Cycloadditions, , of 2-pyrones

Posner G. H., Bull D. S. Recent Advances in Control of Absolute Stereochemistry in Diels-Alder Cycloadditions of 2-Pyrones Recent Res. Dev. Org. Chem. 1997 1 259-271... 

Posner G. H. Stereocontrolled Synthesis of Functionalized Cyclohexenes Via Diels-Alder Cycloadditions of 2-Pyrones and 2-Pyridones-Applications to Synthesis of Physiologically Active Compounds in Stereocontrolled Org. Synth. 1994 177, Ed. Trost B. M., Pb. Blackwell Oxford... 

CONTENTS The Synthesis of Seven-Membered-Rings General Strategies and the Design and Development of a New Class of Cycloaddition Reactions, Paul A. Wender and Jennifer A. Love. Recent Advances in Diels -Alder Cycloadditions of 2-Pyrones, Benjamin T. Woodard and Gary H. Posner. The Inter- and Intramolecular [4 + 4] Photocycloaddition of 2-Pyridones and Its Application to Natural Product Synthesis, Scott McN. Sieburth. 3 + 4 Annulations Between Rhodi- J s... 

Intramolecular [4+4] cycloaddition of 2-pyrones with furans has been studied by West and coworkers (Sch. 39). Irradiation of 172, with a furan tethered to C6 of the pyrone, leads to a mixture of trans 173 and cis 174 [115]. Interestingly, the Cope rearrangement of the cis isomer 174 yields an equilibrium mixture with cyclobutane 175. The effect of a stereogenic center on the tether was also determined in this study. With the furan tethered to C3 of the triflate-substituted pyrone 176, the trans isomer 177 was formed with good selectivity and isolated in very good yield [116]. 

Pyrones can serve either as dienes (47r-component) or as dienophiles (2ff-component) in [4 + 2]-annulation reaction, depending on their reaction partner and their substituents. However, 4-pyrones, coumarins, and chro-mones react only as the 27r-component (dienophile) with electron-rich dienes. A review of the Diels-Alder cycloaddition of 2-pyrones and related compounds has appeared (92T9111). 

Often CO2 extrusion occurs at high reaction temperatures in the cycloaddition of 2-pyrones to dienophiles. Further elimination to form regiospeci-fically substituted aromatic compounds may also take place. (84JOC4033, 84JOC4045). (See Scheme 61.)... 

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-... 

Woodard, B. T., Posner, G. H. Recent advances in Diels-Alder cycloadditions of 2-pyrones. Adv. Cycloadd. 1999, 5,47-83. 

The asymmetric Diels-Alder cycloadditions of 2-pyrones have recently attracted much attention. Albeit much remains to be explored, a few successful strategies have been reported, which have been systematically addressed in a recent review <99AC47>. 

Scbeme 42. Synthesis and IMDA cycloadditions of 2-pyrones tethered with acrylamide... 

Cycloaddition of 2-pyrones 5 to vinyl ethers 6 serves a key reaction in the synthesis of the important chiron 9, being an A-ring precursor to la-hydroxyvitamin D3. The best diastereose-lectivities were obtained when the appropriate chiral auxiliary was used in the diene component22 or a stereochemically matched pair of chiral diene and chiral catalyst was applied22. 

The impact of (2 + 2)-cycloaddition and (2 + 2)-cycloreversion reactions of heterocyclic compounds on organic chemistry over the last 10 years is clearly illustrated by several examples. Various members of the important /Hactam antibiotics, penicillin and cephalosporin C, as well as structurally related heterobicyclic compounds have been obtained by (2+ 2)-cycloaddition of heterocycles with ketenes (Section II,D,l).n Intramolecular photochemical (2 + 2)-cycloadditions of 2-pyrones yield 2-oxabicyclo 2.2.01hex-5-en-3-ones, which upon further irradiation afford cyclobutadienes (Section III,D,2).12 Intermolecular (2 + 2)-cyclo-additions of vinylene carbonates with olefins and with acetylenes offer a simple route to cyclobutanes and cyclobutenes, respectively (Sections III,B,3 and 5).13 (2 + 2)-Cycloaddition and (2 + 2)-cycloreversion reactions have contributed substantially to the development of the chemistry... 

Of special interest are the photochemical intramolecular (2 + 2)-cycloadditions of 2-pyrones and 2-pyridones because the resulting 2-oxa- or 2-azabicyclo[2.2.0]hex-5-en-3-ones are precursors of cyclobutadiene and stable complexes of cyclobutadiene.12,238-261 In 1964 Corey and Streith238 reported the photochemical conversion of 2-pyrone (222) and N-methyl-2-pyridone (223) into the corresponding bicyclic isomers (224 and 225) at —20°. More recent studies performed at a temperature as low as 8°K with 2-pyrones in a matrix of argon or nitrogen showed that the intramolecular (2 + 2)-cycloaddition is only a minor photochemical pathway.12,260-262 The major reaction proved to be... 

RECENT ADVANCES IN DIELS-ALDER CYCLOADDITIONS OF 2-PYRONES... 

This review of recent progress in [4+2] cycloaddition of 2-pyrone dienes emphasizes control of relative and absolute stereochemistry. Discussion includes the development of mild reaction conditions and electronic matching of reaction partners, allowing the isolation of bicyclic lactones with control of relative stereochemistry. Control of absolute stereochemistry in these cycloadditions features a stereo-chemical control element on one or more of the following a pyrone diene, a dienophile, or a Lewis acid promoter. Applications of this methodology to synthesizing complex organic molecules are mentioned. 

It is noteworthy that the high pressure cycloaddition of 2-pyrone with fullerene CgQ, which yields isolable bicyclic lactone adducts in the absence of Lewis acid, has recently been reported. ... 

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