Carbon dioxide 4-pyrones

Hiphenylketene has been reported to react with 2,6-dimethyl-pyrone affording the diphenylmethylenepyran (28, R = R = Ph) after splitting off carbon dioxide. 

The only example of the preparation of a benzazepine from a l//-azepine involves the loss of carbon dioxide from the initially formed [4 + 2] cycloadduct of methyl 2-pyrone-5-carboxylate (8) with ethyl l//-azepine-l-carboxylate (7).106 3-Ethyl 7-methyl 3//-3-benzazepine-3,7-dicar-boxylate (9) is formed as the minor product along with the [6 + 6] 7i-dimer 10 of the l//-azepine. 

Whereas electronically activated 2-pyrones can react thermally in both normal and inverse electron-demand Diels-Alder cycloaddition, 2-pyrone by itself requires thermal conditions that are so vigorous that they cause spontaneous extrusion of carbon dioxide from the bicyclic cycloadduct [61]. 

The [2-I-2-I-2] cycloaddition reaction of diynes 40 and carbon dioxide 41 were successfully catalysed by a NHC-nickel (Scheme 5.12) [15]. The NHC-Ni complex was prepared in situ from [NiCCOD) ] and two equivalents of carbene. Pyrones 42 were obtained in excellent yields at atmospheric pressure of CO and mild reaction conditions. 

Pyrones are useful dienes, although they are not particularly reactive. The adducts have the potential for elimination of carbon dioxide, resulting in the formation of an aromatic ring. Pyrones react best with electron-rich dienophiles. Vinyl ethers are frequently used as dienophiles with pyrones. The regiochemical preference places the dienophile donor ortho to the pyrone carbonyl. 

So far, carbon dioxide insertion has found limited use to form C—C bonds. Dialkyl pyrones have been obtained in low yield (up to 9%) from alkylacetylenes, probably via C02 insertion into a metallacycle 198), e.g. ... 

Isochromones lose carbon dioxide on heating via retro-Diels-Alder pathway to result in o-quinodimethanes (equation 81)1241,129. An isochromone route to podophyllotoxin derivative has been described (equation 82)130. Diels-Alder adducts of a-pyrone readily extrude carbon dioxide on thermal activation to furnish cyclohexadienes, which are useful substrates in tandem Diels-Alder reactions (equation 83)131. 

Guitian and colleagues126 performed some Diels-Alder reactions between in situ generated cyclohexyne and several a-pyrones. The reactions were performed at 100° C which resulted in immediate loss of carbon dioxide from the primary cycloadducts. Reaction yields were generally above 80%. The reaction between 184 and cyclohexyne, derived from 185, to give 186 has been depicted in equation 51. 

An ytterbium binaphthol catalyst was successfully applied in the cycloaddition reactions of 3-carbomethoxy-2-pyrone (454) with O- and S-subsli luted olefins like 455 and 280d. Upon heating, the products lost carbon dioxide to yield chiral cyclohexadienes 456 (equation 136). S -substituted olefins generally gave higher ee values than the corresponding O-substituted ones. 

The indole a-pyrones 376 (R = Me or Et) acted as dienophiles with diethylacetylenedicarboxylate producing the 1-substituted carbazole-2,3-diesters in good yield by loss of carbon dioxide from the initial adduct. ... 

Photolysis of 2-oxetanones gives decarboxylative cleavage to alkenes, similar to pyrolysis, but subsequent photoaddition reactions of the alkene product may lead to complex reaction mixtures. A very useful example of 2-oxetanone photolysis is that of 5-oxabicyclo[2.2.0]oct-2-en-6-one, the photoisomer of a-pyrone when it was irradiated in a argon matrix at 80 K, carbon dioxide and cyclobutadiene were formed (equation 7) (73JA1337). 

Rh(diphos)(fj-BPh4) has been found to be a methylacetylene oligomerization catalyst which produces a variety of linear and branched dimers, as well as linear and cyclic trimers. When the reaction was carried out in the presence of carbon dioxide, a small quantity of 4,6-dimethyl-2-pyrone was also produced (115). 

