Furanose fragmentation

Figure 11.10 Comparison of extracted ion pyrograms of fragment ion m/z 217 of two samples collected from the paint surfaces of Universal Judgement and Stories of Holy Fathers (Monumental Cemetery of Pisa, Italy, painted by Buffalmacco, fourteenth century) with nitro cellulose, starch and arabic gum. 1, Unidentified compound 2, 1,2,3,5 tetrakis (O TMS) xylo furanose 3, tri (O TMS) levoglucosane 4, isomer of 1,2,3,5 tetrakis (O TMS) xylofuranose. Pyrogram obtained with a resistively heated filament at 60CPC in the presence of HMDS [30]...

A similar series of reactions was performed on a 6-azido-6-deoxy-2-ketose.246,252 In most of the foregoing syntheses, the piperidine formed is accompanied by the tautomeric furanose. The tautomers can, however, be separated without occurrence of equilibration (which can be induced by changing the pH of the medium). For the ketose derivative 183, the equilibrium mixture includes246 compound 184 and a dehydration product (182). The conformation and anomeric effect in these piperidines have been studied by n.m.r. spectroscopy,253 and their fragmentation pathways have been determined by mass spectrometry.254... 

Although the characteristic differences discussed above permit ready distinction between furanose and pyranose derivatives, their fragmentation patterns have much in common. The fragments of the A, C, and J series are formed in a similar manner from both of the two types of derivatives, although there is a difference in the contribution of some of the isomeric ions.1 87... 

The oxidation of 1,2-O-isopropylidene-D-glucofuranose with lead tetraacetate was found by Criegee to yield formaldehyde,43 which confirmed the presence of a furanose ring in the sugar derivative. The major oxidation fragment, l,2-0-isopropylidene-5-oZde/q/do-D-a yZo-pentodialdose, was characterized subsequently by Iwadare,88 and has recently been shown to crys-... 

The details on the preparation of the glycone synthon 16 have been described in the earlier report on 3 (24). The key reactions in this synthesis is the Suarez radical fragmentation of the 1,2-O-isopropylidene furanose 17 to give the 1-O-acetyl-1,2-isopropylidene 18 (35), and acetal exchange of 18 to the phenylthio monothioacetal 16 (Scheme 2). Overall, 16 is easily obtained on multi-gram scale (ca. 20 g), in 60% yield, over five steps from commercially available D-lyxose. 

Scheme 29 DIB/h-mediated oxidative fS-fragmentation of a furanose derivative followed by side chain introduction...

Similar principles have been used to explain the fragmentation patterns of monoisopropylidenealdo-furanoses and -pyranoses.24,67 71 O-Ethylidene acetals15 display cleavage patterns analogous to those of isopropylidene acetals, except that both [M — H ] and [M — CHj]+ are prevalent. 

Figure 3-4. Overlay of symmetry-independent molecules in 4 by keeping the furanose ring as the fixed fragment (Cerius ). The C3—C5 boronate ester overlays nicely (left side) but the Cl—C2 portion has different orientations (right side). The orthorhombic form is shown in black and the two symmetry-independent molecules of the monoclinic form are shown in grey...

Fig. 9.13. Polar diagram of the puckering phase 0 (°) vs. the puckering amplitude (A) of 181 non-fused furanose rings in fragments of type 11 found in the CSD release July 1991 (no disorder, no error flag, i <0.08). The numbers show the position of some selected ring conformations 1(0°) envelope (e) with mirror plane through C(2 ) endo 2(144°) e C(3 ) endo 3(324°) e C(3 ) exo 4(180°) e C(2 ) exo 5(126°) twist with C2 axis through C(T) 6(72°) e O(l ) endo...

Soon, it was pointed out by Fuchs et al. (89) that the equivalence of oxygen lone electron pairs is not consistent with X-ray structures of carbohydrates. They scrutinized 111 structures and found torsion angles O in Oendo—C—O—R fragments appreciably larger than 60°. If lone pairs were equivalent, one might expect 0 = 60° to be preferred whereas nonequivalence implies 0 > 60°. Indeed, Cosse-Barbi et al. (69,257) have recently shown that the endo anomeric effect is stronger in furanoses than in pyranoses. They... 

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