Oxygen 2-pyrone derivatives

The catalytic reaction of complex 49 with dtbcH2 and dioxygen is less selective than that of 50. In addition to the formation of dtbq and 3,5-di-ter -butyl-l-oxa-cyclohepta-3,5-diene-2,7-dione, some muconic acid anhydride and 2-pyrone derivatives are also produced as a result of both intra- and extradiol insertion of one of the oxygen atoms from dioxygen. Such products are consistent with the mechanism in Scheme 10, with the proviso that both the rhodium and iridium systems share the same mechanism, at least in the initial stages of the reaction. 

The lactone-peroxides 17 are derived from the corresponding ot-pyrones. Singlet oxygenation at low temperature affords the unstable a-pyrone endoperoxides which, on warming up, readily decarboxylate into 1,2-diacylethylenes. However, subambient diimide reduction leads to the desired lactone peroxides 17 (Eq. 12)29). 

Intermediates such as 224 resulting from the nudeophilic addition of C,H-acidic compounds to allenyl ketones such as 222 do not only yield simple addition products such as 225 by proton transfer (Scheme 7.34) [259]. If the C,H-acidic compound contains at least one carbonyl group, a ring dosure is also possible to give pyran derivatives such as 226. The reaction of a similar allenyl ketone with dimethyl mal-onate, methyl acetoacetate or methyl cyanoacetate leads to a-pyrones by an analogous route however, the yields are low (20-32%) [260], The formation of oxaphos-pholenes 229 from ketones 227 and trivalent phosphorus compounds 228 can similarly be explained by nucleophilic attack at the central carbon atom of the allene followed by a second attack of the oxygen atom of the ketone at the phosphorus atom [261, 262], Treatment of the allenic ester 230 with copper(I) chloride and tributyltin hydride in N-methylpyrrolidone (NMP) affords the cephalosporin derivative 232 [263], The authors postulated a Michael addition of copper(I) hydride to the electron-... 

Both 1,2- and 1,4-pyrones (91 and 92, respectively) should be mentioned. It also seems appropriate to mention oxygen derivatives of this type, because some properties of these molecules are to a large extent determined by their topology. Properties of the C=Y bonds, the heats of combustion, the UV and IR spectra,88 and dipole moment data... 

The phthaleins derived from di- or polyhydric phenols are all anhydrides formed by the elimination of water from two phenolic hydroxyls, attached to two different benzene rings. These anhydride phthaleins are known as pyronines, since they contain, like the pyrones, a six-membered oxygen-containing ring. 

The pyrylium salts (76) are the six-membered oxygen heteroaromatic compounds. The 2- and 4-pyrones [(77) and (78) respectively] and the corresponding benzo analogues [coumarins (79) and chromones (80)] are represented by examples which further illustrate the usefulness of retrosynthetic analysis in the derivation of preparative methods. 

The pyrilium cation 9.1, 2-pyrone 9.2, 4-pyrone 9.3, and their benzo-fused analogues the benzopyrilium cation 9.4, coumarin 9.5, chromone 9.6, are the parent structures of a series of six-membered ring heterocycles containing one oxygen atom. The impetus for research in this area comes from the enormous number of plant-derived natural products based on the benzopyrilium, coumarin, and chromone structures. 

The reaction of N-(terf-butoxycarbonyl)leucinal 2-41 a by Danishefsky et al. with l-methoxy-3-trimethylsilyloxy-l,3-butadiene 2-10 gave the pyrones 2-42 and 2-43 with an induced diastereoselectivity of 9 1 in favour of the syn-com-pound in the presence of Eu(hfc)3 [96]. Later Garner [97] used a N-Boc-serine derived aldehyde 2-41 b and Danishefsky s diene 2-10. In both cases a chelation-control forming a complex between the nitrogen and the oxygen could explain the obtained selectivity. In the presence of HMPA chelation is minimized to give a higher extent of the anfi-product 2-43 (Fig. 2-12) [97]. 

NH group as well as in pyrones and pyrans by the ring-nitrogen atom - to form pyridine derivatives, are also known. It is difficult to say whether, in the course of caramelization, alicyclic nitrogen compounds are formed prior to their aromatization, by subsequent dehydration, or whether azaheterocycles are formed by replacement of oxygen atoms by nitrogen atoms in the Yurev, and Baeyer, reactions, respectively. 

Psoralens, also known as furocoumarins, are naturally occurring or synthetic tricyclic aromatic compounds and are derived from the condensation of a coumarin nucleus with a furan ring [1-10]. Several new furocoumarins have been isolated from natural sources [10, llj. In addition, the synthetic methods for production of these molecules have been described and reviewed [10]. The synthetic methods can be organized on the basis of the key step used for the formation of the two different oxygenated rings, hi this respect, there are three possibilities (i) formation of the furan ring onto the coumarin, (ii) formation of the pyrone ring onto the benzofuran and... 

Another ingenious route for the generation of the dipolar species involves the carbonyl-0-alkylation or 0-silylation of 3-oxygenated 4-pyrones. The example below shows 0-methylation of a kojic acid derivative, then deprotonation of the 3-hydroxyl using a hindered base to trigger the dipolar cycloaddition. 

An enolic OH derived from the P-diketone structural unit can act as the nucleophile in a nucleophilic acyl substitution to give a six-membered oxygen heterocycle known as a pyrone. 

It was known from 1876 that phthaleins form salts with concentrated sulphuric acid, and A. G. Perkin discovered a series of salts of the yellow colouring matters of the quercitin group. These are all derivatives of y-pyrone (see p. 862). They were usually regarded as molecular compounds, but J. N. Collie and T. Tickle assumed that the bridge oxygen in y-pyrone becomes quadrivalent and basic, forming oxonium salts I analogous to ammonium salts ... 

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