Retro-Diels—Alder reactions photochemical

The non-nitrogenous carbene precursor (102) was used for the photochemical generation of the carbene (103) without complications due to reactions of diazirine or diazo species. In the presence of alkenes, carbene (103) gave rise to cyclopropanes and in the absence of alkenes was proposed to undergo [1,2]-C shift to form (104), which suffered retro-Diels-Alder reaction to give a triene. 

Diels-Alder, imino dienophiles, 65, 2 Diels-Alder, intramolecular, 32, 1 Diels-Alder, maleic anhydride, 4, 1 [4 -h 3], 51, 3 of enones, 44, 2 of ketenes, 45, 2 of nitrones and alkenes, 36, 1 Pauson-Khand, 40, 1 photochemical, 44, 2 retro-Diels-Alder reaction, 52, 1 53, 2 [6-h4], 49, 2 [3-h2], 61, 1 Cyclobutanes, synthesis ... 

In common with other azodicarboxylic acid derivatives, the uses of 4-phenyl-l,2,4-triazoline-3,5-dione are many. It undergoes a Diels-Alder reaction with most dienes11-14 and is, in fact, the most reactive dienophile so far reported.15 16 As with the formation of all Diels-Alder adducts the reaction is reversible, and in the case of the adduct with 3-j3-acetoxy-17-cyano-5,14,16-androstatriene, the reverse reaction can be made to proceed under especially mild conditions.14 An instance has also been reported of the dione photochemically catalyzing other retro Diels-Alder reactions.17 Along with the proven use of azodicarboxylic ester,18-18 the dione should be potentially important in the preparation of strained ring compounds. 

The stereospecificity of methanol addition to neopentylsilenes has been investigated by Jones and Bates68. The mild thermal retro-Diels-Alder reaction (at ca 200 °C) of E and Z anthracene [4 + 2] cycloadducts 110 liberates stereospecifically the corresponding silenes 111, which are trapped by methanol. The ratio of the diastereomeric products 396a/396b coincides with the E/Z ratio of the precursors 110 (equation 117). In photochemical reactions of similar silene precursors, alcohols were used also to probe the decomposition mechanism69. 

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 final series of five procedures presents optimized preparations of a variety of useful organic compounds. The first procedure in this group describes the preparation of 3-BROMO-2(H)-PYRAN-2-ONE, a heterodiene useful for (4+2] cycloaddition reactions. An optimized large scale preparation of 1,3,5-CYCLOOCTATRIENE, another diene useful for [4+2] cycloaddition, is detailed from the readily available 1,5-cyclooctadiene. Previously, the availability of this material has depended on the commercial availability of cyclooctatetraene at reasonable cost. A simple large scale procedure for the preparation of 3-PYRROLINE is then presented via initial alkylation of hexamethylenetetramine with (Z)-1,4-dichloro-2-butene. This material serves as an intermediate for the preparation of 2,5-disubstituted pyrroles and pyrrolidines via heteroatom-directed metalation and alkylation of suitable derivatives. The preparation of extremely acid- and base-sensitive materials by use of the retro Diels-Alder reaction is illustrated in the preparation of 2-CYCLOHEXENE-1.4-DIONE, a useful reactive dienophile and substrate for photochemical [2+2] cycloadditions. Functionalized ferrocene derivatives... 

In contrast to tetracyclo[4.3.0.0 .0 ]non-8-eneJ the corresponding 5-benzhydrylidene and 5-isopropylidene derivatives 15 (R = Ph, Me) do not undergo photochemical [27t + 2o-] cycloaddition but a formal [ 2 + 2 + 23] (retro-ene) reaction to furnish the substituted 5-methylenetricyclo[4.3.0.0 ]nona-3,7-dienes 16. The isopropylidene derivatives 17 also rearrange via a 1,3-sigmatropic shift and retro-Diels-Alder reaction to give the bridged cyclooc-tatrienes 18. 

Photochemical addition of dimethyl acetylenedicarboxylate to the enamine H2C=CE-NH-CE=CH2 (E = COOMe) yields the bridged cyclohexene (90), which decomposes at 90 °C in a retro-Diels-Alder reaction to ethylene and the ester (91). 

There are few other retro Diels-Alder reactions that have been used for the generation of o-quinodimethanes. Examples are shown in Scheme 32. The azo compounds used in the formation of the furan and thiophene 3,4-quinodimethanes lose nitrogen theimally below 0 C or photochemically at low temperature <89JA3659>. 

B, the photochemical precursor to SB, undergoes a retro Diels-Alder reaction to benzene and acetylene with log k = 14.27 - 41 700/2.3RT ... 

Intramolecular cycloaddition also played a central role in the synthesis of Dewar benzene (145, equation 12.89), the bicycloheptenol 146 (equation 12.90), and the tricyclic keto ethers 147 and 148 (equation 12.91). The photochemical retro Diels-Alder reaction was also us for a synthesis of bullvalene (149) from cyclooctatetraene dimer (equation 12.92). ... 

The Diels-Alder adduct formed by treating furan with hexafluorobut-2-yne has been subjected to a retro-Diels-Alder reaction acetylene is eliminated and the reaction provides a route to 3,4-bis(trifluoromethyl)furan. The corresponding cycloadduct (154) formed from l,l-dimethyl-2,5-diphenyl-sllacyclopentadiene and the butyne was decomposed in an attempt to generate dimethylsilylene by a symmetry-allowed cheletropic fragmentation (Scheme 50) 836 evidence for silylene formation could be obtained unless the decomposition was effected photochemically or in refluxing cumene, under which conditions the addition of tolan enabled dimethylsilylene to be trapped... 

Photochemical addition of dimethyl acetylenedicarboxylate to the enamine H2Ca C -NH-CE=>CH2 (E => COOMe) yields the bridged cyclohexene (90), irtiich decomposes at 90 °C in a retro-Diels-Alder reaction to ethylene and the ester (91). The adducts (92 R = PhCO or COOBu ") of perf luorobut-2-yne, CP CHCCF, to N-substituted pyrroles have been degraded to 3,4-bls(trlfluoromethyl)pyrroles (93) by two methods in the former, the adducts were first hydrogenated at the unsubstituted double bond and then heated to eliminate ethylene,idille in the latter, the adducts were treated with mesitonitrile oxide, 2,4,S-Me C H CEN -0, which removed the ethylene unit in the form of the isoxazole (94). ... 

The reaction types known to produce disilenes are summarized in Chart 1 and apply regardless of the ultimate stability of the product. Historically, thermal 4 + 2 cycloreversion of complex l,2-disilacyclohex-4-enes, i.e. a retro-Diels-Alder fragmentation came first8 -14, followed by silylene dimerization15 -17. This early work produced only indirect evidence for the formation of disilenes as reactive intermediates, but in retrospect it is clear that these species were indeed produced. The early history of the subject is discussed in Reference 1. The first directly observable and isolable disilene resulted from the dimerization of photochemically produced dimesitylsilylene18, ushering in a new era in disilene chemistry. These more recent developments are described in Reference 2. 

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