2-Pyrones 5-carboxy

Die Reduzierbarkeit der 2H-Pyrone hangt stark von ihrer Struktur ab. Wahrend 6-Carboxy-2H-pyron z. B. in der Kalte mit Lithiumalanat selektiv zu 2-Hydroxy-6-carb-oxy-2H-pyran (100° d. Th.) reduziert wird4, erhalt man i. a. aus 2H-Pyronen unge-sattigte Carbonsauren (s.S. 444). 

Treatment of bicyclic lactones 66, derived from Diels-Alder reaction of 3-carboxy-2-pyrone under standard radical conditions using (TMSlsSiH, leads to bridged lactones 67, which can smoothly be converted to bicyclo[3.3.0]-lactones 68 (Scheme 10). For X = CHaOMe, this cascade of rearrangements took place in a 78% overall yield, providing 68 in diastereomerically pure form. Three additional steps provided a novel route toward Corey s lactone 69. 

Carboxy-, 5-dimethyl nitrobenzene, AP11 Carboxyferrocene, ATl6 2-Carboxy-3-hydroxy- -pyrone,... 

The distal extradiol cleavage of L-dopa 12, catalyzed by an iron-dependent dioxygenase, gives an alanyl muconic semialdehyde derivative 18 which, on cyclization and lactol oxidation, yields stizolobic acid 16. The pyrone ring is then ammonolyzed24 to give 3-(6-carboxy-2-oxo-4-pyridyl)alanine 17 (Scheme 3). 

A biomimetic catechol cleavage reaction was carried out on dihydro-caffeic acid derivative 1922 using the iron(III)-catalyzed peracetic acid cleavage process of Pandell.25 This produced the cyclized muconic acid derivative 20 which could be transformed into pyrone 21 by treatment with hydrochloric acid. Bromination followed by azide displacement gave key intermediate 22 which could either be reduced directly to (i)-stizolobic acid 16 or ammonolyzed and then reduced to give ( )-3-(6-carboxy-2-oxo-4-pyridyl)alanine 17 (Scheme 4). 

Pyrimidine 5-(2-Carboxy-ethenyl)-4-hydroxy- E9b/1, 48 (5-COOR — 2H-pyron + Formamidine) Toluol... 

Iron ( 4-Carboxy-cyclobutadien)-tricarbonyl- E17f, 758 [3-COOR - 2H-pyron/Fe(CO)5]... 

A different approach can be found in the reaction of diketene 44 with Meldrum acid (45). Treatment of the resulting derivative 46 with TsOH in benzene forms 3-carboxy-4-hydroxy-6-methyl-2-pyrone (47) (84SC265). 

Citreoviridin 1, C23H30O6CH3OH, was shown to contain one methoxy group and six double bonds. Permanganate oxidation of this compound in pyridine afiTorded a carboxylic acid (2), which contained one methyl, one methoxy, and one carboxy group. Infrared (ir) and ultraviolet (uv) absorption spectra of 2 indicated that this acid contained an a-pyrone moiety. The substitution on the pyrone ring was evident from the nuclear magnetic resonance (NMR) spectral data. 

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