Retro Dids-Alder reaction

Diels-Alder reactions are reversible cyclohexenes can dissociate to a diene and an alkene by a retro Dids-Alder reaction. 

Interestingly, we were intrigued by the ESI mass spectrum of the compound, as the observed base peak consisted of [M-S02+Na]+. This led us to explore a thermal retro-Diels-Alder reaction that could afford the desired enone 69. It is noteworthy that the chemistry of cyclic enol-sulfites would appear to be an under-explored area with a few references reporting their isolation being found [57]. At last, we were also able to prepare epoxy ketone 70 from 69 in three steps, albeit epoxidation did not take place unless the TES group was removed. Spartan models reaffirmed our initial conformational assessment of enone 69 and epoxy ketone 70, which contain sp3-hybridized C8a and s/r-hybridized C8b (p s e u d o-. v/r - h y b r i d i zed C8b for 70) at the AB-ring junction (Fig. 8.12) and displayed the desired twisted-boat conformation in A-ring. 

Thiophene 1,1-dioxide did not undergo cycloaddition with electron-deficient dienophiles. In most of the cases the dihydrobenzothiophene derivative 109 was obtained as the major product. This shows that self-dimerization is faster than cycloaddition with a different molecule. In the case of dimethyl acetylenedicarboxylate (DMAD) and 4-phenyl-3//-l,2,4-triazole-3,5(4//)-dione (PTAD), the Diels-Alder adducts 111 and 112 of 109 were obtained <1997JA9077>. However, cyclopentadiene gave the Diels-Alder adduct 113 with thiophene 1,1-dioxide. The DMAD adduct 111 on thermolysis undergoes a retro-Diels-Alder reaction to give dimethyl phthalate and thiophene 1,1-dioxide. Azulene was isolated in the thermolysis of 108 in the presence of 6-(dimethylamino)-fulvene this was the result of a [4-1-6] cycloaddition of the thiophene 1,1-dioxide formed in the reaction followed by elimination of SO2 and dimethylamine (Scheme 28) <1999BCJ1919>. 

Attempts to take into account the retro-Diels-Alder reaction in the treatment of the experimental data did not 0ve a constant kinetic order as observed in the case of the reaction of (V,f) with 1,4-naphthoquinone (see below). On the other hand, the dissociation of cyclopentadienedimeric derivatives is practically ne igible in the range of temperature explored, i. e. 50—90 °C. This condudon is indirectly supported by the absence of information in the literature about this possibility. Tlu results obtained are reported in Table 21 and are plotted in Fig. 24 according to an Arrhenius diagram in the case of (V,f). The value of k = 2.0 10 e i ... 

The electron impact mass spectra of (62) and (63) revealed that these compounds are fairly stable to El, and ring contraction reactions are characteristic <840MS(19)433>. Furthermore, retro Diels-Alder reactions have been observed with the 7-oxo derivative (62), whereas this did not occur with... 

The effect of pressure on the rate and product distribution of oxazole Diels-Alder reactions has been studied for 5-methoxy-2-methyl-4-(4-nitrophenyl)oxazole 61, a 5-alkoxyoxazole (Fig. 3.18). " " At atmospheric pressure, 61 gave low yields of cycloaddition products with A -phenyl maleimide and did not react at all with dimethyl maleate or dimethyl fumarate, even when heated at 110°C for 100 h. However, after 10 min at 10 kbar, a benzene solution of 61 and iV-phenyl maleimide provided a 4 1 mixture of endo (62) and exo (63) adducts in quantitative yield. Longer reaction times result in equilibration of 62 and 63, which is postulated to occur via a retro-Diels-Alder reaction, producing a 65 35 ratio after 25 h at 10 kbar. If the reaction is carried out at 10 kbar for 25 h using methanol as the solvent, hydroxypyridine 64 is isolated in quantitative yield. Dimethyl maleate could be induced to react with 61 only at elevated temperature and pressure (10 kbar, 60°C, lOOh, CgHg) to give a 71% yield of hydroxypyridine 65. However, even these... 

In contrast, palladium(0)-catalyzed coupling of the requisite starting oxazole for the synthesis of hippadine 219 with trimethylsilylacetylene at 80°C did not afford the expected oxazole-alkyne 220 (Fig. 3.64). Instead, they isolated the tricyclic furan 221 derived from a Diels-Alder retro-Diels-Alder reaction in 77% yield. Thermolysis of 221 at 320°C effected an intramolecular Diels-Alder reaction with concomitant desUylation. Subsequent DDQ oxidation of this product (not shown) then provided hippadine. 

Subsequent work on the parent molecule revealed less inversion at Cl. In one study at 276°C, 76 4% was observed, and in the other at 312°C, 89% inversion was observed. Both studies examined the pyrolysis of xo, xo-6,7-dideuteriobi-cyclo[3.2.0]hept-2-ene, and found that starting material did not epimerize significantly at Cl under the reaction conditions (subsequently a small amount was measured ), and that the norbomene produced rapidly underwent a retro Diels-Alder reaction so that at most 2% of it was present at any time (Scheme 8.46). 

Harmata and Huang also accomplished a one-pot decomposition and intermolecular cycloaddition under gold catalysis to afford good yields of cycloadducts such as 102 and 103 directly (Scheme 18.22) [27]. The exact structure of the allyl cations or dienophiles in these reactions have not been delineated however, the gold-catalyzed reaction probably did not proceed via a retro-Diels-Alder reaction of 101 to generate sUoxyacrolein 87a in situ, because the cycloaddition of 87a required 24 h of reaction time under gold catalysis for... 

Retro-Diels-Alder reactions yielding thioketones did not occur, contrary to the previous related results. ... 

Pyrolysis of the spiro-compound (6) and its congeners did not result in the expected retro-Diels-Alder reaction but furnished thiophens, fluorene, and 2-(9-fluorenyl)thiophens. The anhydride (7) gave the retro-Diels-Alder product on heating, together with a cycloheptatriene formed by a rearrangement for which there is an earlier precedent. As these reactions are not given by the corresponding diester, subtle structural differences in the systems (7) clearly have a profound influence on the course of the thermolysis. 

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