Aldoheptoses derivatives

Emil Fischer s initial studies [1] focused on dithioacetal formation from a variety of simple aldopentoses, aldohexoses, and aldoheptoses. The products were obtained by simply treating the sugar with an excess of ethanethiol in concentrated hydrochloric acid at low temperature ( 0°C). Many of these dithioacetals have low water solubility, crystallize spontaneously from the reaction mixtures, and can be isolated by simple filtration and washing with odd water. Fischer s key paper [1] also outlined most of the major reaction modes of the diethyl dithioacetal derivatives, later developed in detail, and thus paved the way for subsequent studies into this important class of sugar derivative. 

N. K. Khare, R. K. Sood, and G. O. Aspinall, Diastereoselectivity in the synthesis of D-g/ycero-D-aldoheptoses by 2-(trimethylsilyl)thiazole homologation from hexodialdo-1,5-pyranose derivatives, Can. J. Chem. 72 237 (1994). 

Only two 1,7-anhydrohexopyranoses have been described thus far l,7-anhydro-D-g/ycero-/J-D-gw/0-heptopyranose (58a)247,250 and the corresponding D-ido isomer (58b).247 They appear in the reaction mixture with 1,6-anhydro derivatives after acid treatment of solutions of the parent aldoheptoses. 

This work was followed by the detection of an aldoheptose in Escherichia coli products and the subsequent isolation of jj-glycero-i>-manno-heptoee by Weidel, again in connection with phage-receptor studies. The sugar was obtained from the lipocarbohydrate fraction of Escherichia coli B cell-membranes and was isolated, after acid hydrolysis, on a cellulose-powder column. In this case, sufficient of the product was obtained to show that the diethyl dithioacetal and its hexaacetate had equal but opposite optical rotations, as compared with those of the same derivatives prepared from authentic D-glyc o-n-manno-heptose. 

Such cyclization methods found general utility as in the case of the preparation of nucleoside analogue 10 from an aldoheptose 8 and septanose 9, reported by Gos-selin and coworkers (Scheme 13.3a) [13]. In this instance, 5-deoxy septanoside was obtained. On the contrary, a 2-deoxy septanoside derivative was obtained by oxidation of alcohol 11, followed by oxidation to aldehyde and acid-catalyzed cyclization to afford 12 (Scheme 13.3b) [14]. 

Whereas cyclization of aldohexose and aldoheptose proceeded septanoside formation, implementing aldol-type condensation was also devised to derive septanoside systems. An early report by Baschang illustrated that a dialdehyde, such as 17, obtained by periodate oxidation of a pyranoside, upon Knoevenagel condensation afforded 3-deoxy-3-nitroseptanoside 18 (Scheme 13.5a) [16], On the contrary, such a reaction on dialdehyde 19 with ethyl nitroacetate afforded hij-nitro-septanoside derivatives 20 and 21 (Scheme 13.5b) [17]. 

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