Tetronic acid ring

Chart 10. Reaction with alkali of conjugated tetronic acid ring... 

Table 1 Hsts the polyether antibiotics arranged by the number of carbons in the skeleton. Many of these compounds were isolated independendy in separate laboratories and thus have more than one designation. The groups are subdivided depending on the number of spiroketals. Two classes fall outside this scheme the pyrrole ether type containing a heterocycHc ring, and the acyltetronic acid type, that has an acyHdene tetronic acid instead of a carboxyHc acid. These compounds are ionophores and because of their common features are included as polyethers.

Ketal 73 can be formed in a yield of about 60 % by refluxing a solution of tetronic acid (36), ethylene glycol, and a catalytic amount of para-toluenesulfonic acid in benzene for approximately 12 hours. With only one electrophilic site, 73 reacts smoothly with Dibal-H to give lactol 35 in 84% yield. Compound 35, a participant in a ring-chain tautomeric equilibrium process,18 should be regarded as a latent aldehyde. This substance can, in fact, serve as... 

Effenberger and coworkers have utilized the tolerance of methyl ketones by the recombinant enzyme to develop an alternative synthesis of tetronic acids and their amino derivatives, as shown in Figure 5.18. Treatment of O-acyl cyanohydrins with lithium disilazide resulted in base-induced ring closure to amino tetronic acid derivatives. Alternatively, the cyanohydrins could be converted to a-hydroxy esters prior to acylation, and the same base-induced cyclization then led to tetronic acid derivatives [89]. 

This review of furan chemistry is meant to continue the earlier survey by Bosshard and Eugster1 and concentrates upon the period 1968 to the end of 1979. Like the earlier review, this one is limited to the chemistry of the monocyclic furan nucleus and does not deal, except incidentally, with fused rings such as benzofuran or its quinones. Nor does it deal in detail with dihydro- or tetrahydrofurans, nor with compounds like furylpyridine that contain some other heterocyclic nucleus as well. Some butenolides and tetronic acids are admitted to consideration since they are the carbonyl equivalents of hydroxyfurans regarded as enols, but side-chain reactions are wholly excluded unless the furan nucleus clearly affects them in some important way. 

Basler, B., Schuster, O., and Bach, T. (2005) Conformationally constrained p-amino acid derivatives by intramolecular [2 + 2]-photocycloaddition of a tetronic acid amide and subsequent lactone ring opening. Journal of Organic Chemistry, 70, 9798-9808. 

Additional evidence for the stereospecific conversion of fi-O-4 and arylglycerol lignin models to tetronic acids under conditions that selectively oxidize aryl rings also comes from acidic permanganate oxidation (Tsutsumi et al. 1990). This method gives essentially the same products as ozonation. 

Nishimoto et al. have shown that the photorearrangement of the aziridine (125) to the oxazolinc (126) occurs via a complex of the aziridine and the bromo-compounds (127) in the absence of the latter, the oxazoline ring is not formed. An electron transfer mechanism appears likely. The tetronic acid... 

The system is very sensitive to traces of acid or to light. The l3C-NMR spectra show the equilibrium to be solvent dependent. Terrestric acid and carlic acid behave similarly.1 0 Evidently, the butenolide ring system is entirely compatible with an exo double bond at its 2-position, and accordingly the condensation of tetronic acid itself with aromatic aldehydes readily affords 3-arylidenefurandiones (73) as mixtures of two geometric isomers.155 The similar condensation with 4-hydroxycoumarin is better known but... 

The simplest member (1) of the 2,3,4-furantriones is the one obtained by oxidizing the hydroxy tetronic acid (2) (Scheme 1). The interest in that ring system was increased after the discovery of L-ascorbic acid (3, vitamin C) and its oxidation product dehydro-L-ascorbic acid (DHA). The latter was formulated as the traditional compound 4, possessing the furantrione ring, but it was found to be equilibrated with other forms as will be discussed later. The role of 3 in biological systems arises from its function in the oxidation-reduction processes. The ratio of 3 with 4 may be related to cell division and therefore may have a critical role in growth regulation in addition to its use as antioxidant in foodstuffs. 

Not all fungal tetronic acids are derived by this pathway. The biosynthesis of multicolic acid (6.17), a metabolite of P. multicolor, has been shown by carbon-13 labelling experiments to follow entirely a polyketide pathway. The proposed biosynthetic pathway (Scheme 6.2) involves the intermediacy of a 6-pentylresorcylic acid (6.16) and the cleavage of an aromatic ring. 

A Stille coupling of the vinyl iodides derived from furan-3-carboxaldehyde with the vinyl stannane 1 obtained from tetronic acid is accompanied by electrocyclic generation of a pyran ring when carried out in the dark. The polycyclic pyran product, the core unit of the diterpenoid saudin, is delivered as a single diastereoisomer <05OL2413>. 

Products related in structure to 249 and 250 could be accessed through condensation of benzyl amine with tetronic acid (256) followed by aza-annulation with acrylic anhydride to give 258 (eq. 52).58 Stereoselective generation of the cis ring fusion of 259 was accomplished by catalytic hydrogenation. 

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