Recently we have developed a more general approach to molecules exemplified by III. Thus the Diels-Alder cycloaddition of alkyne II and ct-pyrone, followed by aromatization by loss of carbon dioxide, led to the isolation of III (72%) (5). Alkyne II was obtained in high yields, in two steps from dichloroacetylene and triethylphosphite via Arbuzov-type reactions (5). Since the intermediate chloroalkyne phosphonate I was isolable (90%), phosphorus nucleophiles other than triethylphosphite could be used to give unsymmetrical alkyne diphosphoryl species. We have demonstrated this approach by the reaction of I with PhaPOEt and PhP(OEt)2 (5). 

Pyrones behave as dienes and react with bismaleimides giving biscycload-ducts [62-65]. By heating, carbon dioxide extrusion takes place with formation of bisdienes. This reaction was used to prepare a polyimide with a bicyclooctene structure [51] (Fig. 15). 

The dialkyne/carbon dioxide copolymerisation is controlled by the relative rate of inter- and intramolecular cyclisations of the dialkyne the former is favoured when the number of methylene groups in the monomer R C C (CH2)X C = C—R is equal to 3,4 or 5 (x 3—5), but the intermolecular cyclisation of the dialkyne is favoured to effect 1 1 cycloaddition copolymerisation of the dialkyne and CO2 to a poly(2-pyrone) when the number x has other values [91 96]. 

The mechanism of ring formation from monoalkyne and heterocumulenes, catalysed by Ni(0) complexes, Lx Ni(0), has been proposed to involve one-step cycloaddition scheme (10) [103] and scheme (11) [104, 105] show the formation of the 2-pyrone ring in the alkyne reaction with carbon dioxide and the 2-pyridone ring in the alkyne reaction with isocyanate respectively ... 

This has been essentially accomplished in a case of the diene-dienophile monomer polymerization (6, 7, 8, 16). This excellent work makes use of the reaction of such dienes as cyclopentadienones, a-pyrones, and thiophene dioxides with dienophiles. The resulting adduct, a protected or inactive diene, loses carbon monoxide, carbon dioxide, or sulfur dioxide at elevated temperatures to form a diene. Thus when a Ws-malei-mide reacts with one mole of a cyclopentadienone, the resulting adduct will lose carbon monoxide at 150—260° in an inert solvent to form an active intermediate diene-dienophilic monomer which readily poly-... 

The most successful polymerizations carried out by using a Diels-Alder step-growth reaction are those which generate a highly reactive A-B monomer in situ by the reaction of a bismaleimide with cyclopenta-dienone (12), 2-pyrone (6, 13), or thiophene dioxide (5) derivatives. The intermediate 1 1 adduct loses carbon monoxide, carbon dioxide, or sulfur dioxide, respectively, all to generate the same type of reactive AB monomer, which is converted rapidly to polymer. High molecular weight polymers are obtained (Reaction 4). 

The examples given in Scheme 2.121 are typical of the general approach used to prepare six-membered rings bearing functional groups at any position via the Diels-Alder route from a diversified set of dienes and dienophiles. The flexibility of this protocol is further illustrated in Scheme 2.122. Utilization of 2-pyrone derivatives as diene components opens an entry toward the preparation of 1,3-cyclohexadienes via Diels-Alder reactions followed by a ready elimination of carbon dioxide from the initally formed adduct. For example, reaction of 338... 

Another type of oxacyclic diene are the pyrones, e.g. (120), which react with typical dienophiles at temperatures ranging from 80 to 205 C with spontaneous loss of carbon dioxide (Scheme 31). 

An intramolecular version of this process has been described, leading to bicyclic 2-pyrones. Diynes in which both alkyne functions are internal and are linked by three-, four- or five-atom chains cycloadd to carbon dioxide in the presence of catalytic Ni° and various trialkylphosphines (equation 51). Terminal diynes require stoichiometric metal and give lower yields, however. Extensive studies of ligand effects on yield and chemoselectivity have established a broad scope for the process and pointed out important practical differences between it and the intermolecular reactions described above. ... 

