Glucopyranose 4,6-0-ethylidene

Methyl 2,6-anhydro-a-D-altropyranoside-2 5B g l,4-Anhydro-ot-D-allopyranose-Bi,4 9 1,2-0-Ethylidene-a-D-glucopyranose- S3 10 p-L-Altropyranose-2So... 

Imidazole or pyridine mediated silylation of l,2-0-[l-exo-ethoxy )ethylidene]-oc-D-glucopyranose failed to give a high yield of the 6-silyl ether due to some polymerization and side reactions. Activation of hydroxyl groups via a tributylstannyl intermediate followed by the tetrabutylammonium bromide catalyzed reaction with tert-butyl-chlorodiphenylsilane was more successful [231], the 6-0-silyl derivative being isolated in 87 % yield.. The lability of this protecting group under benzylation with benzyl bromide and sodium hydride at 0 °C has been observed [449]. 

Di-0-dichloroacetyl-l-0-benzyloxycarbonyl-(4,6-0-ethylidene)-p-D-glucopyranose Palladium on carbon 4 -Demethyl-epi-podophyllotoxin Zinc acetate dihydrate Boron trifluoride etherate Trimethylsilyl trifluoromethane sulfonate Celite/basic alumina column... 

Demethyl-epi-podophyllotoxin (1 mmol) and 2,3-di-0-dichloroacetyl-(4,6-0-ethyiidene)-p-D-giucopyranose (2 mmol) were introduced into dry dichloromethane under anhydrous condition. When the temperature was stabilized to -20°C to -30°C, boron trifluoride etherate (1.5 mmol) was added slowly with stirring. Reaction was continued at this temperature and monitored by thin layer chromatography. After the completion of the reaction as evidenced by TLC, the solution was washed with water, dried over anhydrous sodium sulfate and concentrated under reduced pressure to afford the crude intermediate product 4 -demethyl-epi-podophyllotoxin-4-(2,3-di-0-dichloroacetyl-4,6-0-ethylidene)-p-D-glucopyranose. This crude product was then converted to etoposide by following the procedure as above described. The yield of final product etoposide was about 60%. 

By reduction of the intermediate acetoxonium ion, Hodge and coworkers77 prepared (and separated) diastereoisomeric forms of per-acetates of 1,2-O-benzylidene-, 1,2-O-ethylidene-, and 1,2 4,6-di-O-ethylidene-a-D-glucopyranose. For the 1,2-O-ethylidene ring, the exo methine proton resonates at 8 4.87-4.83, and the endo at 8 5.47-5.24 (signals as quartets in benzene-d6). 

One of the earliest publications on gas-liquid chromatography of carbohydrate derivatives showed that l,2,3-tri-0-acetyl-4,6-0-ethylidene-a, S-D-glucopyranose and l,2 5,6-di-0-isopropylidene-a-D-glucofuranose are... 

D-Glucose, treated at room temperature with paraldehyde and sulfuric acid, gives 4,6-0-ethylidene-a-D-glucopyranose. An improved method of preparation has been recommended by Barker and MacDonald. A similar preparation of the l isomer has been reported. An uncharacterized isomer, m.p. 175°, has been isolated from the mother liquors of a preparation of the 4,6-0-ethylidene acetal this is, most probably, the 1,2-0-ethylidene acetal. Two di-O-ethylidene-D-glucoses of unknown constitution have been reported by Mellies and co workers. ... 

Cold acetic anhydride-pyridine has been shown to give the crystalline a-triacetate with 4,6-O-ethylidene-0 -D-glucopyranose whereas previous work has given non-crystalline mixtures of a- and /3-anomers or only /3-anomer with hot acetic anhydride-sodium acetate. However, cold benzoyl chloride-pyridine gave Q ,/3-anomeric mixtures. ... 

Neighbouring group participation in trichloroacetimidate glycosylations glycosylation of 4,6-0-benzylidene-l,2-0-(/ /5 )-ethylidene-a-D-glucopyranose with 2,3,4,6-tetra-O-benzoyl-a-D-mannopyranosyl trichloroacetimidate [39]. 

When a carbohydrate reacts with a vinyl ether such as 2-methoxy-l-propene, the product is a ketal. The reaction of a-d-glucopyranose (71) with this vinyl ether in the presence of an acid catalyst leads to 85. This reaction essentially protects the C4- and C6-OH units as a 1,3-dioxane (see Chapter 18, Section 18.6.6). (Protecting groups were introduced in Chapter 25, Section 25.4.3.) The name of 102 reflects the presence of this ketal it is 4,6-0-isopropylidene-a-d-glucopyranose. The isopropylidene group is the CMe2 unit, and the 0-isopropylidene name refers to the presence of the acetone ketal unit. When 71 reacts with acetaldehyde in the presence of an acid catalyst, the product is 4,6-0-ethylidene-a-d-glucopyranose, 86. The ethylidene term refers to the CHEt unit and benzylidene refers to the CHPh unit. 

The synthesis of p-1 —> 3-linked cyclo-D-glucohexaose was achieved by using l,2-o-benzilidene-4,6-ethylidene-a-D-glucopyranose and the formation of 2-o-benzoyl-4,6-ethylidene-D-glucopyranosyl bromide by photobromination. This brontide was sequentially condensed to give a p-1 3-linked-a-hexaosyl bromide, which was cyclized by silver triflate catalyst to produce the protected 3-1 —> 3 cyclolaminarihexaose [65]. 

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