Heat treatment Diels-Alder reaction

Alkyl and silyl nitronates are, in principle, /V-alkoxy and /V-silyloxynitrones, and they can react with alkenes in 1,3-dipolar cycloadditions to form /V-alkoxy- or /V-silyloxyisoxaz.olidine (see Scheme 8.25). The alkoxy and silyloxy groups can be eliminated from the adduct on heating or by acid treatment to form 2-isoxazolines. It should be noticed that isoxazolines are also obtained by the reaction of nitrile oxides with alkenes thus, nitronates can be considered as synthetic equivalents of nitrile oxides. Since the pioneering work by Torssell et al. on the development of silyl nitronates, this type of reaction has become a useful synthetic tool. Recent development for generation of cyclic nitronates by hetero Diels-Alder reactions of nitroalkenes is discussed in Section 8.3. 

Similarly, /V-sulfonyl-protected vinylimidazole 597 reacts with PTAD to provide the cycloaddition reaction product 598 which easily undergoes the retro-Diels-Alder reaction upon heating or with acid treatment. The primary product is easily isomerized using a base to the aromatized condensed imidazole 599 (Scheme 95) <1998TL4561>. 

The policyclic triene monomer undergoes metathesis polymerisation exclusively by the cyclobutene double bond under mild conditions (in toluene solution at 20 °C) to give a soluble precursor polymer. This polyacetylene precursor can be purified and characterised prior to its conversion at elevated temperature, via thermally initiated symmetry-allowed elimination (retro Diels-Alder reaction), to polyacetylene (a heat treatment of the product also results in isomerisation of the initial cis form to a more stable turns form) [150],... 

Fascinating rearrangements of aziridines have been applied to the synthesis of diterpene alkaloids by Wiesner and co-workers (Scheme 40). For example, the ester 217, prepared from cyclopentadiene carboxylate (215) and the benzyne precursor 216 by a Diels-Alder reaction, was converted to the aziridine 218 by treatment with benzenesulfonyl azide in 83% yield. When the aziridine 218 was heated with water for 24 h, the hydroxy ester 219 was obtained in 97% yield subsequent oxidation with the Jones reagent afforded the ketone 220. This rearrangement is analogous to that of the benzenesulfonylaziridine of norbomene. ... 

Further general route to 2-alkyl-2-butenolactones via the retro Diels-Alder reaction was developed. Fig, 5, [23]. Hydrolysis of a known lactone (15), subsequent esterification with diazomethane, and treatment with dihydropyran afforded a stereoisomeric mixture of pyranyl ether. Alkylation of the pyranyl ether with various alkyl halide yielded alkylated products (16). The retro Diels-Alder reaction of 16 was performed by heating at 200-280 C in a sealed tube to give 2-alkyl-2-butenolactones (17). The utility of the butenolactones was demonstrated by the synthesis of hinokinin (18) and dehydrolappaol A dimethyl ether (19). Similarly rather mild, simple synthesis of a-methylene-y-alkylbutyrolactones was deviced using dimethylfulvene as a protective group for the double bond, Fig. 6. [24], The... 

Diels-Alder reaction to give a dehydrodimer of the hydroxymethylphenol.2 The dienone (2) when irradiated (high-pressure mercury lamp Phillips HPK 125W) at room temperature aromatizes in high yield to the substituted salicylaldehyde (3). The isomerization can also be effected, but in low yield, by heat treatment at 120°. 

The nse of a, S-nnsaturated carboxylic acids as a nncleophUe provides the corresponding 2-(a, S-nnsaturated acyloxy)cyclohexa-2,4-dienones, which on heating undergo an intramolecular Diels-Alder reaction to afford the bicyclo[2.2.2]octenones ° . 2,6-Dimethylphenol (206) was oxidized with Pb(OAc)4 in the presence of unsaturated acids 847 and 848 and then heated in boiling benzene to afford the corresponding bicy-clo[2.2.2]octenones 849 and 850 in ca 40% overall yields, respectively (Scheme 171). The best yield was obtained with a 4 1 molar ratio of the unsaturated acid and Pb(OAc)4. Further treatment of the latter with aq. NaOH provided the lactonic acid 851. 

A -(Aroylamino)norbornene carboxylates (317) are cyclized by treatment with thionyl chloride and triethylamine. The tricyclic 1,3-oxazinones (318) thus formed undergo retro-Diels-Alder reactions when heated at 150°C to give 2-aryl-l,3-oxazin-6-ones (319) and cyclopentadiene (Scheme 91) <84S345>. 

This hetero Diels-Alder reaction allows easy access to a series of pyrans with structures similar to those of the daunosamines. Thus, heating mixtures of A-acylenaminones 335 and enol ethers leads to the isomer mixture 336 and 337, Scheme 94 (86TL6181 91CB881). In all transformations, the major cis isomers 336 result from kinetic control, which is favored by low temperatures and high pressures. The thermodynamically more stable trans products 337 can be obtained by treatment of 336 with Lewis acids (88JA4065). 

Intermediate 50 was subsequently heated under reflux in triisopropylbenzene (232 °C) for 1.5 to 20 h to provide the basic canthine skeleton 51. Recently, Lindsley et al. reported a rapid MW-mediated procedure for synthesis of 51 [95]. This reaction, performed in a monomode MW reactor at 180 °C, required a reaction time of only 5 min. Even more interesting, treatment of the acryl hydrazide-tethered indole input, with benzil in the presence of 10 equiv. NH4OAC delivered not only the expected triazine 50 but also, directly, the 1,2-diphenyl canthine derivative 51 (Scheme 17.37, reaction path b). The products were formed in a 9 1 ratio of 50 and 51, respectively. In the one-pot reaction, the indole underwent a three-component condensation to generate 50 followed by an intramolecular inverse-electron-demand Diels-Alder reaction and subsequent chelotropic expulsion of N2 to generate the 1,2-diphenyl canthine 51. 

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