Alkenes retro-Diels-Alder reaction

The reaction of o-iodophenol, norbornadiene and CO proceeds via alkene and CO insertions to afford the lactone 562, which is converted into coumarin (563) by the retro-Diels-Alder reaction. In this coumarin synthesis, norbona-diene behaves as a masked acetylene[4)3],... 

The primary and secondary products of photolysis of common diazirines are collected in Table 4. According to the table secondary reactions include not only isomerization of alkenes and hydrogen elimination to alkynes, but also a retro-Diels-Alder reaction of vibrationally excited cyclohexene, as well as obvious radical reactions in the case of excited propene. 

Retro-Diels-Alder reactions can be used to regenerate dienes or alkenes from Diels-Alder protected cyclohexene derivatives under pyrolytic conditions144. Most of the synthetic utility of this reaction comes from releasing the alkene by diene-deprotection. However, tetralin undergoes cycloreversion via the retro-Diels-Alder pathway to generate o-quinodimethane under laser photolysis (equation 89)145. A precursor of lysergic acid has been obtained by deprotection of the conjugated double bond and intramolecular Diels Alder reaction (equation 90)146. 

The term Diels-Alder reaction in a general sense refers to the reaction between a diene and a dienophile. Retro Diels-Alder reaction is a process that, under certain conditions, produces diene and olefin or a compound containing a C=C bond. The application of flash vacuum pyrolysis to effect the retro Diels-Alder reaction, as shown in Schemes 5-46 and 5-47, has become the standard procedure since the introduction of the method by Stork et al.74 in the 1970s. Therefore, alkenes that are difficult to access by conventional methods may be obtained via retro Diels-Alder reactions.75 In particular, this reaction allows the preparation of thermodynamically less stable compounds such as 4,5-dialkyl cyclopenta-2-en-one. In this case, the alkene functional group can be regarded as being protected by cyclopentadiene (as shown in 154 or 157), which, after subsequent reaction, can easily be removed through quick pyrolysis. 

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 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 ... 

Cyclic alkenes usually show a distinct molecular ion peak. A unique mode of cleavage is the retro-Diels-Alder reaction shown by limonene ... 

Several types of 1,2-oxazines undergo thermal pericyclic reactions in which the N—O bond is cleaved. Thus (506, R = Me, Ph) undergo a thermal retro Diels-Alder reaction on heating to give the corresponding nitrile and o-benzoquinone methide, which can be intercepted by alkenes (Scheme 56) (90JA5341, 94TL7273). 

The first product of the reaction of 1,2,4,5-tetrazines (39) with alkenes (142) is the bicyclic compound (143), which has never been isolated since it loses nitrogen immediately by a retro-Diels-Alder reaction to give 4,5-dihydropyridazine (144). These compounds have been isolated in a few cases but usually a product is obtained which is formed from (144) in one of four different ways ... 

Alkenes have also been prepared by retro-Diels-Alder reaction of resin-bound cyclohexenes. Figure 3.40 depicts an interesting example of this strategy, in which the retro-Diels-Alder reaction is induced by conversion of a thermally stable, resin-bound 7-oxanorbornadiene into a thermally unstable 7-oxanorbornene [795]. 

Figure 3.40. Retro-Diels-Alder reaction for the release of alkenes from cross-linked polystyrene [795],...

Strained cyclohexenes, such as norbornene derivatives, can undergo retro-Diels-Alder reactions even at relatively low temperatures, and this reaction can be used to prepare 1,3-dienes and alkenes (e.g. synthesis of cyclopentadiene by thermolysis of... 

Cyclic alkenes usually show a distinct molecular ion peak. A unique mode of cleavage is the retro-Diels-Alder reaction. This reaction is illustrated with limonene in Scheme 1.11. A retro-Diels-Alder reaction in this example gives two isoprene molecules. Since the reaction is an example of a rearrangement, one of the isoprene moieties is a neutral molecule. 

Anhydro-4-hydroxyoxazoIium hydroxides, such as compound (231), behave as carbonyl ylides (232) in cycloaddition reactions, yielding bicyclic adducts with alkenes and carbonyl compounds (Scheme 24). The adducts produced by combination with alkynes fragment spontaneously in a retro-Diels-Alder reaction, giving furans (equation 57). The formation of a furan by the action of DMAD on the 4(5//)-oxazolone (233) shows that the latter exists in equilibrium with the mesoionic tautomer (234 equation 58) (79JOC626). 

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. 

Oxazoles represent the most widely recognized heteroaromatic azadiene capable of [4 + 2] cycloaddition reactions. The course of the oxazole Diels-Alder reaction and the facility with which it proceeds are dependent upon the dienophile structure (alkene, alkyne), the oxazole and dienophile substitution, and the reaction conditions. Alkene dienophiles provide pyridine products derived from fragmentation of the [4 + 2] cycloadducts which subsequently aromatize through a variety of reaction pathways to provide the substituted pyridines (Scheme 14). In comparison, alkyne dienophiles provide substituted fiirans that arise from the retro Diels-Alder reaction with loss of R CN from the initial [4 + 2] cycloadduct (Scheme 15,206 Representative applications of the [4 + 2] cycloaddition reactions of oxazoles are summarized in Table 14. Selected examples of additional five-membered heteroaromatic azadienes participatiitg in [4 + 2] cycloaddition reactions have been detailed and include the Diels-Alder reactions of thiazoles, - 1,3,4-oxadiazoles, isoxazoles, pyrroles and imidazoles. ... 

Retro Diels-Alder reaction (Section 16.14B) The reverse of a Diels-Alder reaction in which a cyclohexene is cleaved to give a 1,3-diene and an alkene. 

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