4-Pyrone ring 2,3-dihydro

The 5,6-dihydro-a-pyrone ring is present in several biologically active molecules697,698. This ring system may be usefully prepared, in a one-pot procedure, via a directed aldol-type condensation followed by cyclization (equation 194)6". The products are usefully functionalized with a carboxylic ester group in the 3-position which allows further synthetic elaboration. 

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. 

Expansion of the pyrone ring of chromanones has been achieved by reaction with ethyl diazoacetate and BuLi at —78°C [to produce the a-diazo-ester (154)] followed by acid-catalysed conversion, at room temperature, into the benzoxepin (155). De-ethoxycarbonylation led to the parent 2,3-dihydro-l-benzoxepin-4-(5jF/)-one (156). By a similar sequence, isochroman-4-one (157) was converted into the hitherto unknown l,5-dihydro-2-benzoxepin-4(3H)-one (158). ... 

Pyrimido[4,5-d]pyrimidine-2,4,5,7-tetrones 26,678 2H-Pyrimido[l,2-a]pyrimid-2-ones 26,485 2-Pyrone ring, 3,4-dihydro-... 

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]. 

Even on prolonged hydrogenation under these conditions, only 23 per cent of the product was the 2,3-dihydro compound. When Raney nickel (p. 90) or platinum oxide [159] is present, reduction of the double bond at the 2,3-positions frequently occurs. When ethyl 6-chlorochromone-2-carboxylate was reduced with zinc dust and acetic acid, a dimeric chromanone was obtained [25a]. Hydrogenolysis of the chlorine and reduction of the pyrone ring occurred with hydrogen and palladium-charcoal [25a]. 

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. 

Rotenone 30, the toxic principle from the roots of derris elliptica, is a natural insecticide. Its structure shows an unique combination of benzannelated dihydropyran, dihydro-4-pyrone and dihydrofuran ring systems. 

Substituted derivatives of 5,6-dihydro-a-pyrones (dihydropyran-2-ones or more specifically 2H-dihydropyran-2-ones) occur widely in nature, particularly in plants and bacteria. They possess an a,P-unsaturated-5-lactone ring (1) with an alkyl, alkenyl or aryl substituent at C-6 and occasionally a varied substitution pattern around the ring. Many of these compounds are biologically active, exhibiting phytotoxi-... 

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