Pyrones, maleimides

Keywords double [4+2]photocycloaddition, 4-acyloxy-2-pyrone, maleimide... 

Isocyanates are precursors of various heterocycles such as pyrones, maleimides [100], and 1,2,3-triazolidines, [101]. 

Kottner and Klemm115 studied the Diels-Alder polymerization of bismaleimides with 4,4/-dimethyl-6,6/-(octamethylene)di-2-pyrone. When the maleimide units were connected... 

As shown in Equation (4), the sensitized irradiation of a mixture of 4,6-dimethyl-2-pyrone and maleic anhydride, or of 4,6-dimethyl-5-ethoxycarbonyl-2-pyrone and maleimide, gives the mixed photoadducts (23) and (24) in ca. 20% yields (90BCJ3456, 92BCJ354). As with many other [2 + 2] cyclocondensation reactions involving heterocycles, these are thought to proceed via biradical intermediates. 

A similar reaction occurs with Srs-maleimides and a-pyrones (5). Under the same reaction conditions as employed for the cyclopenta-... 

Olefinic dienophiles such as maleic anhydride and maleimide add to 4,6-dimethyl-2-pyrone to afford mono Diels-Alder adducts. These on irradiation afford, by a photo-decarboxylation, a diene which can be trapped by a dienophile to afford the products (311). - - Acetone-sensitized photoaddition of the oxazolones (312) to maleic and dimethyl maleic anhydride yields the adducts (313). - ... 

Irradiation through Pyrex of crystalline mixtures of the pyrone (171) and maleimide (172) results in the formation of the (2 + 2) cycloadduct (173) which is different from the solution phase behaviour when products arising from a (2 + 4)-cycloaddition mode are obtained. The (2 + 4)-cycloadduct is unstable under the experimental conditions and rearranges into the (2 + 2)-cycloadduct... 

In addition decarboxylation of the (2 + 4)-adduct occurs to give a diene that is trapped as (175) by a second addition of maleimide. A further report by the same group states that the cycloaddition of maleimides with 2-pyrone carboxylates in the solid-state yields endo adducts. This is in contrast to the sensitized cycloaddition that leads exclusively to the exo products. 

Obata. T. Shimo, T. Yasutake. M. Shinmyozu, T. Kawaminami, M. Yoshida. R. Somekawa, K. Remarkable interaction effects of molecular packing on site- and stereoselectivity in photocycloaddition of 2-pyrones with maleimide in the solid state. Tetrahedron 2001, 57. 1531 -1541. 

In addition to metal-based catalysts, organocatalysts are also selective promoters of asymmetric Diels-Alder reactions. Several groups reported the use of cinchona alkaloid catalysts in standard Diels-Alder reactions. Deng combined 2-pyrones with a,P unsaturated ketones, while Bernard and Ricci focused on the reactions of vinylindoles with quinones and maleimides. Lectka reported enantioselective inverse electron demand hetero Diels-Alder reactions of ketene enolates and o-benzoquininone diimides catalyzed by a combination of benzoylquinidine and zinc triflate. For example, subjecting diimide 51 to the standard reaction conditions yields cycloadduct 52 as a single stereoisomer, which can be easily converted to... 

Recent advances in the control of absolute stereochemistry in Diels-Alder reactions of 2-pyrones with dienophiles have been reviewed. The position of the electron-withdrawing group on simple y-pyrones is a decisive factor in the 4 - - 2-cycloaddition of y-pyrones with Danishefsky s diene and with azomethine ylides. The solid-state photo-4 -I- 2-cycloadditions of 2-pyrone-5-carboxylates with maleimides give only the... 

Tetramethyldiphenylmethane)]-bis(maleimide) polymers with 4,6-dimethyl-5-carbethoxy-a-pyrone, 107 with 2,5-dimethyl-3,4-diphenylcydopentadienone, 113 Tetramethyldisiloxane diol, polymers with bis-... 

Irradiation of the 1 1 complex crystals of 4-methoxy-6-methyl-2-pyrone 35 and maleimide 36 gives 37 in 54% yield as a sole product, which is entirely different from the solution products 39 and 40. The peri-, site-, and stereospecific photoreaction in the solid state is considered to be brought about by two sets of the hydrogen bonding and CT stacking between 35 and 36 in the complex. 

Obata, X, Shimo, T., Yoshimoto, S., Somekawa, K., and Kawaminami, M., Peri-, site- and stereo-controUed photocycloaddition of 4-methoxy-6-methyl-2-pyrone with maleimide induced by the hydrogen bond and CT stacking in the sohd state, Chem. Lett., 181—182,1999. 

Photosensitized reaction of 15c with maleimide 19g at low temperature (-10 to -20°C) affords the exo-[4-1-2]-adduct 16 as a major product, together with the [2-1-2]-adduct 17 (Scheme 11). The same reaction was carried out at room temperature to afford the bis-adduct 27, which was formed via decarboxylation of 16cg followed by addition of another molecule of 19g, in addition to the formation of 17cg. The low-temperature photoreactions of 2-pyrones 15a, 15b with 19g or 19g also gives [4-1-2]-adducts 16ag, 16bg, and [2-1-2]-adduct 17bg. Similar photoreactions of 2-pyrones 15b, 15c with maleic anhydride afford exo-[4-1-2]-adducts (major products) and/or [2-1-2]-adducts. Photosensitized reaction of 15a with dimethyl acetylenedicarboxylate affords 1, 2, 4-tris(methoxycarbonyl)benzene 28 in 50% yield byway of decarboxylation of the [4-1-2]-adduct, which was difficult to isolate even from the reaction mixture obtained from the low-temperature irradiation (Scheme 12)." Similar reactions of 15a with... 

The x-ray crystallographic analysis shows that the distances between the reacting double bonds to yield 63 and 64 are ca. 4.2A and 4.0A, respectively,. The solid-state photoreaction of 2-pyrone-5-carbox-ylate 15 with maleimides 19 yields the endo-[4+2]-cycloadducts 65. This is in contrast to the sensitized photoreaction that leads exclusively to the exo-[4+2]-cycloadducts 66 (Scheme 24). Solid-state photoirradiations of 1 1 complex crystals formed between the 2-pyrone 15d and 4-((0-arylalkyloxy)- 2-pyrones 66a through 66e and maleimide 19g gives [2+2]-cycloadducts 67dg and 67a through 67e with exclusive stereoselectivity (Scheme 25). > ... 

These highly stereoselective cycloadditions also occur on photoirradiation of grind up mixtures of crystals of 2-pyrones with maleimide. On the other hand, the photoreactions in solution afford another type of [2-1-2] -cycloadduct 68 or bis-adducts 69 as major products. The latter products are formed through decarboxylation of the [4-1-2]-cycloadducts, followed by addition of another molecule of 19g. 

Figure 82.5 was reprinted from Tetrahedron, 58, T. Shimo, T. Uezono, T. Obata, M. Yasutake, T. Shinmyozu, and K. Somekawa, x-ray and MO analysis of highly stereoselective solid-state photocycloadditions of 2-pyrones with maleimide, p. 6114, 2002, with permission from Elsevier Science. 

Obata, T., Shimo, T., Suishu, T. and Somekawa, K., Stereoselective photo [4-1-2]-cycloadditions of 2-pyrone-5-carboxylate with maleimides in the soMd state and in solution, /. Heterocyclic Chem., 35, 1361, 1998. 

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