VI. Summary

Stereoselectivity is the preferential formation or consumption of one stereoisomer rather than another in a chemical reaction. In radical reactions stereoselectivity is con­trolled by a combination of conformational, steric, stereo­electronic, and tor­sional effects. The stereoselectivity caused by these effects is generally increased by conducting reactions at lower temperature. For radicals not centered on C-1, steric effects direct reaction to occur along the least-hindered pathway. A primary factor in deter­mining this pathway is the way in which various groups shield a radical center.

As the steric size of a molecule reacting with a carbohydrate radical increases, the extent to which the least-hindered pathway is followed also increases. As this size of reacting molecules becomes smaller, stereo­selec­tivity decreases but does not completely dis­appear; rather, a low level of selectivity remains due to torsional interactions.

Stereoelectronic effects operate in conjunction with conformational effects to determine ster­e­oselectivity in reactions of pyranos-1-yl radicals. The critical fac­tor in forming a particular ster­e­o­isomer in a reaction is the ability of the reac­tants to maintain in the transition state a stabilizing inter­action between orbitals on C-1 and the ring oxygen atom. Maintaining this inter­action causes different conformations of a radical to yield stereo­iso­merically different products. This stereo­electronic, con­for­mation-dependent, transition-state stabilization gives rise to a phenomenon known as the kinetic anomeric effect. This effect provides a basis for predicting and rationalizing stereoselectivity of pyranos-1-yl radical reactions.

Radical cyclization places additional requirements on reaction stereo­se­lectivity. Prominent among these is that in most situations a reaction proceeds through a chair-like transition state that has substituents located in pseudo­equatorial positions. In some instances a boat-like transition state is lower in energy than a chair-like one. This is often the case when structural features such as allylic strain or pseudo-1,3-diaxial interactions destabilize a chair-like transition state. The stere­o­selectivity of radical reactions provides the basis for several synthetic processes. These include the synthesis of β-glycosides, the use carbohydrates as chiral auxiliaries, and the incorporation of carbohydrates into enantioselective syntheses.