[tubez]

Hi, I'm looking for the mechanism mentioned in the title. More specifically, I'm interested in the borohydride intermediates formed in the reaction, because I need to know how to determine stereochemistry for similar reactions (i.e. determining the major product in Kiliani-Fischer Synthesis)
Thanks!

[billnotgatez]

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[tubez]

Understood.

Kiliani-Fischer synthesis occurs from the addition of cyanide to the aldehyde group on a sugar, generally known as the anomeric carbon. I know that it attacks the carbonyl while the sugar is in a chair-like conformation, and the CN group has two modes of attack; the top or the bottom side, depending on certain conditions. I was unsure if these conditions are dictated by the anomeric effect or if they were mainly dictated by steric hindrance, as I cannot find any information on the actual mechanism of the reaction. My class notes seem to imply that this mechanism is selective much like sodium borohydride's reduction of aldoses.

[orgopete]

If you were to react racemic glyceraldehyde with cyanide, would you get equal amounts of the four stereoisomers?

[tubez]

For D-glyceraldehyde, I would expect the cyanide to attack the carbonyl opposite of the C2 OH due to electrostatic repulsion of the hydroxyl group, yielding the S configuration at the new stereocenter. I'm unsure about what happens when the carbohydrate is configured in a ring structure, however. Looking at glucose, where all hydroxyls are axial, there is no obvious steric or electrostatic reasoning for selectivity, as observed here.

[Dan]

The reaction proceeds via the open chain aldehyde form of the aldose and is largely controlled by the configuration at C2.

Have you heard of, for example, the Felkin-Ahn model?