Pentopyranoside methyl 2-

Pentopyranoside, methyl 2-deoxy-3,4-0-isopropylidene-/S-D-ery[Pg.427]

Dideoxy- -L-ribo-pentopyranosid Methyl-3,4-0-isopropyliden-2-(-methyl)- E19a, 175 (2-OAc -> 2-H)... 

A number of pentopyranoside phenylboronates are known already through the work of Ferrier and Prasad (26). The mobility of methyl... 

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. 

Figure 1. Mass spectrum of methyl 3-deoxy-p-n-threo-pentopyranoside...

The most striking difference between the mass spectra of methyl 3-deoxy- and 4-deoxy-/ -D-f/ireo-pentopyranosides, 4 (Figure 1) and 5 (Figure 2), respectively, is the prominence of m/e 104 and 44 in the former and of m/e 60 [62] in the latter. The loss of C-2-C-3 from 5 leads to m/e 60 (see Equation 9), whereas its loss from 4 leads to m/e 44 (see Equation 7). The peak at m/e 104 [105] in Figure 2 shows that the elimination of C-3-C-4 also occurs from isomer 5. 

Methyl 4-deoxy-4,4-difluoro-2,3-O-i opropylidene-0-D-eryrAro-pentopyranoside... 

Hydroboration was also employed"0 in the synthesis of methyl 2,3-dideoxy-4-0-methyl-a-DL-g/ /C(TO-pentopyranoside (182) from 3,4-di-hydro-2-methoxy-2H-pyran (181). Catalytic dealcoholation of 182 afforded 3,4-dihydro-3-methoxy-2f/-pyran (183), which, on bromination in methanol, gave"0 methyl 2-broino-2,3-dideoxy-4-0-methyl-a-DL-en/t/iro-pentopyranoside (184) as the main product. 

The stereoselectivity in the Grignard reaction,97 complementary to that in the methyllithium109 and diazomethane156 reactions, of methyl 3,4-0-isopropylidene-a-L-en/thro-pentopyranosid-2-ulose (62), and that between the Grignard and diazomethane reactions of the / ano-mer107 of 62, methyl 2,3-0-isopropylidene-/ -L-en/thro-pentopyranosid-... 

Application of mass spectrometry to stereochemical analysis in the monosaccharide series is limited by the lack of experimental studies, by the absence of a generalizing, theoretical concept based upon modem conformational analysis, and by lack of precise knowledge of the mechanism and stereochemistry of the reactions taking place in the mass spectrometer. The only investigation in which a thorough study has been made of the effect of the stereochemical features of carbohydrates upon their mass spectra is that of Heyns and Scharmann, who measured the mass spectra of a number of permcthylated methyl pentopyranosides, namely, methyl tri-0-methyl-j8-D-ribo-(13), -arabino-(14), -xylo-(lS), and -lyxo-pyrano-sides (16), and of methyl tri-Omethyl-a-D-lyxopyranoside (17). 

A detailed study of the methyl glycosidation of the four aldopentoses has been made by Bishop and Cooper,16 42 with the aid of gas chromatography of the esters of the resulting pentofuranosides and pentopyranosides. Three competing reactions were established for the formation of the furanosides (1) an irreversible formation of furanosides from the free sugars (2) an anomerization of the furanosides and (3) conversion of the furanosides into the pyranosides. (A fourth reaction, anomerization of the pyranosides, will not be considered here.)... 

---