5-0- methyl-1,2-O-isopropylidene

Methyl 4,6-0-(diphenylmethylidene)-Methyl 4,6-O-isopropylidene-Methyl 2,3 4,6-di-O-isopropvlidene-Methyl 4,6-O-dodecanylidene-Methyl 4,6-O-methylene-... 

Like sodium hydroxide and sodium methoxide, sodium methyl mercaptide merely detosylates an isolated secondary sulfonyloxy group, as in methyl 6-deoxy-3,4-0-isopropylidene-2-0-tosyl-a-D-galactoside226 and methyl 4,6-O-benzylidene-3-0-methyl-2-0-tosyl-a-D-galactoside.162... 

The less acidic hydroxyl groups being more reactive in this alkylation sequence [1661, the 3-benzyl ethers dominate in the reaction of 2,3-diols, and 4-benzyl ethers in the case of 4,6-diols. Thus, 95 % of methyl 3-0-benzyl-4,6-O-benzylidene-a-D-manno-pyranoside together with 5 % of unreacted starting material have been obtained from methyl 4,6-O-benzylidene-a-D-mannopyranoside. Methyl 2,3-0-isopropylidene-sodium hydride in 1,2-dimethoxy-... 

Fig. 2-17. Examples of acetals. 1, 1,2-O-lsopropylidene-a-D-glucofuranose 2, 1,2 5,6-di-0-isopropylidene-a-D-glucofuranose 3, methyl 4,6-O-benzylidene-a-D-glucopyranoside.

An analogous methyl 2,3-dideoxy-3-C-methylene-a-L-en/thro-hexopyranoside (20) was obtained10 from methyl 4,6-O-benzylidene-2-deoxy-a-L-eri/t/iro-hexopyranosid-3-ulose (19) it was used in the synthesis of olivomycose (see p. 242). There was also reported32,34 the preparation of 3-deoxy-l,2 5,6-di-0-isopropylidene-3-C-methyl-ene-a-D-rifco-hexofuranose (22) from 1,2 5,6-di-0-isopropylidene-a-D-nfeo-hexofuranos-3-ulose (21) in about 60% yield, followed by transformations into branched-chain sugar derivatives, including... 

In the field of natural red algal galactans, MS was used for the first time in the structural analysis of odonthalan.353 Galactose and 6-O-methylgalactose, the hydrolysis products of the polysaccharide, were separated by column chromatography and identified by MS in the form of their di-O-isopropylidene derivatives. Cyclic derivatives of this type had been shown previously to be especially suitable for mass-spectrometric characterization of the structure and (in certain instances) stereochemistry of monosaccharides.349,354 A combination of GLC and MS was also used for identification of methyl 4.6-O-0 -methoxycarbonylethylidene)-D-galactosides (as their trifluoroacetates) in the methanolysis products of Ji-carrageenan from Petrocelis middendorfii,355... 

The model monosaccharides just listed were prepared from common precursor IV.l (Scheme 39), which was readily obtained by azidonitration of 3,4,6-tri-O-acetyl-D-galactal followed by deacetylation with sodium methoxide. Treatment of IV.l with acetone and toluene p-sulfonic acid monohydrate at room temperature led to predominant formation of the thermodynamically favored 3,4-O-isopropylidene (IV.2) in 61% yield while also producing 27% of the 4,6-O-isopropylidene derivative IV.3. The position of the isopropylidene IV.2 was verified by the use of NMR chemical shift analysis to confirm the position of the acetate group in the resultant acetylated adduct IV.4. Synthesis of the 4-O-sulfate derivative (IV.7) from IV.2 utilized a step that differentiated the 3-OH and 4-OH positions after benzylation and de-isopro-pylidination of IV.2, a selective methylation at the 3-OH of diol IV.5 was achieved via a tin procedure [91] to give methyl glycoside IV.6. Conversion of the azide into... 

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