A-Pyrone, irradiation

Pyridine, 1,2,3,4-tetrahydro-l-methy1-2-phenyl-, 54, 93 a—Pyrone, irradiation of, 50,... 

Propanedithiol, 50,72 Propargyl bromide, coupling with an organo-copper reagent, 50, 98 a-Pyrone, irradiation of, 50, 23... 

Barrelene was obtained via a double Diels-Alder reaction from a-pyrone with methyl acrylate (H.E. Zimmerman, I969A). The primarily forming bicyclic lactone decarboxylates in the heat, and the resulting cyclohexadiene rapidly undergoes another Diels-Alder cyclization. Standard reactions have then been used to eliminate the methoxycarbonyl groups and to introduce C—C double bonds. Irradiation of barrelene produces semibullvalene and cyclooctatetraene (H.E. Zimmerman. 1969B). 

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). 

Irradiation of the epoxide (53) gave a pyrone (54) and it has been noted that the mass spectra of the two compounds were almost identical at 40° (Bursey et al., 1967). 

Cyclobutadieneiron tricarbonyl may also be produced through reaction of 3,4-dichlorocyclobutene with Na2Fe(CO)4,5 and by irradiation of a-pyrone followed by treatment with Fe2(CO)9 . The method outlined here is the most convenient procedure especially when considerable quantities (10 g. or more) of cyclobutadieneiron tricarbonyl are required. The analogous reaction of derivatives of 3,4-dihalocyclobutenes with Fe2(CO)9 affords the corresponding cyclobutadieneiron tricarbonyl complexes. Cyclobutadieneiron tricarbonyl can be oxidized to generate cyclobutadiene in situ.7... 

The irradiation of a-pyrone in the presence of Fe(CO)5 results in formation of (cyclobutadiene)tricarbonyliron and the yellow (a-pyrone)-tricarbonyliron, m.p. 140°-141°C (141). This latter complex can also be... 

The reverse reaction has also been observed Irradiation of the Dewar a-pyrone in an argon atmosphere affords cyclobutadiene which is dimerized to tricyclo-[4.2.0.02-5]octadiene upon heating or photolyzed to acetylene (127) 134 135). 

Irradiation in methanol of 14 -hydroxy-17/ -a-pyrones, e.g. bufalin (11) and its 16j3-acetoxy- and lla-hydroxy-derivatives, bufotalin and gamabufotalin, gives rise to the 14, 21-oxides, e.g. (36), whose structures were confirmed by synthesis. Oxide formation is impossible in the absence of a 14/ff-hydroxyl and thus, with resibufogenin (10) and 14a-artebufogenin (37), the reaction is terminated by the addition of solvent leading to the vinyl ethers (38) and (39) respectively. 

Linearly Conjugated Dienones. - Two reaction modes have been detected following a study of the photoreactivity of 4,6-dimethyl-oc-pyrone in a matrix. Irradiation with A > 315 nm brings about both ring-opening to yield a ketene and valence isomerization to produce l,5-dimethyl-2-oxa-3-oxobicyclo[2.2.0]-hex-5-ene. ... 

The photochemistry of a-pyrone (57a) (Scheme 10) was the subject of some of the earliest matrix-isolation studies of organic species. This system still has some interest for further study, however, especially on account of the success that DFT computations have had in assisting with the assignments of matrix IR spectra. Recently, the matrix photolyses of both a-pyrone and its 4,6-di-methyl derivative (57b) have been investigated with the aid of DFT calculations. When (57a) is irradiated (A > 285 nm) in Ar or Kr matrices, rapid formation of rotamers, (58a) and (59a), of the Z isomer of the ring-opened... 

It is severely angle-strained in addition to being antiaromatic. Cram and co-workers generated 52 inside 15 by irradiating a-pyrone hemicarceplex 15053. 

Substituted cyclobutadienes have been made by irradiation of the appropriately substituted derivatives of a-pyrone [57,68], (X)... 

Irradiation of 4,6-dimethyl-a-pyrone immobilized in a guanidinium-sulfonate-calixarene gives rise to a 4,6-dimethyl-Dewar-p-lactone, a carboxyl zwitterion and 1,3-dimethylcyclobutadiene, both in the solid crystalline state and in aqueous solution. Diarylmethyl carbocations have been generated within the cavities of non-protic zeolites (LiY, NaY, KY, RbY, CsY, and NaX) via laser-mediated decarboxylation of diarylacetic acids. 

Cram and coworkers generated 92 inside 31 by irradiating a-pyrone hemicarceplex 31 93. 

Cyclobutadienetricarbonyliron can also be prepared by irradiation of photo-a-pyrone with Fe(CO)5. It is a pale yellow solid, m.p. 26°C. 

