Is that top boron reagent a mistake? Should it not be Et2B(OMe).
The top example involves external hydride delivery via an axial attack. This transition state goes straight into a Chair. Equatorial attack proceeds through the unfavourable twist boat. The conformation is locked by R2 being equatorial.Second example is of internal hydride delivery. A chair transition state gives the correct product when you lock R2 in a equatorial position.
Mitch Andre Garcia's Chemical Forums 2003-Present.
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