Alloxan rearrangement

Secondary rearrangement of the uric acid-4,5-diol derived from theobromine or caffeine to give a methylated alloxan is shown in IV - XXII and X XXIII, Fig. 13 respectively. 

The prototype reaction is the conversion of glyoxal into glycolic acid (equation 2), and here the benzilic acid rearrangement mechanism coincides with that for an intramolecular Cannizzaro reaction. The reaction is observed with other purely aliphatic a-diketones such as f-butyl 2,3-dioxobutyrate and cyclohexane-1,2-dione (equations 3 and 4), but the scope is limited in the aliphatic series by competing (c.g. aldol) reactions. Suitably constructed heterocyclic systems also rearrange, and the conversion of alloxan (3) into alloxanic acid (4) was among the first of the benzilic acid rearrangements to be discovered (equation 5). ... 

Relatively little attention has been paid to this important area, and much of the definitive work has centered upon studies of the rearrangement of alloxan and substituted alloxans (equation 5). The essen-... 

Our research group has reported the ring contraction of 1,2,6-thiadiazine 1,1 -dioxides 43a to 1,2,5-thiadiazole 1,1 -dioxides 89a, which has been assumed to proceed through an alloxan-alloxanic acidlike rearrangement (Scheme 50) (86JCS(P1)643). 

Early work" " on the reaction of 2-substituted amino-3-amino-pyridines (54) with alloxan (55) in neutral solution has been corrected. The initial yellow product from the condensation is a ureide of formula 56, which readily rearranges to the colorless spirohydantoin 57 on treatment with acid or base. Tire isomeric pyridine 49 yields a second series of ureides and hydantoins. When 2,3-diaminopyridine itself is used, a mixture of products is obtained in which the 3-oxo compound 56 (R = H) predominates over its 2-oxo isomer. Alkaline hydrolysis followed by decarboxylation and methylation served to confirm the orientation (see Section II2B). If the reaction with alloxan is carried out in alkaline solution, the only product obtained is the 3-oxo-2-carboxylic acid." ... 

Very recently (Kwart et al. 1961) the rearrangements of alloxan and several of its derivatives have been shown to take place with nitrogen-to-carbon rather than with carbon-to-carbon rearrangement. Thus, alloxan-5-i C [24a, = R = H] was subjected to rearrangement... 

Since isovioluric acid represents an unusual oxime of alloxan, it is not surprising to find that dilute hydrochloric add rearranges it to violuric acid. The simplest mechanism which suggests itself for this reaction involves two steps (1) the hydrolysis of isovioliuic add to alloxan and hydroxylamine, and (2) the recondensation of these to form violuric add. 

The action of nitrous acid on isobarbituric acid yields isovioluric acid (alloxan-6-oxime), which is rearranged to violuric acid (alloxan-5-oxime) by means of add, but otherwise behaves much like alloxan. 

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