When the lactone (Chapter 9) 2H-pyran-2-one (2-oxo-2H-pyran, a-pyrone) is irradiated below 290 nm (with a pyrex filter to form a bicyclic lactone and, subsequently, without a filter to let in higher energy radiation) while frozen in an argon (Ar) matrix at 8 K, carbon dioxide (CO2) is eliminated and a species is formed whose IR spectrum (with only C-H and C-C bands and a weak band at 1523 cm ) is in concert with a rectangular (not square planar) structure. On warming, decomposition to dimer (mdo-tricyclo[2.0 . 0 ]octa-2,6-diene) and ethyne (acetylene, HC=CH) occurs. 

When irradiated with UV light, a-pyrone was known to liberate CO2 to form cyclobutadiene ... 

The anti-aromaticity of cyclobutadiene (46) has made this molecule a topic of choice in research by matrix isolation, where it occupies an important chapter. It was initially obtained by matrix photodecomposition of a-pyrone by a prolonged irradiation at 20.4 K in argon matrix via a series of intermediates (Scheme 6.21). The peaks marked in the unexpectedly simple spectrum obtained were attributed to such species (Fig. 6.30). To finally establish the structure of this molecule, rectangular or square, many further experiments were required, in particular the generation from a different precursor and testing the effect of specific deuteration in the spectrum [8]. 

Fig. 6.30 IR spectrum of richest portion of photolyzed compound affra- 75 h of irradiation of a sample of a-pyrone in an argon matrix. Reprinted with permission from [8]...

The photodecarboxylation of lactones usually involves homolysis of the OC bond (i to the carbonyl group, to generate a biradical after loss of COj (see Scheme 1). " This process has been used to make strained ring systems (such as cyclopropanes, cyclobutenes, and cyclophanes) via intramolecular CC coupling of the biradicals formed. Some applications of this reaction are shown in Scheme 3. Thus, upon irradiation of a-pyrone, cyclobutadiene is formed by a two-step decarboxylative process (Scheme 3A). Likewise, the natural product aubergenone has been synthesized from a-santonin using the photodecarboxylation of a y-lactone to a cyclopropane as the key step (Scheme 3B). Photolysis of lactones has also been used for the preparation of cyclophanes (Scheme 3C). ... 

A pentafulvalene has been trapped as a diiron complex using Fe(CO)5. Thiophene dioxide has also been stabilized as its tricarbonyliron complex. The irradiation of a-pyrone (45) in the presence of Fe(CO)5 gives cyclobutadiene stabilized by complexation with the Fe(CO)3 moiety (46), together with the (ct-pyrone)Fe(CO)3 complex (47) (eq 29). ... 

Diels-Alder cycloaddition of 5-bromo-2-pyrone with the electron-rich tert-butyldi-methylsilyl (TBS) enol ether of acetaldehyde, using superheated dichloromethane as solvent, has been investigated by Joullie and coworkers (Scheme 6.90) [188]. While the reaction in a sealed tube at 95 °C required 5 days to reach completion, the anticipated oxabicyclo[2.2.2]octenone core was obtained within 6 h by microwave irradiation at 100 °C. The endo adduct was obtained as the main product. Similar results and selectivities were also obtained with a more elaborate bis-olefin, although the desired product was obtained in diminished yield. Related cydoaddition reactions involving 2-pyrones have been discussed in Section 2.5.3 (see Scheme 2.4) [189]. 

A 2-step route to oxygenated bicyclo[n.3.0]alkanes via bicyclic 4-pyrones such as 133 has been reported (equation 2)62. Irradiation in hydroxy lie solvents caused ring contraction to the zwitterion, followed by solvent incorporation to give fused bicyclic cyclopentenone 134. Good regioselectivity in favor of solvent capture at the more substituted oxyallyl terminus was seen in differentially substituted examples, presumably due to increased... 

As noted above, formation of a furan [4 + 3]-cycloadduct during irradiation of a 4-pyrone was advanced as evidence for the zwitterionic intermediate. This process can be moderately efficient (equation 4)68, and can be envisioned as an approach to substituted cyclooctanoids. Besides the formation of three new carbon-carbon bonds, an additional attractive feature is the complete diastereoselectivity, arising from a compact [4 + 3]-cycloaddition transition state with approach from the face opposite the epoxide. However, the generality of the intermolecular reaction is limited, as competing [2 + 21-photodimerization, solvent trapping and rearrangement often predominate58. 

A unimolecular pyrolysis/rearrangement sequence leading to stable reaction products without the need for nucleophiles was reported by Katritzky et al. Thus, 6-(2-oxoalkyl)-l,3-dioxin-4-ones, when heated or irradiated, gave 6-substituted-4-hydroxy-2-pyrones in good yields <2005JOC4854>. a,7-Dioxoketenes are discussed as intermediates (Scheme 25). 

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