Cycloaddition reactions retro Diels—Alder reaction

SCHEME 7. Competition between cycloadditions and retro- Diels -Alder reactions in the reaction of dicyanoacetylene and ll-methylene-l,6-methano[10]annulene... 

Although both double bonds can react in the [3 + 2] cycloaddition, the retro Diels—Alder reaction of the intermediates occurs rapidly to give 1,4,5-tri-substituted-1,2,3-triazole predominantly. 

Takeshita H., Mori A., Tian G. R. Carbon-Carbon Double Bond Formation by Means of High-Pressure Cycloaddition-Retro-Diels-Alder Reaction Between 2,3-Bis(Methoxycarbonyl)-7-Oxanorbornadiene and Dienes Yuki Gosei Kagaku Kyo-kaishi 1990 48 132-143... 

Cycloadditions selectively afford the adducts on the 6,6-ring junctions [65], and the products occasionally undergo a facile retro-Diels-Alder reaction as a consequence of the low thermodynamic stability of the adduct. Very stable Diels-Alder cycloadducts have, however, been prepared by use of different substituted o-quinodimethanes, probably because of stabilization by aromatization of the resulting adducts [66],... 

A convenient synthetic method for 1,2,3-triazoles unsubstituted at C-4 and C-5 utilizes a reaction of azides with norbornadiene, for example, Scheme 29 <2004JOC1081>. The process is performed in refluxing dioxane. In the first step, norbornadiene undergoes 1,3-dipolar cycloaddition to glucose-derived azide 225 to give triazoline 226. The following retro Diels-Alder reaction results in the elimination of cyclopentadiene to furnish triazole derivative 227 in 79% yield. 

The first stannenes were obtained as reactive intermediates that were identified by their reaction products.570,571 For example, Me2Sn=C(SiMe3)2 (Scheme 20) was prepared by the 1,2-elimination of LiBr, or by a retro-Diels-Alder reaction, and was characterized by cyclodimerization, by ene reactions with alkenes, and by cycloaddition with 1,4-dienes.572... 

The analogous transformation of 125, also realized by flash vacuum pyrolysis, gave rise to allenic oximes 126 [165], which are not directly accessible by the classical route starting from allenyl ketones and hydroxylamine (see Section 7.3.2) [122], Because compounds 125 are prepared from allenyl ketones and furan by [4 + 2]-cycloaddition followed by treatment with hydroxylamine, the retro-Diels-Alder reaction 125 —> 126 is in principle the removal of a protecting group (see also Scheme 7.46). 

Generation of the highly strained tricyclic 91 (Sch. 18) can be accomplished by transannular photo-[4+2] cycloaddition of 1,3,6-cyclo-octatrience 90 by direct (unsensitized) irradiation, albeit in modest yield [60]. Structures such as 91 can, however, be unstable toward a thermal retro-Diels-Alder reaction [61]. 

In an analogous reaction of (2) with norbomadiene, the tetracyclic adduct (116) is formed which on heating undergoes a retro Diels-Alder reaction with formation of the thiophene derivative (117). Further cycloaddition reactions of mesoionic 1,3-dithiolones have been carried out with cyclopentadiene, 1,3-cyclohexadiene and 1,5-cyclooctadiene (78CB3037). 

Several additional examples of the intramolecular cycloaddition of unactivated acetylenes with isomiinchnones were reported by Maier [30]. This cycloaddition approach represents an efficient method for providing rapid access to annulated furans present in several sesqui- and diterpenes, such as the panicu-lides [51],furanonaphthoquinones [52],furodysin,andfurodysinin [53,54].The decomposition of acyclic acetylenic diazoimides 102 and 103 with Rh2(OAc)4 resulted in cycloaddition and retro-Diels-Alder extrusion of methyl isocyanate to give annulated furans 104 and 105 in good yield. The overall transformation is closely related to the intramolecular Diels-Alder reactions of acetylenic oxa-zoles extensively studied by Jacobi and coworkers [55]. 

If D-A adduct 1 contains some 1 2 adducts as impurities, 1,4-benzoquinone is formed by a retro-Diels-Alder reaction during the pyrolytic distillations. In this case, a dark yellow solid of benzoquinone can be seen on the walls of the air condenser, and the distillate has a deeper yellow color. Contamination with a small amount of 1,4-benzoquinone apparently does not interfere with photochemical [2+2] cycloadditions of enedione 3 with alkenes and alkynes, an important application of 3. Fractional distillation of the benzoquinone-contaminated 3 as described for the second distillation of 3 can remove the benzoquinone with some loss of enedione 3. The benzoquinone deposits initially as a dark yellow solid on the walls of the distillation head and air condenser during early fractions. 

The decomposition of dichlorosilacyclopent-3-ene has been proved to be unimolecular by the study of inert dilution. The mass spectral study suggested that the decomposition was concerted, and, in the opinion of the investigators, it corresponded formally to the retro Diels-Alder reaction 13, 29). Thus the reaction in Eq. (38) was believed to proceed via a concerted 1,4-cycloaddition. 

A clever approach to lysergic acid (78) has been described by Oppolzer and coworkers (equation 26). Their approach involved an initial retro Diels-Alder reaction of (75) to give diene (76). TTiis compound then underwent intramolecular cycloaddition with the oxime ether to afford adduct (77) as a 3 2 mixture of diastereomers (67% yield). This mixture could be converted to lysergic acid by a short sequence of reactions. 

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