Y-Pyrone ring

These investigations shed considerable light on the synthesis of the Y-pyrone ring however, it is unlikely that any conclusions regarding the biosynthetic process could be drawn from them. 

Kojic acid is a -y-pyrone. The conventional way of numbering the atoms in the Y-pyrone ring of kojic acid is shown in structure II. For convenience... 

Lown and Sondhi were interested in the synthesis of chromophores of the anthracycline antibiotics in which the quinone ring c was replaced by a y-pyrone. The y-pyrone ring was prepared by an oxidative cyclization mediated by DDQ, presumably proceeding via the quinone (Scheme 27). 

Some further aspects of the chemistry and biological activity of the insecticidal substances cinnzeylanine and cinnzeylanol have been described. The labelling pattern of colletotrichin (129), biosynthesized from [1- C]- and [1,2- C2]-acetate, has been examined and indicates that it is a nor-diterpenoid with an additional y-pyrone ring. 

Rifamycin G [36] and 16,17-dehydrorifamycin G [37] are examples in the rifamycin series where C-1 is replaced by an oxygen atom. A y-pyrone ring is therefore present, instead of the quinone or dihydro-quinone moiety of other rifamycin type ansamycin anibiotics. 

As other rifamycin derivatives obtained from the culture broth of Nocardia mediterranei, rifamycin G (12) was isolated and characterized [36]. Rifamycin G (12) possesses a y-pyrone ring as a partial structure of its chromophore instead of the quinone ring of rifamycin S (2), and the double bond at C16-C17 of the ansa chain was hydrogenated. 

In a continuation of our work on pinnatoxins, a novel marine alkaloid, pinnamine (10), was isolated from the Okinawan bivalve P. muricata. Pinnamine exhibited acute toxicity against mice, with characteristic toxic symptoms, such as scurrying around and convulsion (LD99 0.5 mg/kg) [17]. The structure of pinnamine (10) was determined to be a unique alkaloid containing a 9-azabicyclo[4.2.1]nonane moiety and a dihydro-y-pyrone ring. The absolute stereostructure was determined by an analysis of the circular dichroism spectrum [18]. 

Pyronotropane alkaloids are natural compounds with y-pyrone ring condensed with tropane ring at C-3 and C-4 (Fig. 12.4). 

Baeyer and Piccard were the first to prepare crystalline monocyclic pyrylium salts without hydroxy or alkoxy substituents, from y-pyrones and Grignard reagents in 1911. They ascribed a correct structure to these salts, although the bonds in the ring and the valency of the oxygen heteroatom remained contested topics for the next 20 years. The discussions around the formula of pyrylium... 

The cleavage of kojic acid by hot alkali, to fragments containing one to three carbon atoms, could not be definitely shown to commence at the ring oxygen atom, but what was said earlier about y-pyrones in general renders this conclusion probable. 

Another enantioselective synthesis of longifolene, shown in Scheme 13.27, uses an intramolecular Diels-Alder reaction as a key step. The alcohol intermediate is resolved in sequence B by formation and separation of a menthyl carbonate. After oxidation, the pyrone ring is introduced by y addition of the ester enolate of methyl 3-methylbutenoate. 

New congeners of cytotoxic nor-diterpenoid dilactones in Podocarpus nagi three highly polar components with a-pyrone ring. Hayashi, Y. Yuki, Y. Matsumoto, T. Sakan, T. Tetrahedron Lett. 1977,34,2953-2956. 

However, chromones react differently, because the phenolic hydroxy group in the ring-opened intermediate is unreactive. Thus, isoxazoles (264) result from the reaction with hydroxylamine, and a pyrazole is formed with hydrazine. y-Pyrones also give pyrazoles with hydrazine. 

Industrially, a selectivity to DAA of between 90—95% can be achieved (64). The principal by-products are mesityl oxide and acetone trimers. j W-Triacetone dialcohol [3682-91-5] can form by condensation of acetone with diacetone alcohol (116). Dehydration of ry/ -triacetone dialcohol can yield semiphorone [5857-71-6] (6-hydroxy-2,6-dimethyl-2-hepten-4-one), which may in turn ring close to form 2,2,6,6-tetramethyl-y-pyrone [1197-66-6/, or ultimately dehydrate to phorone [504-20-1] (2,6-dimethyl-2,5-heptadien-4-one) (146). Similarly, an unsymmetrical acetone trimer can also be formed which dehydrates to 2,4-dimethyl-2,4-heptadiene-6-one. These impurities complicate the high purity recovery of DAA, and are thought to be responsible for a yellow discoloration of DAA. The addition of dibasic acid (147) or nitrogen containing carboxylic or phosphonic acids (148) has been patented as refined product stabilizing agents. 

Much more stable are the 2- or 4-oxides, i.e., a-pyrones (unsaturated lactones) 20 and y-pyrones, which could, in principle, have aromatic character owing to their zwitterionic resonance structures (20A-20C). However, although protonated pyrones are definitely aromatic, the neutral compounds appear to have very little aromaticity. Bird s aromaticity index I6 for pyrylium is only 65.8% in comparison with benzene, whereas for 4-pyrone it is 37.2% and for 2-pyrone it is only 32.9%, as seen in Table 4 [22], In agreement with Table 2, the ring 0-(C = O) bond in 20A is a type X-Z bond, whereas the ring O = (C-O ) bond in the two other resonance formulas is a Y-Y bond. 

Kizhner377,378 showed that 2,6-dimethyl-y-pyrone and hydrazine react in a different way, the pyrone ring opening and a more stable... 

A dihydro-derivative (4) of strobiline (M+ 193) was also obtained. In the mass spectrometer, the pyrrolidine ring is opened to give fragments of m/z 165 and 164 (base peak), followed by a retro-Diels-Alder reaction to give fragments of m/z 137 and 136, respectively. The location of the two additional hydrogen atoms was deduced from the relative u.v. extinctions of (3) and (4), which is consistent with the dihydro-y-pyrone skeleton in the latter compound. This was confirmed by 3H n.m.r. spectra that showed no vinylic protons. 

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