Galactopyranose reactions

From the rearrangement of tetra-O-acetyl-2-hydroxy-D-galactal in boiling acetic acid it was possible to isolate l,2,4,6-tetra-0-acetyl-2,3-dide-h.ydro-3-deoxy-a-jy-threo-hexose (32) (58%) and a small amount of 1,2,3,4,6-penta-O-acetyl-jS-D-galactopyranose. In the reaction mixture the presence of some a-pentaacetate was demonstrated chromatographically but NMR spectroscopy indicated no resonances corresponding to the / anomer of compound 32. These spectral measurements indicate that compound 32 constituted 80% of the mixture of products. 

Starting from (—)-29, carba-a-D-galactopyranose (7) and carba-) -D-glu-copyranose (94) have been synthesized by a reaction analogous to that employed in the preparation of the racemates. - ... 

An alternative approach, in which the anomeric centre of the sugar is reacted with a carbon nucleophile, but the amino group is subsequently inserted at the other side or in both sides at the same time, is reported in Fig. 41 for the synthesis of a- and (3-homogalactonojirimycin.72 Reaction of 2,3,4,6-tetra-O-benzyl-p-D-galactopyranose with methylenetriphenylphosphorane generated an enitol on which the amino group was inserted as phtalimide by a double inversion process of the 5-OH, under Mitsunobu s conditions. 

Scheme 10.—Proposed Mechanism for Type II Reaction of l,2 3,4-Di-0-isopropyli-dene-6-O-pyruvoyl-a-D-galactopyranose (20).

When the esters listed in Table IX are irradiated, typical carbonyl reactions result that is, each of these compounds (22-27) experiences both a-cleavage and hydrogen-abstraction reactions. For the esters of l,2 3,4-di-0-isopropylidene-a-D-galactopyranose (22-26), the hydro-gen-abstraction reaction is internal, and leads69 to a Type II reaction... 

The photochemical reactions of xanthides are quite complex. They are solvent-, concentration-, temperature-, wavelength-, and time-de-pendent.130 The most thoroughly studied of these compounds is compound 64, whose irradiation (through a Corex filter) in cyclohexane under nitrogen produces tetrasulfide 69 (37% yield), xanthate 70 (35%), l,2 3,4-di-0-isopropylidene-a-D-galactopyranose (71, 13%), sulfur, and carbonyl sulfide. Irradiation of a dilute solution of 64 produced only 70 (in 74% yield). The most intriguing finding from irradiation of the xanthides 64-66 is the fact that 66 produces a xanthate... 

In a comparative study of fluorination of l,2 3,4-di-0-isopropyli-dene-6-O-p-tolylsulfonyl-a-D-galactopyranose with tetrabutylammonium fluoride in a variety of dipolar, aprotic solvents (as well as 1,2-eth-anediol, in which no reaction was observed), acetonitrile was found to give the highest proportion of substitution of the sulfonic esters relative to their elimination.106 Elimination is the major, competing reaction in these nucleophilic-substitution reactions, because of the high basicity and low nucleophilicity of the fluoride ion or, in terms of the... 

The Arbuzov reaction has been applied for the synthesis of fluorinated carbohydrates. 1,2 3,4-Di-O-isopropylidene-a-D-galactopyran-ose 6-(N,N-diethyl-P-methylphosphonamidite) was treated with ethyl fluoroacetate, to afford 6-deoxy-6-fluoro-l,2 3,4-di-O-isopropylidene-a-D-galactopyranose in 19% yield.241 The corresponding 6-deoxy-6-fluoride was obtained in 60% yield by treatment of l,2 3,4-di-0-isopropylidene-a-D-galactopyranose 6-(dipropylphosphinite) with hexafluoropropene.242 The mechanism of this reaction has been discussed.243,244 In contrast, treatment of 1,2 -.3,4-di-O-isopropylidene-a-D-galactopyranose 6-(tetraethylphosphorodiamidite) with benzoyl fluoride yielded the corresponding 6-benzoate, not the 6-deoxy-6-flu-oride expected.245... 

Xanthates, on the other hand, are photolyzed to yield the corresponding alcohols, and the yield reported, for instance for l,2 3,4-di-0-iso-propylidene-6-0-[(methylthio)thiocarbamoyl]-a-D-galactopyranose (50%, see Scheme 13) is apparently lowered by partial removal of isopropyli-dene groups under the reaction conditions. 

In order to obviate the formation of a formic ester, the reaction of l,2 3,4-di-0-isopropylidene-a-D-galactopyranose (42) was performed in p-dioxane. The 6-chloro-6-deoxy derivative (45) was again obtained, but only in low yield the major product was assigned the novel structure of 6-0-(4,6-dichloro-l,3,5-triazin-2-yl)-l,2 3,4-di-0-isopropylidene-a-D-galactopyranose (54). 

O-isopropylidene-D-xylose diethyl dithioacetal.7 The same reaction was observed for 2-deoxy-D-arafoino-hexopyranose and 4,6-dichloro-4,6-dideoxy-D-galactopyranose, giving acetals 16 and 17, respectively. 

The first polymerizations reported by Kops and Schuerch147 were those of l,4-anhydro-2,3,6-tri-0-methyl-/3-D-galactopyranose and 1,4-anhydro-2,3-di-0-methyl-a -L-arabinopyranose. The latter compound was slightly contaminated with l,4-anhydro-2,3-di-0-methyl-a-D-xy-lopyranose, but the course of the polymerization could nevertheless be monitored reasonably accurately. For the most part, the polymerizations were conducted at 10% concentration (g/mL) in dichloro-methane, or aromatic hydrocarbons, with 1-5 mol% of phosphorus pentafluoride, or boron trifluoride etherate. At low temperature (—78 to —97°), the d.p. of both polymers produced was —90 at increasing temperatures of polymerization, termination processes became more severe, and the d.p. lower. Usually, the reaction times were long (perhaps unnecessarily so), and the conversions were 50 to 90%. The specific rotations of the D-galactans prepared at —28 and —90° differ by only —10° ( — 85 to — 95°), but those of the L-arabinans varied from + 6... 

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