difference between the favoured and disfavoured transition state.
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