4.6- Dimethyl-2-pyrone, reaction with methyl

This ability of ethyl and methyl l//-azepine-l-carboxylate to participate as an enophile and a dienophile is demonstrated in a further report describing their reactions with methyl pyrone-3-carboxylate and its 5-isomer <84BCJ3483>. On heating at 80°C for 68 hours, methyl 1/f-azepine-l-carboxylate and the pyrone-3-isomer yielded the [2 + 4]7t adduct (23) and the azepine [6 + 4]tc dimer. Under similar conditions ethyl 1/f-azepine-l-carboxylate and the corresponding pyrone-5-isomer gave a mixture of the [2 + 4]7t and [6 + 4]7t adducts (24) and (25) in yields of 25% and 20%, respectively. At 110°C the products consisted of the azepine [6 + 6[n dimer and a small amount of 3-ethyl 7-methyl 3/7-3-benzazepine-3,7-dicarboxylate (26), arising from (24) by extrusion of carbon dioxide. In a further reaction the same authors have demonstrated the formation of (27) by reaction with tetrachlorocyclopentadienone dimethyl acetal. 

In the hands of Collie and Tickle in 1899 this reaction gave the first crystalline pyrylium salts. The salt character of the compounds was proved by conductivity measurements the basicity of 2,6-dimethylpyrone was found to be a little higher than that of urea. Basicities of other pyrones decrease in the order 2,6-dimethyl-> 2-phenyl-6-methyl-> 2,6-diphenylpyrone, paralleling the dipole moments. These hydroxypyrylium salts hydrolyze in water to pyrones. " The formation of salts of 2,6-dimethylpyrone with organic acids was investigated by Kendall,and with mineral acids by Cook. 11 ... 

Thiopyrones and selenopyrones can be alkylated more readily than pyrones. Thus 2,6-dimethyl-4/f-pyran-4-thionc (4,6-dimethyl-4-thiopyrone) (23, Y = S) reacts rapidly with methyl iodide yielding a 4-methylmercaptopyrylium iodide (24, Y = S, R = Me, X = I). Many alkylating agents were investigated by King et al. The kinetics of the reaction between 2,6-dimethyl-4-thiopyrone and substituted phenacyl bromides was found to be described by the Hammett... 

In order to prepare the cyclohexenaldehyde 8, 3-hydroxy-2-pyrone 14 and ethyl 4-hydroxy-2-methyl-2-butenoate 15 are subjected to a Diels-Alder reaction in the presence of phenylboronic acid which arranges both reactants to the mixed boro-nate ester 19 as a template to enable a more efficient intramolecular Diels-Alder reaction with optimal control of the regiochemical course of the reaction. Refluxing in benzene affords the tricyclic boronate 20 as primary product. This liberates the intermediate cycloadduct 21 upon transesterification with 2,2-dimethylpropane-l,3-diol which, on its part, relaxes to the lactone 22. Excessive i-butyldimethyl-silyltriflate (TBSTf) in dichloromethane with 2,6-lutidine and 4-7V,A-dimethyl-aminopyridine (DMAP) as acylation catalysts protects both OH goups so that the primary alcohol 23 is obtained by subsequent reduction with lithiumaluminum-hydride in ether. 

Both compounds are synthetically accessible through carboxylation of the corresponding anionic species. The propiolate obtained in the mono-carboxylation was not directly isolated, but esterified in situ with methyl tosylate. The radical scavenger 2,6-di-tert-butyl-p-cresol was added to minimize the risk of radiation-induced polymerization reactions. Literature reports of reactions on isolated carbon-14 labeled propiolates or acetylenedi-carboxylates are few, an exception being that of the catalytic hydrogenation of dimethyl [2,3- " C2]acetylenedicarboxylate , but the suitabUity and versatility of their unlabeled and carbon-13 labeled counterparts have been confirmed in numerous publications. Their reaction with a-pyrones, for example, could furnish doubly labeled aromatic systems with well defined substitution patterns, as demonstrated by the synthesis of methyl [ 1,3,a- C3]-salicylate (25), an intermediate in the preparation of [ CsJpindolol . 

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-... 

Anionic [ReOBr3(L)] and neutral [ReOBr(L)2] complexes are formed by reactions of [NBu4][ReOBr4] with 2-methyl-3-oxy-4-pyrone or l,2-dimethyl-3-oxy-4-pyridinone which act as monobasic ligands. The monoligand complex can be regarded as an intermediate in the formation of the bis-chelate complex. 

Pyrone derivatives, 3,6-dimethyl-4-hydro5g -2-pyrone and 4-meth-oxy-6-methyl-2-pyrone, were hydrogenated over 5% Pd-titania modified with Cnd and Cn. During the reaction the quinuclidine iV-atom of the alkaloid modifier can interact with the acidic OH group of 3,6-dimethyl-4-hydro5g -2-pyrone... 

To a flask equipped with a condenser were added 70 g 2-methyl-5-methoxy y-pyrone (0.2 mol) and 31 g dimethyl sulfate (0.25 mol). The mixture was heated at 50°C for 2 h, and was then poured into 3 eq. of ice-cooled 20% HCIO4. After 2 h, the pyronium perchlorate salt was filtered and added to 175 mL 10% ammonium carbonate solution saturated with ammonium sulfate. After the reaction, the product was extracted with EtOAc. The product was obtained via evaporation of solvent and vacuum distillation, b.p. 75-80° C (1 mmHg). The product can also be converted to hydrochloride salt, m.p. 164°C. 

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