Cyclobutanes 1.4- cyclohexadienes

Endo product (86) is thought to result from thermal addition and is probably not a photoproduct. Cyclohexadiene yields cyclobutanes (87)—(89) and 1,4-cycloaddition product (90) with dimethylmaleic anhydride(87> ... 

Cyclobutyl-cis-4-trans-5-dimethyl- 1,3-dioxolane, by reaction of erythro-3-methane-sulfonyloxy-2-butyl cyclo-butanecarboxylate with sodium borohydride, 51, 12 hydrolysis to cyclobutane-carboxaldehyde, 51, 13 3,5—CYCLOHEXADIENE—1, 2-DI CARBOXYLIC ACID, 50, 50 A1 a-Cyclohexaneacetaldehyde, 53, 104... 

Apparently the fusion of a benzocyclopropene to two cyclobutanes represents the upper limit for isolable benzocyclopropenes. Several attempts to synthesize dicyclopropabenzenes failed. Thus, attempted aromatization of the carbene adducts of the isomeric cyclohexadienes, 99 and 101 afforded none of the expected 100 and 102, respectively, and attempted aromatization of 103 resulted only in uncharac-terizable decomposition products without any evidence for the intermediacy of 104. ... 

These quenching methods can provide indirect evidence about the involvement of CIP and SSIP in PET reactions. We will briefly discuss the dimerization of 1,3-cyclohexadienes exemplarily, leading either to Diels-Alder products or cyclobutane adducts [37]. 

The y radiolysis of 1,3-cyclohexadiene was reported, in 1969, to yield two sets of dimers [38]. One set was the familiar pair of syn and anti cyclobutane dimers which had previously been obtained via triplet sensitized photodimerization. The other set... 

In 1981, this cation radical Diels-Alder cyclodimerization of 1,3-cyclohexadiene was shown to be more cleanly (only 1 % of the cyclobutane dimers is produced), conveniently (in a synthetic organic context), and efficiently (70 % yield) carried out by chemical ionization of the diene, using 3+ (Scheme 15) [39]. 

An intriguing competition arises in the context of cation radical cycloadditions (as in the context of Diels-Alder cycloadditions) which involve at least one conjugated diene component. Since both cyclobutanation and Diels-Alder addition are extremely facile reactions on the cation radical potential energy surface, it would not be surprising to find a mixture of cyclobutane (CB) and Diels-Alder (DA) addition to the diene component in such cases. Even in the cyclodimerization of 1,3-cyclohexadiene, syn and anti cyclobutadimers are observed as 1 % of the total dimeric product. Incidentally, the DA dimers have been shown not to arise indirectly via the CB dimers in this case [58]. The cross addition of tw 5-anethole to 1,3-cyclohexadiene also proceeds directly and essentially exclusively to the Diels-Alder adducts [endo > exo). Similarly, additions to 1,3-cyclopentadiene yield essentially only Diels-Alder adducts. However, additions to acyclic dienes, which typically exist predominantly in the s-trans conformation which is inherently unsuitable for Diels Alder cycloaddition, can yield either exclusively CB adducts, a mixture of CB and DA adducts or essentially exclusively DA adducts (Scheme 26) [59]. 

The three systems are (1) 1,3-cyclohexadiene with all three geometric isomers of 2,4-hexadiene (2) 1,3-cyclopentadiene with cis- and trani-diaryloxyethenes and (3) 2,3-dimethyl-1,3-butadiene with cis- and Irani-1,2-diaryloxyethenes. Further, stereospecific cyclobutanation has been observed in two distinct systems (1) cyclobutadimerization of cis- and trani-anethole (in both dichloromethane and acetonitrile) and (2) 2,3-dimethyl-l,3-butadiene with cis- and trans-, 2-diaryloxyethanes. 

VCH = vinyl cyclohexene, COD = cyclohexadiene, CDT = cyclododecatriene, DVCB = divinyl cyclobutane)... 

Cycloadditions only proceeding after electron transfer activation via the radical cation of one partner are illustrated by the final examples. According to K. Mizono various bis-enolethers tethered by long chains (polyether or alkyl) can be cyclisized to bicyclic cyclobutanes using electron transfer sensitizer like dicyanonaphthalene or dicyano-anthracene. Note that this type of dimerization starting from enol ethers are not possible under triplet sensitization or by direct irradiation. Only the intramolecular cyclization ci the silane-bridged 2>. s-styrene can be carried out under direct photolysis. E. Steckhan made use of this procedure to perform an intermolecular [4+2] cycloaddition of indole to a chiral 1,3-cyclohexadiene. He has used successfully the sensitizer triphenylpyrylium salt in many examples. Here, the reaction follows a general course which has been developed Bauld and which may be called "hole catalyzed Diels-Alder reaction". 

Cyclohexadiene-1,4-dione, 2,5-dibromo- cyclobutane synthon in cubane synthesis, 78... 

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 . 

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