Pyranosid-4-ulose

Stoicheiometric RuOyCCl was also used to oxidise several furanoses, partially acylated glycosides and l,4 3,6-dianhydrohexitols [317] pyranosides to pyrano-siduloses [313] methyl 2,3,6-tri-O-benzoyl-a-D-glucopyranoside and its C-4 epimer to the a-D-xy/o-hexapyranosid-4-ulose (Table 2.3) [317], and methyl 2,3,6-trideoxy-a-D-e 7f/tro-hexapyranoside to the -a-D-,g/yceri9-hexa-pyranosid-4-ulose, an intermediate in the synthesis of forosamine [318], It was also used to oxidise benzyl 6-deoxy-2,3-0-isopropylidene-a-L-mannopyranoside to the a-L-/yxo-hexapyranosid-4-ulose [319] and for oxidation of isolated secondary alcohol functions, e.g. in the conversion of l,6-anhydro-2,3-0-isopropylidene-P-D-man-nopyranose to the-P-D-/yxo-hexa-pyranos-4-ulose mannopyranose (Fig. 2.16, Table 2.3 [20, 320, 324]). 

Irradiation of pyranosid-4 ulose derivatives such as 65 undergo decarbonylation to yield furanoside 66 by preferential C-3, C-4 cleavage [66]. This is confirmed by the separation of its epimer 67 containing a 2-3 transfused system derived from a diradical intermediate Scheme 35). 

Benzyl glycoside Benzyl a-L-ihreo-pento-pyranosid-4-ulose C12H14O5 238.24... 

Vinyl Magnesium Route. A direct route to dieno-p osides or VI) is shown in Figure 3b, introduction of the vinyl residue is effect by reacdon of vinyl magnesium bromide and a pyranoside-ulose VII (, follow by eliminadon of the tertiary hydroxyl group in VIII. This route could, in pnnciple, be common for both methyl, and desmethyl dienic systems.. ... 

By monitoring the intensity of the carbonyl absorption it was observed that oxidation of methyl 4,6-0-benzylidene-2-deoxy-a-D-Zt/ ro-hexopyrano-side with chromium trioxide-pyridine at room temperature gave initially the hexopyranosid-3-ulose (2) in low concentration, but attempts to increase this yield resulted in elimination of methanol to give compound 3. However, when methyl 4,6-0-benzylidene-2-deoxy-a-D-Zt/ ro-hexo-pyranoside is oxidized by ruthenium tetroxide in either carbon tetrachloride or methylene dichloride it affords compound 2 without concomitant elimination. When compound 2 was heated for 30 minutes in pyridine which was 0.1 M in either perchloric acid or hydrochloric acid it afforded compound 3, but in pyridine alone it was recoverable unchanged (2). Another example of this type of elimination, leading to the introduction of unsaturation into a glycopyranoid ring, was observed... 

The selectivity of the reduction of methyl a-DL-ald-2-enopyranosid-4-ulose 326 and, consequently, the low availability of the 2,3-unsatu-rated pyranosides of the a-threo (329) configuration, required the inversion of the configuration of C-4 (C-5 in the glyeulopyranosides) for completion of several syntheses. The benzoic acid-diethyl azodicar-boxylate-triphenylphosphine reagent was reported to effect the esterification specifically, with inversion of the configuration the yields were significantly higher than those obtained in the two-step, sulfonic-ester, displacement procedure.229... 

For such hex-3-enopyranosid-2-uloses as 127 or 129, reaction with lithium dimethylcuprate,261 or with anions of malonic ester-type, methylene components in the presence of bis(2,4-pentanedionate)-Ni(II) catalyst,263 affords, in each case, the 1,4-addition products (128 and 130, respectively), in which the branching group is in the axial orientation. The methyl-branched pyranosides 126 and 128, readily accessible in this way, have been of use as chiral precursors for the synthesis of multistria-tins.264-285... 

To avoid Norrish II photolysis of pyranosid-2-uloses derivatives containing aglycones derived from primary and secondary alcohols, t-butoxy derivatives 58 can be successfully photolyzed by a Norrish I reaction to yield, by C-l, C-2 cleavage and diradical formation, compounds 59 [30 %) and 60 [40 %) (Scheme 32) [62],... 

Baker and Buss425 used the Pfitzner-Moffatt procedure (phosphoric acid-A /V -dicyclohexylcarbodiimide dimethyl sulfoxide) for the preparation of methyl 3-benzamido-4,6-0-benzylidene-3-deoxy-Q -D-uraZ) o-hexopyranosid-2-ulose and methyl 2-benzamido-4,6-(3-benzylidene-2-deoxy-a-i>n/ >o-hexo-pyranosid-3-ulose in yields of 90%. 

Reaction of methyl 3-0-benzyl-4,6-0-benzylidene-/f-D-ura6mu-hexo-pyranosid-2-ulose (252a) with DAST in dichloromethane at room temperature affords the difluorinated compound 253 in 80% yield. However, the a anomer 252b did not react at room temperature, and by heating in benzene the migration product 254 was obtained as a mixture of anomers. 

Recently, the 4-methyl ethers of (90A) and (91A), methyl 4-0-methyl-d-D-ara ino-hexopyranosidulose and methyl 4-0-methyl- 8-D-ri6o-hexo-pyranosid-3-ulose, respectively, have also been prepared by using chromium trioxide/acetone oxidation. The latter, which was preponderant, was obtained crystalline. 

Methyl 2,3,6-trideoxy-a-D-hex-2-enopyranosid-4-ulose, F2, was readily prepared from triacetyl glucal FI [67], as summarized in Scheme 20. Reaction of this material with Danishefsky s diene [68] gave the annulated pyranoside F3 in 93% yield as a single diastereomer, in keeping with precedents. 

An improved synthesis of l,6-anhydro-2,3-di-0-benzyl-P-D-x7/o-hexopyranos-4-ulose involves 1,6-anhydride formation from methyl 2,3-di-O-benzyl-a-D-gluco-pyranoside and subsequent Swem oxidation. ... 

Ozonolysis of the model compound methyl 4-O-ethyl-p-D-glucopyranoside has led to authentic samples of methyl, 3,6-di-0-acetyl-4-0-ethyl-P-D-arafei/io-hexo-pyranosid-2-ulose, methyl 2,6-di-0-acetyl-4-0-ethyl- 3-D-rifco-hexopyranosid-3-ulose, and methyl 2,3-di-0-acetyl-4,0-ethyl-P-D-glucohexodialdo-l, 5-pyranoside. The isomerization of 2-oxo- and 3-oxo-glycosides in pyridine has been elucidated. Methyl a-D-aroZ>i/u>-hexopyranosid-2-ulose 4 undergoes an intramolecular hydride shift leading to the a-D-arai>OTo-hexopyranosid-3-ulose 5 which then converts to the thermodynamically more stable a-D-r/Z>o-isomer 7 via an enediol intermediate 6 which also forms directly from 4 to a lesser extent. The... 

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