Galactose 3,4-0-isopropylidene-6-0-methyl

The synthesis of halodeoxy sugars has also been achieved by reaction of sugar phosphorodiamido and phosphonamido derivatives with alkyl halides (83). Heating equimolar amounts of 6-(tetraethylphosphoro-diamido)-l,2 3,4-di-0 isopropylidene-D-galactose with methyl iodide (and benzyl bromide) at 140°C. for 4 hours afforded the 6-deoxy-6-iodo (74b) (75%) and 6-bromo-6-deoxy (74c) (56%) derivatives, respectively. 

Deoxy-2-[ F]fluoro-D-galactose can be prepared through an addition reaction to tri-O-acetyl-D-galactal (465), but better through Sn2 reaction (K F-Kryptofix 222 in MeCN) of methyl 3,4-0-isopropylidene-2-0-triflyl-6-0-trityl-y -D-talopyranoside (220 see Section 11,2), according to the cold synthesis. 

This carbocylization method has been used successfully with a range of hex-5-enopyranose derivatives, and good yields of hydroxyketones of various configurations have been recorded [4]. It is less satisfactory with the 6-deoxy-5-ene derived from l,2 3,4-di-0-isopropylidene-a-D-galactose, giving only 40% of compound 51 [26], but good yields of the cyclohexanones have been obtained from substituted methyl galactopyranosides [25]. The reaction does not afford cyclopentanones when applied to 5-deoxy-4-enofuranose derivatives [27]. 

The easily prepared 3,4-isopropylidene derivatives of the a and j3 methyl-D-galactopyranosides form the starting points of the synthesis.7,9 Methylation of either substance, followed by hydrolytic removal of first the isopropylidene and then the glycosidic methyl radicals leads to crystalline 2,6-dimethyl-D-galactose. Since the above mentioned syn-... 

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... 

When the proportion of acetone is restricted, the main products are mono-O-isopropylidene-D-galactoses. Thus, the furanoid 5,6- and pyranoid 3,4- and 4,6-isopropylidene acetals were obtained in yields of 22, 15, and 13%, respectively. The absence of 1,2-acetals is significant, and is attributed to an effect of A/,/V-dimethylfonnamide.23-24 A similar, low activity of the anomeric hydroxyl group has been noted with the 2,2-dimethoxypropane-A/,A/-di methyl form amide reagent.25... 

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