The use of Diels-Alder-type cycloaddition reactions is the most intensively investigated cycloaddition approach to the design of ladder polymers in a concerted process. Another methodology was published by Tsuda and coworkers [52, 53, 54]. They developed a nickel (0)-catalyzed [2 + 2 -l- 2] cycloaddition copolymerization of cyclic diynes 38 with heterocumulenes (like carbon dioxide or isocyanates 39). The soluble ladder-type products - poly(2-pyrone)s and poly(2-pyridone)s 40 - possess molecular weights M of up to 60000, corresponding to a Dp > 200. Unfortunately, the products formed were contaminated by nickel salts originating from the catalyst used Ni(COD)2. 

The transformation of a-pyrones to benzene and carbon dioxide upon their cycloaddition with acetylene, and the reaction of 2//-pyrans to yield benzoic acid derivatives when exposed to acetylenecarboxylates are conceptually related processes (see Scheme 2.3). These are, strictly speaking, retro Diels-Alder reactions," some of which embody interesting mechanistic problems. 

Ditertiary phosphane complexes of nickel were found to be effective in the formation of pyrone 108 by cyclocotrimerization of alkynes with carbon dioxide. The formation of the nickelacyclopentadiene 105 from two moles of alkyne and a nickel complex is followed by CO2 insertion into a nickel-carbon bond to give the oxanickelacycloheptadienone 106, which then eliminates 108 with intramolecular C—O coupling. Another route involving [4 + 2] cycloadditions of 105 with CO2 in a Diels - Alder reaction to give 107 cannot be ruled out but is less probable because CO2 does not undergo [4 + 2] cycloaddition with dienes. Addition of another alkyne to 105 results in the formation of a benzene derivative (Scheme 38). ... 

The pressure-promoted [4 + 2]-cycloaddition of 2-pyrone 318 with cyclo-propenone ketal 377 (25 C, 6.2 kbar) affords a mixture of reaction products exo-adduct 378a, cycloheptatrienone ketal 379, and cycloheptatrienone 380 (resulting from Si02 hydrolysis of 379), each representing the product derived from the Diels-Alder reaction of 2-pyrone 318 with cyclopropen-one ketal 377 (86JA6695). The mdo-adduct 378b loses carbon dioxide upon depressurization, while the exo-adduct 378a is thermally stable. 3-... 

The Pd-catalyzed carbonylation of aryl halides (cf Section 2.1.2) occurs with high turnover numbers and reaction rates in SCCO2 as the solvent using standard precursor complexes and commercially available phosphine or phosphite ligands [30]. The generally better performance of the phosphite-based catalysts was attributed to their better solubility in the reaction mixture, but the formation of Pd carbonyl complexes was also mentioned as a possibility. The [Ni(cod)2]/dppb system (dppb = l,4-bis(diphenylphosphino)butane) was investigated in an early study as a catalyst for the synthesis of pyrones from alkynes and CO2 under conditions beyond the critical data of carbon dioxide [31]. Replacing dppb with PMcs results in a system with better solubility and catalytic performance, albeit catalyst deactivation remains a problem [3 c, 15]. 

The photoisomer of a-pyrone was synthesized in order to prepare a cyclobutadiene.64 It was pyrophoric in air at room temperature and its mass spectrum was similar to that of furan. Flash thermolysis was reported to give many compounds, as shown in Scheme 26.65 No evidence for the formation of cyclobutadiene was obtained, but carbon dioxide and acetylene products seem to support its intervention. 

Corey and Streith found that the photolysis of pyrone and pyridone yields bicyclo[2.2.0]hexane analogs 133). They suggested that the product has a structure formed by formal addition of carbon dioxide to cyclobutadiene (126). 

Reaction of the pyrones 4 and 5 with the cyclopropenone acetal 6 gave cycloheptatrienes 7 and 8, respectively, by a similar sequence of cycloaddition followed by elimination of carbon dioxide. 

---