2-Pyrone imines

Isoamyl nitrite hydrogen chloride 2-Pyrone imines... 

Hexamethyl-2,4-cyclohexadienone in ether, through which methylamine has been bubbled 45 min. at room temp., irradiated 3.5 hrs. with the Pyrex-filtered light of a 450 w. Hanovia lamp dienamide (Y 89%) in glacial acetic acid flushed with Ng, satd. with HCl, isoamyl nitrite added dropwise, and heated 4.5 hrs. at 100° -> permethyl-2-pyrone imine (Y 67%) heated 4 hrs. at 160-180° with 5 3 aq. 20%-NaOH-methanol permethyl-2-pyridone (Y 74%). H. Hart, D. A. Dickinson, and W. Y. Li, Tetrah. Let. 1975, 2253 ring opening s. a. G. Quinkert et al., B. 109, 1332 (1976). 

Concerning the transformation of substituents, a special note should be made on a series of ring-closure reactions carried out on the side chain of some [l,2,4]triazolo[4,3-r]benzo[l,2,3]triazines published by Moustafa <2001SC97>. The results are summarized in Scheme 18. This scheme shows that by transformation of the R group attached to the sulfur atom of derivative 97 a fairly large set of cyclic products - involving thiazolidone 97a, [l,2,4]triazole 97b, coumarone and its imine 97c and 97d, respectively, benzoxazylpyrane, 97e, thiophene 97f, and cyclopenta- or cyclohexa-fused protected pyrone 97g substituents - have been obtained. 

There is little or no well-defined chemistry of 2-amino-pyrones, -pyrylium salts, etc., because of the ease with which these systems undergo ANRORC reactions to give substituted pyridones (equation 88). 4-Aminopyrylium salts are reversibly deprotonated to the imines (equation 89). Amino groups in the 3- and 5-positions of pyrones can be diazotized normally. 

Nitron 7Z)-N-Methyl-C-(4-nitro-phenyl)- E14b, 1463 (aus Imin) Pyrimidine 5-(2-Carboxy-ethenyl)-4-hydroxy-2-methyl- E9b/2, 48 (5-COOR — 2H — pyron + Acetamidine)... 

Certain ylides react with arylcyclopropenones via 1,4-addition to give a-pyrone and its heterocyclic derivatives the formation of the l,3-oxazin-6-one 336 from reaction of the pyridine-iV-imine 335 with 14 is representative. Pyrimidones, pyridines and related... 

The useful conversion of 4-pyrones into 4-imines on reaction with tosyl isocyanate may involve a 2 + 2 cycloadduct, as shown, from which carbon dioxide is then lost. 5 ... 

Diels-Alder reactions have also been reported in which phosphaal-kynes function as the dienophile component to yield phosphaben-zenes. An AMI semiempirical study comparing aU nes and phosphaalkynes as dienophiles has been published recently. It has also been reported that ethyl cyanoformate and p-toluenesulfonylcy-anide undergo Diels-Alder reaction with some pyrones to yield pyrido[3,4- >]-indoles 14 and isoquinolines 15. This methodology was limited in scope, however, and it could not be extended to other nitriles (R = COPH, Me, Ph, Py, MejN) nor to imines. 

Carbon-heteroatom double bonds can also participate in this reaction. These include both carbonyl compounds (Scheme 11.37) and imines (Scheme 11.38). Addition to aldehydes is co-catalysed by tin(II) or indium(III) salts. Under these conditions, tetrahydrofiirans are obtained. The presence or absence of the co-catalyst can also switch the reaction from one mode to another (Scheme 11.39). An indium cocatalysed cycloaddition to a 7-pyrone aldehyde 11.117 was used in a synthesis of aureothin 11.122 and A-acetylaureothamine 11.123 (Scheme 11.40). Cross-metathesis of the exo-cyc ic alkene 11.118 allowed a subsequent Suzuki coupling with a gem-dibromide 11.120 that showed the expected selectivity (Section 2.1.4.2). This reaction required the use of thallium ethoxide as the Lewis base to suppress the formation of side products. A Negishi coupling completed the synthesis of aureothin 11.122. Reduction and acylation of the nitro group yielded A-acetylaureothamine 11.123. The latter compound is active digainst Helicobacter pylori, a bacterium behind stomach ulcers. 

Cycloaddition Reactions. Dichloroketene, A-sulfonyl-imines, diazomethane, and aroylketenes add to (1) to provide good yields of a cyclobutenone, 2-azetines, a pyrazole, and 4-pyrones, respectively. Cooligomerization of three molecules of (1) with one molecule of carbon dioxide proceeds in the presence of Ni catalyst, giving a 2-p5Tone derivative in 90% yield. ... 

Thus, we have developed catalytic asymmetric aza Diels-Alder reactions of imines with alkenes using a chiral lanthanide Lewis acid, to afford 8-hydroxyquinoline derivatives in high yields with high diastereo- and enan-tioselectivities. The characteristic points of this reaction are as follows, (i) Asymmetric aza Diels-Alder reactions between achiral azadienes and dienophiles have been achieved using a catalytic amount of a chiral source, (ii) The unique reaction pathway, in which the chiral Lewis acid activates not dienophiles but dienes, is revealed. In most asymmetric Diels-Alder reactions reported using chiral Lewis acids, the Lewis acids activate dienophiles [64, 65]. However, inverse electron-demand asymmetric Diels-Alder reactions of 2-pyrone derivatives have been reported [72]. (iii) A unique lanthanide complex including an azadiene and an additive, which is quite different from the conventional chiral Lewis acids, has been developed. 

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