Retro Diels-Alder rearrangement

Irradiation of the triazoline (70) in methanol gave the ester (71) via the ketene (72), formed by a retro-Diels-Alder rearrangement as shown in Scheme An analogous cycloreversion has been reported in azatricyc-... 

Multicenter fragmentations encompass eliminations, retro Diels-Alder, rearrangement, and other reactions involving the cleavage of more than one bond and the concurrent formation of one or more new bonds. 

The two main rearrangements observed in El are the McLafferty rearranganent and the reaction known as the retro Diels-Alder rearrangement. The ions issued from these rearrangements are easy to identify in mass spectra based on the nitrogen rule (Section 9.6.3). 

The sequence depicted has been suggested as a plausible reaction mechanism. Diazabasketene primarily reacts via a retro-Diels-Alder reaction to give an azine which, after a Cope rearrangement, undergoes a further retro-Diels -Alder reaction to cleave off hydrogen cyanide. The resulting azabicyclo[4.2.0]octatriene finally isomerizes to the target molecule. 

Finally, it should be of interest that the rearrangements of several cyclic benzylic sulfinates have also been described in the literature by Durst46 and Hogeveen47, and seem to proceed by a special two-step mechanism retro-Diels-Alder extrusion of S02, followed by its chelotropic addition to the unstable quinodimethane intermediate (e.g. equation 10). 

When the cyclic acetylene 260 is generated from a suitable precursor, it undergoes an isomerization reaction spontaneously generating naphthalene (263) and benzo-fulvene (264) as the finally isolable products. Very likely the process begins with a retro-Diels-Alder reaction to the [3]cumulene 261, which in a cascade reaction via the semicydic allene 262 rearranges to 263 and 264 [111]. 

As with the McLafferty rearrangement and the retro-Diels-Alder reaction before, the occurrence of the ortho elimination is not restricted to molecular ions. It may equally well proceed in even-electron species. 

Actually, the earliest derivative of a vinylcyclopropane radical cation was a serendipitous discovery. It was formed by an unusual hydrogen shift upon photo-induced electron transfer oxidation of tricyclo[4.1.0.0 ]heptane (26). This result has been questioned on the grounds that the same rearrangement was not observed in a Freon matrix. However, there is no basis for the assumption that radical cation reactions in frozen matrices at cryogenic temperatures should follow the same course as those at room temperature in fluid solution and in the presence of a radical anion, which is potentially a strong base. In several cases, matrix reactions have taken a decidedly different course from those in solution. For example, radiolysis of 8 in a Freon matrix generated the bicyclo[3.2.0]hepta-2,6-diene radical cation (27 ), or caused retro-Diels-Alder cleavage yet, the... 

Reaction of dimethyl acetylenedicarboxylate with A " -compounds in boiling xylene gave the unstable Diels-Alder adducts (65) which rearranged by a retro-Diels-Alder reaction to the aromatic seco-compound (67). Using dicyanoacety-lene at lower temperature, the adducts (66) have been isolated and these were converted, in boiling xylene, into the analogous aromatic seco-compound (68). No... 

The reaction of 77 with alkynes has further been elaborated for the synthesis of substituted phthalonitriles 81. An alternative for the synthesis of these compounds is the cycloaddition reaction of 77 with enamines followed by a retro-Diels-Alder loss of N2 and elimination of the amine (Scheme 16). Generally, more forcing reaction conditions are required and lower yields are obtained in reactions with alkynes than in reactions with enamines, for example, 4-ethyl-5-methylphthalonitrile is obtained in 51% yield from 2-pentyne (xylene, 150°C, 18 days) and in 73% yield from 4-(l-ethylprop-l-en-l-yl)morpholine (CHCI3, 70°C, 7 days) <1998T1809>. The mechanism of the reaction with enamines has been studied in detail. This revealed a [1,5] sigmatropic rearrangement in the cyclohexa-2,4-dien-1-amine intermediates formed after the loss of N2 <1998T10851>. 

At 150 °C, the analogous 2-methylenetricyclo[4.1.0.01,3]heptane (34) equilibrated with 35 and 36 as products of a methylenecyclopropane rearrangement and a subsequent retro-Diels Alder reaction, respectively, until at 180 °C an irreversible rearrangement to bicyclo[4.2.0]octa-l,5-diene and 3,4-dimethylenehexa-l,5-diene (39) via diradical 37 took place.223,224 Increasing pressure favored the formation of the bicyclic system 38 (50% yield at lOOTorr).223... 

Two other important fragmentations in MS also involve cyclic sixmembered transition states (i) the McLalferty rearrangement and (ii) the retro-Diels—Alder reaction. 

Whether this rearrangement reaction is also of value for the synthesis of other 1-acylisobenzofurans does not seem to have been proved. An 1-amidoylisobenzofuran 56 has been prepared by a retro Diels-Alder reaction. It could be isolated as a crystalline compound at -20°C, but decomposed readily. A trapping reaction with dehydrobenzene was reported (89AP565). 

Photolysis of the oxadiazabicyclo[2.2.3]nonadiene 285 gave a rearranged bicyclic system 286 plus 1-ethoxycarbo-nyl-177-azepine 287 (Equation 42) < 1999H(51)141 The latter was shown to form on photolysis of 285, but it was suggested on the basis of kinetic evidence, that some may also be formed directly from 285 by what is formally a retro-Diels-Alder type reaction. 

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