Methyl 3,4-0-benzylidene benzoylation with benzoyl

When maltose was treated with 8 molar equivalents of benzoyl chloride in pyridine at —40°, it afforded octa-0-benzoyl-/3-maltose, l,2,6,2, 3, 4, 6 -hepta-0-benzoyl-/3-maltose, and l,2,6,2, 3, 6 -hexa-0-benzoyl-/3-maltose in yields of 3, 79, and 12%, respectively.69 Similar treatment of methyl /3-maltoside with 7 molar proportions of benzoyl chloride in pyridine gave crystalline methyl 2,6,2, 3, 4, 6 -hexa-0-benzoyl -/3 - maltoside and methyl 2,6,2, 3, 6 -penta-O -benzoyl -/3 -maltoside in yields of 71 and 23%, respectively.69 Analysis of the relative yields of the products of these reactions suggests that, after HO-3, the 4 -hydroxyl group in maltose and methyl /3-maltoside is the most hindered. Benzoylation of methyl 4, 6 -0-benzylidene-/3-maltoside with 10 molar proportions of reagent gave 2,6,2, 3 -tetra-0-benzoyl-4, 6 -0-benzylidene-/3-maltoside (71%) and an equimolar mixture (22%) of the 2,6,2 - and 2,6,3 -tribenzoates. These results indicated that the order of reactivity of the hydroxyl groups towards acid halides in pyridine is HO-2, HO-6 > HO-2 HO-3 > HO-3. 

S. Hanessian 243 6-BR0M0-6-DE0XY HEXOSE DERIVATIVES BY RING-OPENING OF BENZYLIDENE ACETALS WITH N-BROMOSUCCINIMIDE METHYL 4-0-BENZOYL-6-BROMO-6-DEOXY-a-D-GLUCOPYRANOSIDE... 

Methyl 2,3-0-benzylidene-4-C>-methyl-a-L-rhamnopyranoside reacted with NBS in carbon tetrachloride to give methyl 2-0-benzoyl-3-bromo-3,6-dideoxy-4-0-methyl-a-L-altropyranoside. This and the reactions of other benzylidene acetals with NBS, which were used in the synthesis of rare deoxy-sugars, are covered in more detail in Chapter 13. 

This indirect method of selective benzoylation at C-4 is of special interest in the galacto series (47). Reaction of methyl 4,6-O-benzylidene-a-D-galactopyranoside 97 with NBS afforded the corresponding 6-bromo-4-benzoate (98) in over 90% yield. The axial hydroxyl group at C-4... 

Methyl 4,6-O-benzylidene-a-D-altropyranoside showed little selectivity on reaction with a unimolar proportion of benzoyl chloride43 2-and 3-O-benzoyl and 2,3-di-O-benzoyl derivatives, together with starting material, were isolated in the molar ratios of 1 1.1 1.8 1. In contrast, benzoic anhydride in pyridine gave the 2-ester in 35% yield, with only a trace of the 3-benzoate. 

Esterification of several methyl 4,6-O-benzylidene-D-hexo-pyranosides with one molar equivalent of benzoyl chloride-triethylamine in an inert solvent was found to occur with good selectivity, mainly affording 2-benzoates from the a-D-gluco, a-D-allo, and a-D-altro compounds, and the 3-benzoate from the j8-D-gluco compound.52... 

This class of acylating agents has thus far been but little used in carbohydrate chemistry. However, unimolar benzoylation of methyl 4,6-O-benzylidene-a-D-glucopyranoside, methyl 4,6-O-benzylidene-a-D-altropyranoside, and benzyl 4,6-O-benzylidene-jS-D-galactopy-ranoside with benzoyl cyanide gave good yields of the corre-... 

Methyl 3-benzamido-4,6-di-0-benzoyl-2,3-dideoxy-2-fluoro-a-D-al-tropyranoside (87) was obtained by treatment of methyl 2-benzamido-4,6-0-benzylidene-3-0-p-tolylsulfonyl-a-D-glucopyranoside (84) with tetrabutylammonium fluoride in hexamethylphosphoric triamide, followed by O-debenzylidenation and benzoylation.85 The reaction is... 

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 first example in the pyranoid series was afforded153 by methyl 2-0-benzoyl-3,4-0-benzylidene-j8-D-arabinopyranoside (92), which gave the corresponding benzoxonium ion (93) this ion was shown to be in equilibrium158 with its regio- and stereo-isomer 94, the latter... 

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