Mutarotation 4-nitrophenyl

The concept of bifunctional catalysis was first introduced to account for the unusually large catalytic efficiency of 2-hydroxypyridine in the mutarotation of tetramethylglucose (42). In this reaction, phenol acts as an acid catalyst and pyridine as a basic catalyst and it was, therefore, concluded that a compound with the phenolic hydroxyl and the basic nitrogen at the proper spacing should be able to produce a concerted attack on the sensitive bond of the reactive molecule, with a corresponding reduction of the required activation energy. A similar effect was invoked to explain the unusual pH dependence of the hydrolysis of p-nitrophenyl acetate in the presence of poly-4(5)-vinylimidazole (PVI) (43). 

Deoxy-D-x2/Zo-hexos-5-ulose reduced cold Fehling solution and readily formed a bis[(p-nitrophenyl)hydrazone] at room temperature and was rather alkali-labile. It showed mutarotation. The structure was deduced from elemental analysis of the compound and its derivative, and from the mode of preparation. 

The original purpose was to cyclize this compound to a cyclitol derivative by treatment with potassium in xylene. However, a crystalline diene derivative (115) was formed which, after hydrolysis of the methylene groups, gave crystalline l,6-dideoxy-D-(/ireo-2,5-hexodiulose (116). It showed no indications of the presence of a double bond and reduced Fehling solution in the cold. The location of the two keto groups at C-2 and C-5 in this compound and in the diulose (118), obtained later, was proved by ozonization of their parent diene compound (115) to yield D-threaric acid. The compound (116) showed no mutarotation and was converted into a crystalline bis[(p-nitrophenyl)hydrazone] and di-(p-nitrobenzoate). 

Fig. 1 Properties of SXA-catalyzed hydrolysis of 4-nitrophenyl- 3-D-xylopyranoside (4NPX). a Stereochemistry. SXA catalyzes hydrolysis of 4NPX with inversion of anomeric configuration. Mutarotation, off the enzyme, converts a-D-xylose to its equilibrium mixture (a P ratio of 1 2.5) with a half-life of 1 h [3]. R = 4-nitrophenyl. b Diprotic model. pX,s 5 and 7, assigned to catalytic base (D14) and catalytic acid (El 86), respectively, govern catalysis and binding of ligands [3]...

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