The Oxidation of Tetronic Acids. I. Structural Criteria for the Formation of 1,2-Diketones

Abstract

No satisfactory mechanism has been devised for the production of 2,3-butanedione by the acid oxidation of 3-methyltetronic acid (I). Wolff proposed (1) an intermediate 3-hydroxytetronic acid (II) whose hydroxylic hydrogen atom was assumed to rearrange to the 5-position with simultaneous loss of carbon dioxide. This hypothesis was based upon his observation that 3-bromo-3methyltetronic acid (III) was slowly hydrolyzed by base to form a solution from which 2,3-butanedione and carbon dioxide were liberated on acidification. To date no further work has appeared on this remarkable reaction.In order to explore the limits of the oxidation reaction, and at the same time to gain information regarding its nature, we have prepared and studied various types of tetronic acids from which -diketones might arise. These fall into three structural classes. In class (a) are acids carrying only 5-substituents, while in (b) the substitution is entirely on the 3-carbon atom, and in (c) both 3-and 5-substitution are present. These compounds and the results of their oxidation are shown in Fig. 2 where, for the sake of uniformity, non-enolic structures are used.It is apparent that the formation of -diketones is not a general reaction, but is dependent upon certain structural features in the parent tetronic acid. The first and most important of these is the presence of methylene hydrogen to permit enolization. Second, if the 3-carbon is monosubstituted, -diketones are produced regardless of substitution on the 5-carbon atom. On the other hand, if the 3-carbon is unsubstituted, the 5-carbon must carry one and only one substituent group. Significant differences also appear with respect to the natu e of the diketones produced. Thus, if the tetronic acid carries one 3-substituent, then regardless of 5-substitution, the over-all oxidation process results in decarboxylation with reduction of the 5-and oxidation of the 3-carbon atoms.3 However, in the case of monosubstitution on the 5-carbon, this rule is exactly reversed. These cases, and also that of tetronic acid itself, will be considered later

N-Phthaloylglycine-Derived Strigol Analogues. Influence of the D-Ring on Seed Germination Activity of the Parasitic Weeds Striga hermonthica and Orobanche crenata

“…The preparation of 4-methoxy-3-methyl-2(5H)-furanone (12a) was accomplished by methylation with dimethyl sulfate of the parent 2-methyltetronic acid (Knight and Pattenden, 1975). The lastmentioned compound was prepared in a one-pot synthesis by bromination of ethyl 2-methylacetoacetate to give the 2-bromo derivative (Conrad, 1896), which rearranged in the presence of hydrobromic acid to the 4-bromo isomer, which on heating gave 2-methyltetronic acid (Reid et al, 1950). The synthesis of 3-ethyl-2(5H)furanone (13a) was carried out, using the concept outlined in Scheme 3.…”

The Oxidation of Tetronic Acids. Ii. Absence of Rearrangement in the Neopentyl System During Decarboxylation

“…Oxidation of S-methyl-S-tert-butyltetronic acid. A ten-gram sample (0.062 mole) was oxidized with an excess of chromium trioxide and dilute sulfuric acid in the standard manner (1). A yellow oil was obtained in the distillate.…”

The First Total Synthesis of (±)-Saudin

“…The synthesis of 4 is outlined in Scheme . Reaction of 5 with 6 led to the exclusive formation of the cis-fused Michael-aldol product 7 . Reaction of ketone 7 with trimethylsilyliodide and hexamethyldisilazide as described by Boeckman and Fang , gave the thermodynamic enol silyl ether 8 .…”