Mutarotation lactones

Bicucine, C20H19O7N, H2O. This alkaloid has m.. 222° (dec.) and — 115 4° (N/10, KHO) but in N/HCl it shows mutarotation — 145° to — 100°,due to the formation of an equilibrium mixture of bicucine and bicuculline. Alkaline permanganate oxidises it to 3 4-methylene-dioxyphthalic acid, isolated as the ethylimide. In view of its formation from bicuculline by the action of alkali, Manske has suggested for its formula (II) or (III), the former representing it as the nomarceine (p. 208) analogue of bicuculline, whilst (III) makes it the hydroxy-acid corresponding to the lactone, bicuculline and is preferred. 

In equation 8.2-6a, the slope of -1 with respect to pH refers to specific hydrogen-ion catalysis (type B, below) and the slope of + 1 refers to specific hydroxyl-ion catalysis (Q if k0 predominates, the slope is 0 (A). Various possible cases are represented schematically in Figure 8.5 (after Wilkinson, 1980, p. 151). In case (a), all three types are evident B at low pH, A at intermediate pH, and C at high pH an example is the mutarotation of glucose. Cases (b), (c), and (d) have corresponding interpretations involving two types in each case examples are, respectively, the hydrolysis of ethyl orthoacetate, of P -lactones, and of y-lactones. Cases (e) and (f) involve only one type each examples are, respectively, the depolymerization of diacetone alcohol, and the inversion of various sugars. 

Since the trimethylglucose has been shown to form a 6-lactone, characterized by its rapid mutarotation in aqueous solution,161-163 and to give rise when methylated and hydrolyzed to 2,3,4,6-tetramethyl-D-gluco-pyranose,189 it follows that C5 is unsubstituted. While it is clear that the third methoxyl group must be present on C6, little direct evidence is available to support this view. The syntheses of the trimethylglucose from methyl 2,3-anhydro-/3-D-mannopyranoside (known to condense with benzaldehyde and therefore to carry free hydroxyl groups on C4 and C6)168 and also from 3-benzyl-D-glucose162 leave, however, no doubt that C6 is methylated. 

Furanose—furanose Interconversions.—The only known case of an anomeric fiiranoid interconversion is for D-mannurono-1,4-lactone. Corrected for hydrolysis of the lactone, the mutarotation was expressed by [a] J = —34.5 x + 92.2 (at pH 4.0). The rate... 

The first figure given indicates the of the original form the second figure given indicates the [a]X of the equilihrium mixture of a and forms after mutarotation. The aidonic acids equilibrate with the lactone. 

Another controversial aspect of the bishydrazone structure concerns the hydrazone residues. The bishydrazone was proposed to have the structure 46, which mutarotates in solution to 47 (47). More recently, on the basis of a comparative study of the spectroscopic properties of the bis(phenylhydrazone) with some related compounds, the bishydrazone was assigned the structure 2,3-dideoxy-3-phenylazo-2-phenylhydrazino-L-threo-hex-2-enone-l,4-lactone (48) (48). However, this latter structure was inconsistent with its NMR spectra (49). 

Lactonizes upon evapn. The lactone forms large sweetish crystals, mp 145-148°. [ ]J —56.0° (shows mutarotation). Sol in water. 

The solution obtained from either form of the carbohydrate, which, on standing some hours shows the normal specific rotation, 52.3 , contains a mixture of the two forms in equilibrium. The view has been put forward that carbohydrates which exhibit mutarotation may exist as aldehydes or as lactones. 

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