Synthesis glycals

The reactions involved in the glycal synthesis have been studied in considerable detail, especially in the conversion of arabinose into 2-des-oxyribose. For this particular conversion the overall yield has been doubled by recently introduced improvements but it is still very low.112 The reduction of the acetobromoaldose to the acetylated glycal by zinc dust and acetic acid was found to proceed in better yield if a few drops of chloroplatinic acid were added at intervals to maintain a vigorous reaction.112 120 127 Moreover, the reaction could then be conducted at lower temperatures (i. e., — 5° to —10°). This was particularly the case in the pentose series, and the simultaneous formation of the pentose tetraacetate by replacement of the bromo group by an acetyl residue, was much reduced.60 112 Hughes148 demonstrated that the maximum yield of triacetylglucal from acetobromoglucose was obtained when the addition of zinc dust and catalytic amounts of chloroplatinic acid was spread over several hours and the reaction mixture was maintained at 0°. 

The synthetic value of this reaction has been enhanced by the development of a wide range of methodologies for glycal synthesis from 0-acylatcd glycosyl halides1 or from noncarbohydrate precursors. By use of the Lewis acid-catalyzed diene-aldehyde cyclocondensation reaction, glycals of natural and non-natural sugars can be obtained (Scheme 4).31... 

Preparation (4, 4S). D-Ribose may be synthesized from D-arabinose by alkaline isomerization, by the glycal synthesis, or through the pyridine-catalyzed epimerization of D-arabonic acid followed by reduction. The sugar also has been prepared by the oxidative degradation of calcium D-altronate (44) and by the nitromethane synthesis from D-erythrose (4 ). 

Glycal Synthesis, This method depends upon the preparation of a glycal (which has a doul)le bond between carbons 1 and 2 of the reducing residue... 

By an ingenious application of stereochemical principles, the structure of turanose has been related to that of maltose, and a direct proof of its structure has been obtained 115). The proof involves the conversion of maltose to epimaltose by means of the glycal synthesis (see p.l27), and the reduction of the epimaltose to the same product (3- or 4-0-a-D-glucosyl-D-mannitol) as that obtained by the reduction of turanose. The symmetrj" of mannitol is such that substitutions at the 3- and 4-positions of mannitol are equivalent substitutions. An outline of the important steps in the synthesis are given in the formulas on the previous page. 

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