Structures of Carbohydrate Boronates

Chemical procedures have to be used with the greatest care, and 

Glycerol ethylboronate produced by the borinate method39 has the five-membered, cyclic structure, consistent with its being a kinetically controlled product, whereas that obtained from 3-deoxy-DL-glycero-tetritol contains a six-membered ring. 

From these findings with triols, it follows that, apart from the expectation that formation of five- and six-membered rings would be favored (see, however, the exceptional compound 19), no general conclusions can be drawn regarding the structures of boronates derived from more-complex polyhydric alcohols. In Table V, alditol boronates are listed with structures when these can be concluded either from the method of synthesis, from physical studies, or by deduction (as with the 1,2 5,6-diesters formed from 3,4-di-O-substi-tuted mannitols). 

In 1956, a ribose diphenylboronate (m.p. 140-142°) was prepared in modest yield by a lusion method,7 but it was later shown26 that an almost quantitative yield of a mono-ester was obtainable by conducting the condensation in hot 2-methoxyethanol, and this was a-D-ribose 2,4-phenylboronate (22), as indicated by p.m.r. and nB-n.m.r. analysis,45 and conversion by selective substitution into the 3-p-toluenesulf-onate.48 It is here suggested that the 2,4-ester (22) might be very sus- 

Condensation of D-ribose with two molar equivalents of phenyl-boronic acid in 2-methoxyethanol gave a diester to which was assigned the l,5 2,3-/3-furanose structure (25) on the basis of as-yet-unreported n.m.r. data.26 

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