Pentopyranoside

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. 

Scheme 7.—Proposed Mechanism for Type I Reaction oftert-Butyl 3,4-O-Isopropyli-dene-a-L-en/thro-pentopyranosid-2-ulose (10).

H- and F-N.m.r. Data for 3-Deoxy-3-lluoro- and 4-Deoxy-4-fluoro-pentopyranoses and -pentopyranosides... 

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

A pentopyranoside-fused butenolide is the key intermediate for the synthesis of the natural micotoxin patulin [226, 227]. Its synthesis involves Wittig olefination of a 3,4-di-O-protected arabinopyran-2-uloside, followed by protecting group removal and dehydration (Scheme 47). In other research, the glucopyranosid-2-uloside 190 was converted into the butenolide derivative 191 by aldol condensation with diethyl malonate and transesterification [228]. The latter was shown to be prone to autoxi-dation, leading to 192. Subsequent Michael addition with hydroxide ion, followed by decarboxylation, furnishes C-branched-chain sugar 193. 

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. 

---