| Abstract: | Substituted 2-deoxy-2-acylamino-4-O-methanesulfonyl-hexopyranosiduronates yield, by mild alkaline mesylate (a, e)-β-elimination, the corresponding 4,5-unsaturated 4-deoxyhexopyranosiduronates VIII. This type of aminosugars proved to be the cyclic enol-ether acetal form of the 2,4-dideoxy-2-acylamino-hexos-5-ulosuronates. The structural principle of the latter can be found e.g. in neuraminic acid. These 4,5-unsaturated 4-deoxy-acylamino-hexopyranosiduronates give by reduction with NaBH4 the corresponding 4,5-unsaturated 2,4-dideoxy-2-acylamino-hexopyranosides IX (with an endocyclic double bond on the glycosidic C atom 5). The isomeric 5,6-unsaturated 2,6-dideoxy-2-acylamino-hexopyranosides XVI (with an exocyclic double bond) are furthermore synthesized according to the method of HELFERICH 14] by elimination of a molecule of HI from the corresponding 2,6-dideoxy-2-acylamino-6-iodo-4-O-acylhexopyranoside derivatives XV. The ring stability of the two types of isomeric unsaturated hexopyranosides mentioned (bearing respectively an exo- and an endocyclic 5-enol-ether linkage) has been examined. In accordance with the stability principle of BROWN 16] – on the base of our preliminary experimental indications – the hexopyranosides with endocyclic double bond have been shown to be more stable than those with an exocyclic double bond: the latter (1) decompose slowly at 20°; (2) the α-glycosidic linkage is very easily split by dil. acetic acid at 20° within a few hours, giving 2, G-dideoxy-2-acyl-amino-D -xylo-hcxofuranos-5-ulose derivatives XX. On the other hand, the hcxopyranosides with endocyclic double bond show in the mass spectrometer, besides other fragmentations, a retro-dien decomposition. Some data on the NMR. spectra (100 and 220 MHz) of the above isomeric unsaturated acylamino-hexopyranosides (and hexopyranosiduronates, resp.) are furnished. The ORD./CD. spectra of the 4,5-unsaturated 2,4-dideoxy-2-acylamino-hexopyranosiduronates, which have two «COTTON centres», have been measured. |
