Diels-Alder dimers, derivatives

Tile behavior of /3-moiiooxo derivatives of 4-chlomaiioiies (27) toward morpholine was rather complex (98JOC9840). Tlius, the proposed thio-ketoiie 5-sulhde intermediates 28 would dimerize into either 1,2,4,5-tetrathianes 29 in a two-step manner or to 1,3,4,5,6-oxatetrathiocins 30 by a [5 + 3] cycloaddition. Meanwhile, the formation of oxadithiins 31 and 1,2,4-trithiolanes 32 is suggestive of the disproportionation of 28 into the thioke-tones 33 and the thioketone 5 -disulhdes 34. Tlie oxadithiins 31 correspond to a Diels-Alder dimer of 33, and the 1,2,4-trithiolanes 32 correspond to cycloadducts of 33 and 34. 

Despite the body of evidence in favor of the Mayo mechanism, the formation of diphenylcyclobutanes (90, 91) must still be accounted for. It is possible that they arise via the 1,4-diradical 94 and it is also conceivable that this diradical is an intermediate in the formation of the Diels-Alder adduct 95 (Scheme 3.64) and could provide a second (minor) source of initiation. Direct initiation by diradicals is suggested in the thermal polymerization of 2,3,4,5,6-pentafluorostyrene where transfer of a fluorine atom from Diels-Alder dimer to monomer seems highly unlikely (high C-F bond strength) and for derivatives which cannot form a Diels-Alder adduct. 

The amine-catalyzed Diels-Alder dimerization reaction of a, 3-unsaturated ketones in water was developed by Barbas et al. to form cyclohexanone derivatives (Eq. 12.12). They believe that the reaction proceeds via the in situ formation of 2-amino-1,3-butadiene and iminium-activated enone, as the diene and dienophile, respectively. 

A review of Diels-Alder reactions of fullerenes with acyclic and cyclic dienes has been presented. The addition of substituted pyrimidine o-quinodimethanes (75) to [60]fullerenes yields novel organofullerenes (76) bearing a pyrimidine nucleus covalently attached to the Ceo cage (Scheme 26). The Diels-Alder dimerization of cyclopenta[/]phenanthrene (77) with isobenzindene (78) yields the dimer (79) in 85% yield (Scheme 27). Further evidence has been supplied to support an early reorganization of the r-network in the dimerization of 2-methoxycarbonylbuta-1,3-diene. The Lewis acid-catalysed Diels-Alder reactions of acrylate derivatives of new carbohydrate-based chiral auxiliaries with cyclohexadiene show excellent endo. exo... 

The Stille reaction on 4-bromoisothiazolin-3-one V-oxides 261 (R = Et, (A)-2-C(Me)Ph) affords the vinyl intermediates 262 ( =1), which immediately undergo a Diels-Alder dimerization yielding adducts 263 via an 3co-transition state (see Section 4.05.2.2). As a confirmation of the high reactivity of 262 ( = 1) in Diels-Alder reactions, the oxidation of ethenyl derivative 262 (R = Et, = 0) (see Section 4.05.7.7) affords directly the dimer endo-263. By the same reaction in the presence of A -phenylmaleimide, compound endo-264 is formed <1999T12313>. 

Styrene (and derivatives) also possesses the rare monomer quality that the neat material, without initiator, may be spontaneously polymerized by simply heating to 80-100°C for 24-48 hr. It is thought that this occurs via the initial Diels-Alder dimerization of styrene to the two diasteomers A and B [14]. The two diastereomers appear to have an extremely labile hydrogen, which is both doubly allylic and tertiary. However, only dimer A has the correct stereochemistry (an axial phenyl), which enables the excess styrene to abstract a hydrogen atom from it, producing two radical species (Eq. 23.5). 

The in situ generation of the a,/3-unsaturated thioaldehyde, thiometha-crolein, has been reported to provide a 2 1 ratio of the Diels-Alder dimerization products 2 and 3 derived from the 4ir and carbon-carbon double bond 2ir participation of thiomethacrolein in a [4 + 2] cycloaddition which proceeds with little regioselectivity [Eq. (3)].5... 

Interestingly, no a,/3-unsaturated thiocarbonyl dimer and no products derived from the 27t thiocarbonyl participation in Diels-Alder reactions were observed although the potential, reversible generation of such products could not be ruled out. In contrast, the low temperature dimerization of a,j8-unsaturated thioketones including methyl vinyl thioketone, generated by flash vacuum pyrolysis (FVP), provided 1 derived from the clean, regiospecific Diels-Alder dimerization with 4tt and 2tt thiocarbonyl participation [Eq. (2)].4... 

The Diels-Alder dimerization of a,/3-unsaturated dithioesters has been reported to proceed with complete regioselectivity and exclusively through an endo [4 + 2] transition state to provide the cycloadducts, e.g., 12, derived from 4ir and 2tt carbon-carbon double bond participation of E-a,/3-unsaturated dithioesters [Eq. (7)].16-18... 

According to the activation volume data the Diels-Alder dimerization of 1,3-butadiene [39] and o-quinodimethane (Table 2.5, entries (3) and (4), respectively) fall into the same class of concerted processes as those discussed for 1 and 7, while the Diels-Alder dimerization of hexamethylbis(methylene)cyclopentane seems to occur in stepwise fashion. According to the activation volume data summarized in Table 2.6 only the Diels-Alder reaction of 1,3-butadiene with a-acetoxyacrylonitrile seems to proceed concertedly while all other Diels-Alder and homo-Diels-Alder adducts are probably formed in stepwise processes comparable to the corresponding competitive [2 + 2] cycloadditions. Stereochemical investigations of the chloroprene and 1,3-butadiene dimerization using specifically deuterated derivatives confirm the conclusions drawn from activation volume data. In the dimerization of ( )-l-deuteriochloroprene (17) the diastereomeric Diels-Alder adducts ISa-Dz and... 

Cyclohexadiene dimerisation is similar, the kinetic thermal product being 80% endo and 20% exo Diels-Alder dimer, while the photosensitised dimerisation gives, together with major amounts of cyclobutane derivatives (4 parts), the exo Diels-Alder dimer (1 part), with only traces of the endo isomer . Exo Diels-Alder adducts were obtained also from photosensitised addition of cyclic dienes to maleic anhydride . 

The reaction of l-chloro-l,4-dihydro-Tarsanaphthalene derivatives 48 with a suspension of DBU in pentane afforded a mixture of 1-arsanaphthalenes 49 and their Diels-Alder dimers SO. Both 49a and 49b are in mobile equilibrium with their head-to-head Diels-Alder dimers 50a and 50b (Scheme 17) <20010M2109>. 

The Diels-Alder dimer of 2,3-dichlorocyclopentadienone formed at 25 °C has been shown to have the endo-configuration as in (199), the result of regiospeciflc and stereoselective cycloaddition. At 170°C cheletropic loss of CO from (199) affords the trienone (200) which itself dimerizes with subsequent loss of HCl to yield the stereoisomeric aromatic derivatives (201) and (202). The attempted dehydrobromination of (203) with l,5-diazabicyclo[5,4,0]undec-5-ene (DBU) gave... 

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