Oxygen-containing pyran-2-ones

Zirconium(IV) isopropoxide, 352 Containing one oxygen—tetrahydro-pyrans... 

Other oxygen-containing systems that have been studied include 2,2,6,6-tetramethyltetrahydropyran-3-one,225 2-alkoxytetrahydropyran-3-ones,226 2-acyl-2,3-dihydro-4W-pyrans,227 cyclic acetals,228 and a wide variety of carbonyl-containing pyranoside derivatives.229... 

The comparison of some oxygen-containing cycles presented in Table 7.11 allows one to reveal a significant effect of the double bond location with respect to the heteroatom. Results with 2,3-dihydrofuran (entry 1) and 3,4-dihydro-2H-pyran (entry 3) show that the double bonds having the nearest location to the oxygen exhibit strong... 

The naming of these compounds is a bit odd. Pyran refers to the six-membered oxygen-containing heterocyclic ring system with two double bonds. It is not aromatic though compounds like pyrones are. The compound with only one double bond is therefore dihydropyran, and the saturated ring system istetrahydropyran. 

Figure 11.17. Examples of unsaturated fatty acids. Large numbers of such alkenes both as ( )- and (Z)-isomers exist. The intermediate in the oxidation of arachidonic acid to (only one shown member of the) prostaglandins is part of a cascade of reactions to an entire family of compounds that vary in oxidation state. Leukotrienes (three conjugated double bonds and no five-membered ring) and thromboxanes with six-membered oxygen-containing rings (oxa-cyclohexane, pyran, oxane) are also derived from C20 polyalkenes.

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-... 

Five-membered oxygen- and sulfur-containing heterocyclic ketones reveal notable reactivity (03RCB961, 03RCB1380). Benzo[fc]furan-3-one 204 with arylidenemalononitriles 30 gives dibenzo[b,d]pyrans 206 instead of expected pyran 205 as a result of Michael reaction and the exchange of methylene components (03RCB961) (Scheme 77). 

Most naturally occurring pyran-4-ones, chromones and flavonoids contain one or more oxygen-linked groups. Alkylation and acylation of hydroxyl groups and dealkylation of methoxyl groups has, therefore, received much attention. 

Pyran A doubly unsaturated six-membered ring compound that contains one oxygen and five carbons. 

Autoxidation of colupulone gives a similar mixture of products to that obtained by oxidation with sodium or ammonium persulphate. A major product is the dihydrofuran (48) in which one of the isoprenyl side chains is oxidized and cyclized. In other products (49)-(53) alternative cyclizations to furan or pyran rings occur [43]. Japanese workers have described a series of oxidation products of lupulone of which the lupoxes contain one additional oxygen atom (CgeHggOo) and the lupdoxes two (C2eH380e). Colupox a is identical with the dihydrofuran (48) but the identity of the other products has not been confirmed. 

Furan is the five-membered ring heterocycle containing one oxygen. Thiophene is the five-membered ring heterocycle containing one sulfur. Pyran and thiopyran are the six-membered ring derivatives. 

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