4-Pyrones closure

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. 

The analogous open chain carboxylic acid, Z-non-2-en-4-ynoic acid, when treated with 4-iodoanisole in the presence of a palladium-triphenylphosphine catalyst and potassium carbonate gave a mixture of three products, two of which were isolated (4.41.) z) the pyrone derivative arising from the attack of the anisylpalladium complex at the 4-position, followed by ring closure //) the furane derivative (major product) arising from the... 

Bromo-2-pyrones and 3-bromocoumarins give furan- and benzofuran-2-carboxylic acids by ring fission and subsequent closure, e.g. (228) — (229) (116) or (117) — (230). Pyrone rings are opened by aqueous acid in some cases, probably by successive protonation and attack of a water molecule, e.g. dehydroacetic acid (231) gives (233) which immediately forms (232) or (234) with HC1 or H2S04, respectively. 

Schulte and coworkers (62AP801) have prepared a number of 3-propargyl-4-hydroxy-2-pyrone derivatives (210) by condensation of /3-dicarbonyl compounds (208) with propargyl-malonyl chloride (209). On heating (210) with zinc carbonate, ring closure took place to give the corresponding furo[3,2-c]pyrone derivatives (211 Scheme 40). 

Reversibility of 67t-Electron Electrocyclic Ring-Closure. The high diastereoselectivity obtained in these reactions here is likely a result of the reversible 67t-electron electrocyclic ring-closure.20,37 41 The best evidence for the reversibility of this ring-closure is described in Scheme 12. We were able to isolate both the desired major isomer 10 and the minor isomer 44 from the formal cycloaddition reaction of the iminium salt 56 with pyrone 12. 

In an extension to the above methodology, a sequential 1,2-addition, dehydration, and in situ ring closure via a 67t-electron electrocyclic cyclization has been described in the context of the pharmacologically relevant natural products warfarin A, the arisugacins, merulidial, and isovelleral (Scheme 20) <2005CAR1287>. Carbohydrate-derived a,3-unsaturated enals were coupled with 4-hydroxycoumarin 188 and 4-hydroxy-6-methylpyran-2//-one 191 in the presence of proline catalysts to provide pyrone-annulated products of types 190 and 192. Stereoselective electrocyclic ring closure was observed only when hydroxyl functionality (at any of R -k ) on the enal was acyl protected. 

Various substituted cyclobutanes and cyclobutenes can be utilized in the synthesis of 2-pyrones . For example, 2-hydroxyl cyclobutylidenes 230 undergo base-promoted rearrangements to afford 2//-pyran-2-ones 231 via formation and ring closure of an intermediate 1,5-keto ester (Scheme 131) <2005TL8237>. 

The preparation of furans from a-pyrones has been reported.74-76 For ring contraction to occur it is necessary that a suitable leaving group should be present in the 3-position. An example is given by Broy and Mayer,76 where ring opening on 3-hydroxy-a-pyrone occurred in alkaline solution, followed by ring closure to the 2-furoic acid. This reaction was also applied to 3-bromo-a-pyrone. 

Carbomethoxy-2-pyrones. Ketone enolates (NaH or LDA) react with 1 with loss of methanol to give 3-carbomethoxy-2-pyrones (2) in generally good yield. The final ring closure can be promoted if necessary by addition of acetic anhydride or TsOH and elevated temperatures. 

The success of this strategy depends on the availability of open-chain materials containing all the carbon atoms possessing the required functionality. Borsche and Bodenstein adopted this type of closure to prepare pyrones of the Kawa family (29CB2515 62FOR131), but they used open-chain precursors obtained by degradation of the same Kawa pyrones in an attempt to elucidate their structures. 

Interesting variations of the Ol—C2 closure give rise to pyrones con-... 

The cychsation of an unsaturated 1,5-dicarbonyl compound produces pyrylium salts, providing of course that a suitable acidic medium is chosen - it must not contain nucleophihc species that would add to the salt, once formed. Acid-catalysed ring closure of 1,3,5-triketones produces 4-pyrones, as shown 2-pyrones are formed via the constraction of an a, 3-unsaturated ester that has a 5-carbonyl group. 

Pyrones result from the acid-catalysed closure of 1,3,5-tricarbonyl precursors. 

Formation of the 5,6-bond is also involved in the Claisen condensation between diethyl oxalate and an a,/3-unsaturated ester at its 7 position, to generate an intermediate in which ring closure via the ketone enol produces a 2-pyrone. ... 

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