Methyl 3,4-0-benzylidene hydrolysis

Methylation of methyl 2-acetamido-4,6-0-benzylidene-2-deoxy-a-D-altroside, obtained by hydrolysis of the 3-acetate estergave a 3-methyl ether. Hydrolysis of the benzylidene group, and tritylation of the resulting glycoside, led to crystalline derivatives. 

Oxazol-5(2H)-one, 2-benzylidene-4-methyl-tautomerism, 6, 186 Oxazol-5(2ff)-one, 2-methylene-isomerization, 6, 226 Oxazol-5(2H)-one, 2-trifluoromethyl-acylation, 6, 201 Oxazol-5(4ff)-one, 4-allyl-thermal rearrangements, 6, 199 Oxazol-5(4H)-one, 4(arylmethylene)-Friedel-Crafts reactions, 6, 205 geometrical isomerism, 6, 185 Oxazol-5(4ff)-one, 4-benzylidene-2-phenyl-configuration, 6, 185 photorearrangement, 6, 201 Oxazol-5(4ff)-one, 4-benzyl-2-methyl-Friedel-Crafts reactions, 6, 205 Oxazol-5(4ff)-one, 4-methylene-in amino acid synthesis, 6, 203 Oxazol-5(4ff) -one. 2-trifluoromethyl-hydrolysis, 6, 206 Oxazolones... 

Cyclic sulfates rapidly react with the fluoride ion sources to give monofluoro derivatives Thus, the 2,3-cyclic sulfate of methyl-4 6 O benzylidene p D manno pyranoside cleanly reacts with tetramethylammonium fluoride to give methyl 4,6-0-benzylidene 2 deoxy 2 fluoro-p-D-glucopyranoside-3-sulfate Acid hydrolysis followed by acetylation gives 2 deoxy 2 fluoro-P-D-glucopyranoside triacetate in 84% isolated yield [5S] (equation 38)... 

This section is completed with a brief review of the synthesis and properties of this epimer (20) of the precursor of thiazole in bacteria. This pentulose is conveniently accessible by an unconventional route (Scheme 19). Methyl 2,3 4,6-di-O-isopropylidene-a-D-mannopyranoside, readily available from methyl ot-D-mannopyranoside, is converted to the ketonic glycoside by butyllithium in 91% yield, following a method first published by Klemer and Rodemeyer43 and scaled up by Horton and Weckerle.44 This was converted by means of lithium hydroxide in a water-ether mixture into 3,5-0-benzylidene-l-deoxy-D-eryf/iro-2-pen-tulose in 55% yield. Hydrolysis to the free pentulose (20) proceeded in 73% yield in aqueous acetic acid. This product was obtained as a syrup with a characteristic absorption band at 1705 cm 1 as a film. Thus, there is a fair proportion of the open-chain ketone under these conditions, as with the D-threo epimer.45... 

A novel method of opening of oxiranes involves the use of (chlo-romethylene)dimethyliminium chloride (39) [see Section II,2c p. 250], monochlorodeoxy or dichlorodideoxy derivatives are obtained, depending upon the reaction conditions employed.83 Thus, methyl 2,3-anhydro-4,6-0-benzylidene-a-D-allopyranoside (110) reacts with 39 in 1,1,2,2-tetrachloroethane at room temperature to give, upon hydrolysis of the primary adduct 111 with an aqueous solution of sodium hydrogen carbonate, methyl 4,6-0-benzylidene-2-chloro-2-deoxy-3-0-formyl-a-D-altropyranoside (112). If a solution of 39 and 110 in 1,1,2,2-tetrachloroethane is heated at reflux temperature, methyl 3,4-0-benzylidene-2,6-dichloro-2,6-dideoxy-o -D-altropyrano-side (113) is obtained in high yield the n.m.r. spectrum of 113, like that of 47 (see Section II, 2c p. 250), showed the presence of two diastereoisomers which differed in the configuration of the benzyl-idene-acetal carbon atom. 

A branched-chain iodo sugar derivative, l,5-anhydro-4,6-0-benzyl-idene-2,3-dideoxy-3-C-(iodomethyl)-D-rifoo-hex-l-enitol [4,6-O-ben-zylidene-3-deoxy-3-C-(iodomethyl)-D-allal] (200), is one of the products formed on treatment of methyl 4,6-0-benzylidene-2,3-dideoxy-a-D-en/thro-hex-2-enopyranoside (77) with the Simmons-Smith reagent (diiodomethane and zinc-copper couple).123,212 Compound 200 displays high solvolytic reactivity, an observation that has been rationalized by supposing the formation of the highly stabilized carbonium ion213 (201). Thus, under conditions wherein methyl 2,3,4-tri-0-acetyl-6-deoxy-6-iodo-a-D-glucopyranoside required more than 24 hours to react appreciably with an excess of silver nitrate in 50% aqueous p-dioxane buffered with silver carbonate, the iodide 200 was hydrolyzed completely in less than 1 minute the product of hydrolysis of 200 is the cyclopropyl aldehyde 202. Methanolysis of... 

The correct choice of the type of acetal group can be of some importance when routes to partially protected sugars are needed. For example, although the two benzylidene groups in methyl 2,3 4,6-di-0-ben-zylidene-a-D-mannopyranoside are removed37 at similar rates during acid hydrolysis, methyl 2,3 4,6-di-0-isopropylidene-a-D-mannopyran-... 

C-3 (or C-4) from a 1,3-dioxolane actually, this reaction (see Section HI, 14) was observed for 2,2-disubstituted 1,3-dioxolanes, and it may be assumed that, if there is any competition between removal of a proton from C-2 and from C-3 (or C-4), the rate of the first reaction is far greater than that of the second. The possibility of dioxolanylium ions reacting with nucleophilic reagents was first demonstrated by Hanes-sian and Staub,152 and the scope of these reactions was independently investigated by Pedersen and coworkers.153-157 Hanessian and Staub152 treated methyl 5-0-benzoyl-2,3-0-benzylidene-j3-D-ribofuranoside (125) with triphenylmethyl fluoroborate in acetonitrile, and obtained, after hydrolysis, a 1 1 mixture of the two benzoates 127 and 128 (yield 83%), probably through the benzoxonium ion 126. The same reaction... 

The synthesis of the partially methylated glucoside, methyl 2,3-di-O-methyl-a-D-glucopyranoside (34) (Expt 5.121) also utilises the 4,6-O-benzylidene derivative (27), which is first converted into the 2,3-di-O-methyl derivative (33) by reaction with dimethyl sulphate in the presence of sodium hydroxide. Selective removal of the acetal grouping is achieved by mild acid hydrolysis the methyl ether groups are stable under both acidic and basic conditions. 

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