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Summary. I n the benzilic acid type rearrangement of t-butyl a, ,&dioxobutyrate (VII) the intact t-butoxycarbonyl group is shifted t o the p-carbonyl carbon atom.

Dedicated to Prof. Tino Giiumann on the occasion of his 6OCh birthday (25.V11.85) ~~ 4-Aryl-2,3-dioxobutyrdmide hydrates 1 undergo the benzil-benzilic acid rearrangement to form (substituted) benzyltartronate monoamides 2. For compound l a (Ar = Ph), it is demonstrated by isotopic labelling that the reaction occurs exclusively by migration of the CONH, group. Kinetic measurements with la-c and with the cyclic amide quinisatine 6 show that the rearrangement of the carboxamide group, proceeding via an alkali-catalysed step, can reach a plateau in the kOb,./[OH-] diagram (cf: the Fig.), due to complete formation of a mono-anion, and a further increase of rate attributable to the rearrangement of a bis-anion. Comparisons suggest that rearrangements involving an amide group are slower than those involving an ester group, and that, for this effect (as for others), the pre-equilibrium deprotonation of the hydrate is more important than a specific migration tendency.

tert -Butyl a,P-dioxobutyrate (hydrate; Id) undergoes, at medium or high pH, the benzilic-acid rearrangement with exclusive 1,2-shift of the COO(t-Bu) group; the same is true for the corresponding isopropyl ester l c and ethyl ester l b at high pH, whereas at lower pH, the overall picture of these reactions is complicated by concurrent hydrolysis of the ester, followed by a 1,2-shift of the COO-group. Consequently, the shift of these electron-attracting groups cannot be considered to be systematically disfavoured (compared, e.g., with alkylgroup shifts). Kinetic measurements of the rearrangement show for both esters (as well as for the analogous ethyl ester lb, and also for ethyl 3-cyclopropyl-cc,~-dioxopropionate (4)) a characteristic rate profile: at relatively low pH, k is proportional to [HO-1, approaching saturation with increasing [HO-] (interpreted as complete transformation of the substrate into the hydrate monoanion), which is followed at higher pH by another rate increase with k proportional to [HO-] (probably due to the reaction of the hydrate dianion). The similarity of k values for 1 M shows that in the shift of COOR steric hindrance caused by R is negligible.1,2-Shifts of electron-withdrawing groups towards electron-deficient centres might be considered to be disfavoured. They have, however, been demonstrated in a significant number of cases, comprising Wagner-Meerwein, pinacol, and benzilic-acid rearrangements [2]. The course of these shifts depends upon factors more important than electron abundance in the migrating group, factors which have to be established individually for each reaction type.In the case of the benzilic-acid rearrangement, we have shown [3] that the action of alkali on (hydrated) ethyl a$-dioxobutyrate (lb) [4] comprises two different reactions (Scheme 1 ) : A ) a direct 1,2-shift of the unhydrolyzed COOEt group takes place at higher pH ( 3 14), with subsequent hydrolysis of the ester group (3b-2); B ) at lower pH ( < lo), the ester group is hydrolyzed first and a COO-group is shifted. The first reaction corresponds to preferential attack of HO-on the geminal-diol group at C(2) (keto hydrate), the second to preceding attack of HO-on the ester carbonyl group of 1.Both reaction paths yield methyltartronate z3). Correspondingly, (hydrated) tert-butyl a,@-dioxobutyrate (Id) which cannot be hydrolyzed by alkali shows only ester-group migration upon treatment with NaOH forming tert -butyl 2-methyltartronate (3d) [6]. As a result one can state that, in principle, carboxylate as well as ester groups can migrate in the benzilic-acid rearrangement.