Threo compounds thermodynamics

After the formation of the carbocation, two competing processes occur in the moderated systems racemization and Cl+ transfer. In the case of the meso compound, front attack leads to the thermodynamically stable erythro form, whereas in the dl system, the less stable threo diaste-reomer is formed, which readily leads to racemization. 

A synthesis of methyl a-D-sibirosaminide (23) has been reported (Scheme 3), which starts from a 3-C-methylaltropyranoside derivative, and Dyong and Schulte have described the synthesis of a closely related compound (24) from the 2-deoxy-3-ulose (25), which required introduction of the 2-hydroxy-group by means of a [2,3]-sigmatropic rearrangement (Scheme 4). The addition of HCN to the threo-analogue of (25) in the presence of pyridine afforded the kinetic cyanohydrin (axial attack) in 74% yield, whereas in the presence of sodium bicarbonate the thermodynamic product with the opposite configuration was obtained. The two products were isolated as their mesylates, which were... 

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