Idose. configuration

Most substrates, with the exception of hydroxypymvate, have a threo configuration of hydroxyl groups at the C-3 and C-4 positions (139). The new stereocenter formed in TK-catalyzed addition is formed in the threo configuration with high diastereo-selectivity (151). Using TK-catalyzed condensations of hydroxypymvic acid with a number of aldehydes a practical preparative synthesis of L-idose [5934-56-5], L-gulose [6027-89-0], 2-deoxy-L-xylohexose, and... 

The above interesting approach to the asymmetric allyltitanation reaction does, however, have a limitation. Thus, L-glucose is much more expensive that the D-form and, consequently, homoallylic alcohols of the opposite configuration cannot easily be obtained by this method. In an attempt to induce the opposite si face selectivity, other acetonide derivatives of monosaccharides from the xylose, idose, and allose series were tested [42b,42c], The enantiofacial discrimination was, however, much lower than that with DAGOH and both re and si face selective additions to aldehydes were observed. 

Draw the structural formulae of both pyranose forms which would result from ring closure (hemiacetal formation) ofD-idose. What is the relationship between these structures and the corresponding pyranose forms derived from L-idose Check your answer by determining the configuration of the chirality centres according to the RIS nomenclature. 

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