Pyrones 4 + 3 cycloaddition reactions

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. 

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. 

These phenomena can be illustrated by the cycloaddition reactions of fulvenes with electron-deficient a-pyrones. In general, the Diels-Alder reactions of electron-deficient dienes such as 458 with 6-alkyl substituted fulvenes favor addition across one of the endocyclic... 

The first report of a cycloaddition reaction in the presence of an optically active catalyst13 appeared in 198314a. The dienes 14 add to benzaldehyde in the presence of 1 mol% of the chiral lanthanide NMR shift reagent Eu (hfc)3, i.e. tris[3-(heptafluoropropyl-hydroxymethylene)-(+)-camphorato]-europium(III), to give, after treatment with trifluo-roacetic acid, the dihydro-y-pyrone 15 enriched in the (R)-enantiomer, the degree of... 

Two approaches, based on furan, have found wide application in carbohydrate synthesis. Cycloaddition reactions of furan with 2-substituted acrylonitrile or acrolein lead to oxabicycloheptanes which, in tnm, can be transformed to monosaccharides. On the other hand, furfuryl alcohols can be converted—either by the Clauson-Kaas reaction or by mild oxidation—into 5,6-dihydro-4-pyrones, suitable for easy functionalization to sugars. 

It is noteworthy that there had been very few reports of [4 + 2] cycloaddition reactions using y-pyrones as dienophiles when our studies commenced.15 The only examples of 4-//-benzopyran-4-ones employed as dienophiles were 3-acylchromones reported by Wallace,16 while the first examples of 3-acyl-4-//-pyran-4-ones in [4 + 2] cycloaddition reactions were described in the preparation of reduced flavones.17... 

The usage of a,P-unsaturated iminium salts clearly represents a general and efficient solution leading to 2//-pyranyl products exclusively via the C-1,2-addition pathway. The reaction of the pyrone 22 led to the pyran product 23 in a much-improved yield relative to Moreno-Manas s study, and also gave previously unknown products 30b and 30c under these reaction conditions [Scheme 5]. The significance of using preformed a,P-unsaturated iminium salts to control regioselectivity of this formal cycloaddition reaction was recently validated in an account reported by Cravotto.39... 

However, under our conditions20,35,37 as well as a variety of other conditions,53,54 the formal [3 + 3] cycloaddition reaction of the iminium salt 51 with pyrone 12 failed to provide any desired pentacycle 52. This failure prompted us to think that 51 might be sterically too demanding, thereby obstructing the reaction pathway. 

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. 

A DFT study at the B3LYP/6-31G level on the intramolecular 5 + 2-cycloaddition reactions of 3-OR (R = SiMe3, H, CHO, Me)-substituted / -hydroxy-y-pyrones tethered to alkenes and alkynes has been reported. Calculations indicate an initial transfer... 

Recently, the reaction of masked ortho-benzoquinone [92] with C60 was tested [93]. The [4+2] cycloaddition reaction of such electron-deficient dienes with fullerenes resulted in the formation of highly functionalized bicyclo [2.2.2] octenone-fused fullerenes. The reactants were generated in situ by the oxidation of the readily available 2-methoxy phenols with hypervalent iodine agents. For the several different masked ortho-benzoquinones that were tested, it was found that the yield of the cycloadducts depends on the nature of the starting materials and the reaction conditions. Other Diels-Alder reactions of such electron-deficient dienes with electron-poor fullerenes involved tropones [94], 1,3-butadienes substituted with electron-withdrawing groups [95], and 2-pyrone [96]. 

CONTENTS Preface, Mark Lautens. Photocyclization and Photocycloaddition Reactions of 4- and 2-Pyrones, Frederick G. West. Intramolecular [4+3] Cycloaddition Reactions, Michael Harmata. Lewis Acid Catalyzed [2+2] Cycloaddition Reactions of Vinyl Sulfides and Their Analogues Catalytic Asymmetric [2+2] Cycloaddition Reactions, Koichi Narasaka and Yujiro Hayashi. Vinylboranes as Diels-Alder Dienophiles, Daniel A. Singleton. Preparation and Exo-Selective [4+2] Cycloaddition Reactions of Cobaloxime-Substituted 1,3-Dienes, P... 

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... 

The HDA reaction allows for rapid access to chiral six-membered heterocyclic structures that serve as valuable intermediates in organic synthesis. The first highly enantioselective HDA reaction promoted by a chiral hydrogen bond donor was reported from the Rawal laboratory. While investigating the cycloaddition reactions of amino-siloxy diene 115, it was observed that this diene was exceptionally reactive to heterodienophiles, and underwent HDA reactions with various aldehydes at room temperature, even in the absence of any added catalyst (Scheme 6.14). Subsequent treatment of the intermediate cycloadducts (116) with acetyl chloride afforded the corresponding dihydro-4-pyrones (117) in good overall yields [101]. Further studies of this reaction revealed a pronounced solvent effect,... 

Besides the common oleftnic dipolarophiles, other unsaturated systems have been evaluated in cycloaddition reactions of zwitterionic TMM-Pd complexes, including polyenes and acetylenes. While acyclic electron-poor dienes generally gave mixtures of five- and seven-membered rings [48], a limited number of selective [3 + 4] and [3 + 6] cycloaddition reactions have been achieved with cyclic polyenic substrates as illustrated by formation of cycloadducts 41 and 42 from pyrone [49] and tropone [50], respectively (Scheme 16). On the other hand, activated alkynes have failed to produce the corresponding cyclopentene derivatives [51]. 

Pyrones, synthesis via transition metal-aided cycloaddition reactions of alkynes 88CRV1081. 

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. 

Cycloaddition reactions in pyrone derivatives A and D and activated pyrones B, C, and E (Scheme 46) with pyrylium-ion character are important in synthetic methodology because the initial bicycloadducts F-J, which can have up to four contiguous stereocenters as well as variously membered rings (3, 4, 5, 6, 7, etc.), can be formed in one procedure. These bicycload-... 

Hsung 2 used a [4 4-2] cycloaddition reaction of a y-pyrone to synthesize the tetracyclic core of arisugacin, a novel inhibitor of acetylcholinesterase. He noticed an unexpected concentration effect on the stereoselectivity in the reactions of 3-cyano-/-benzopyrone derivatives with electron-rich dienes. When 1-methoxybutadiene (187) reacted with /-benzopyrone 188, for example, the ratio between endo adduct 189 and exo adduct 190 depended on the concentration of 188, as demonstrated by the data given in Table 4 (equation 52). Raising the concentration of 188, while keeping the diene concentration twice as high, caused the reaction to become less endo selective. Variation of the diene concentration, while keeping the /-benzopyrene concentration constant, did not demonstrate a clear trend. 

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