Hexofuranose derivatives, preparation

The mass spectra of methyl 3-deoxy-p-v-tkreo-pentopyrano-side, methyl 4-deoxy-j3-T>-thieo-pentopyranoside, and 5-deoxy-fi-D-xylo-furanoside are discussed and compared fragmentation paths are sufficiently different to allow identification on the basis of their mass spectra. On the other hand, the mass spectra of methyl 2- and 3-deoxy-5-O-methyl-f3-i>-erythro-pentofuranosides do not exhibit fragmentation differences. The mass spectra of 3-deoxy-l,2 5,6-di-O-isopropylidene -d-xylo - hexofuranose, 5- deoxy -1,2-0-isopropylidene-D-xy o-hexofuranose, and 6-deoxy-l,2-0-iso-propylidene-D-glucofuranose show prominent differences, even between the 5- and 6-deoxy isomers. The interpretation of the spectra was aided by metastable-ion peaks, mass spectra of DzO-exchanged analogs, and the mass spectrum of an O-isopropylidene derivative prepared with acetone-d6. 

This Subsection deals with the preparation of 2 - and 4 -ketohexosyl-purines and -pyrimidines, which have proved to be versatile synthetic intermediates. A 5 -keto derivative of a hexofuranose nucleoside is also described. The synthesis of epoxy-, halogeno-, unsaturated, epimino-, and thio-ketonucleosides will be developed in subsequent Subsections and Sections. 

An analogous methyl 2,3-dideoxy-3-C-methylene-a-L-en/thro-hexopyranoside (20) was obtained10 from methyl 4,6-O-benzylidene-2-deoxy-a-L-eri/t/iro-hexopyranosid-3-ulose (19) it was used in the synthesis of olivomycose (see p. 242). There was also reported32,34 the preparation of 3-deoxy-l,2 5,6-di-0-isopropylidene-3-C-methyl-ene-a-D-rifco-hexofuranose (22) from 1,2 5,6-di-0-isopropylidene-a-D-nfeo-hexofuranos-3-ulose (21) in about 60% yield, followed by transformations into branched-chain sugar derivatives, including... 

The formation of the pyridinol is prevented if, in the step 19 to 20, no anion can be eliminated from C-3 this is the case with 5-amino-3,5-dideoxy-l,2-0-isopropylidene-a-D-er /thro-pentofuranose, which, on acid hydrolysis, afFords only the Amadori rearrangement product and no pyridine derivative. The reaction then proceeds, according to the above mechanism, in only one direction from 19. The 3-deoxypentose is prepared, in a manner analogous to the formation of 15, from 3-deoxy-l,2-0-isopropylidene-a-D-riho-hexofuranose through catalytic reduction of the phenylhydrazone of its periodate-oxidation product. ... 

Reference to the use of 6-benzylthio-l,2-0-isopropylidene-3-0-methyl-a-D-x> /o-hexofuranose-5-ulose for preparation of other derivatives is made in Chapter 14. 

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