Cyclobutane ring dienes

In general, 2,3-dialkyl-p-benzoquinones exhibit site selectivity in that they tend to give predominantly Diels-Alder adducts resulting from diene attack on the external rather than the internal double bond. This external site selectivity is, however, dramatically reversed when a (substituted) cyclobutane ring is fused to p-benzoquinone. Paddon-Row and coworkers103 studied the reactions of p-benzoquinones such as 129 with several... 

The initial dimerization presumably leads to a [2.2.2]propellane derivative that undergoes thermolysis to the diene that is the observed product. The dimerization occurs in dilute solutions, but at higher concentrations, polymers are found. This finding is in accord with the proposal that these reactions occur via initial formation of a 1,4-diyl, which closes to form the cyclobutane ring. In this way, the reaction avoids an orbital symmetry disallowed pathway. The diyl could react with the alkene at higher concentrations to give a polymer. 

Isomerization reactions of organo-fluorine compounds are included in Houben-Weyl, Vol. 5/3, pp 346-353, where five types of isomerization reactions are covered (1) intramolecular cycli-zation of dienes, (2) opening of cyclobutene rings, (3) opening of cyclobutane rings, (4) isomerization of polyfluorocyclohexadienes, and (5) intramolecular rearrangement of chlorofluoro-and bromofluorocarbons. Much of the work reported is still of fundamental significance and is covered here, where appropriate. Several more later reviews also deal with various aspects of this area.1... 

Cycloadditions of fluorinated dienes are known to give cyclobutane rings. The original suggestion of the structure of the dimer of perfluoro-1,3-butadiene was perfluorotricyclo[4,2,0,02 5]octane [727]. The compound did not react with potassium permanganate. Fifteen years later, another formula was found correct based on physical methods. Instead of three four-membered rings, the product consists of three five-membered rings. Its name is perfluorotricyclo[3,3,0,02 6]octane [128],... 

The initial scission of cyclobutane ring of available pagodane derivative 37 is achieved as a result of homolytic bromination. The tribromide 38, thus formed, underwent facile bromine elimination-fragmentation (and hydrogenolysis of the C-Br bond at the a-methoxycarbonyl center) under the action of metals to form diene 39. Diimide reduction of 39 affected only one double bond and gave product 40, which served as a common precursor for both 35 and 36. The... 

Reaction of benzynes with acyclic dienes generally gives Diels-Alder products in poor yields because of competitive [2 + 2] cycloadditions and ene-type processes (Scheme 113). The product distribution, obtained from 2,3-dimediyl-l,3-butadiene (484)," is more characteristic of a two-step mechanism than of a concerted addition. Widiout pursuing this argument, it tqrpears diat the extremely poor yield of [4 + 2] cycloadduct (485) results from die very fast reaction of benzyne with the transoid diene (484). Bond formation at C-1 coupled with a hydrogen transfer (- 487) or cyclobutane ring closure (-> 486) seems to occur rapidly before diene (484) can adopt the s-cis conformation. 

The metal-catalyzed dimerization of norbornadiene was discovered in 1961 by two independent groups of workers (3, 4) and since then has received considerable attention by others. The various possible dimers are shown in Table II. Excluding structures in which the cyclobutane ring is fused, trans, 14 dimers (XXIII-XXXVI) are possible, of which seven have been isolated. The structures of six are now rather well established. Initially (3) the structure of one dimer was considered to be (XXXVII). In the meantime NMR measurements (44) and synthetic studies (45, 46) have shown that (XXXVII) is (XXV), the liquid exo-trans-endo dimer. Pyrolysis of both (XXV) and (XXIII) (m.p. 67-68°) yields cyclopentadiene and a mixture of bicyclo[4.2.1]nona-2,4,7-triene (XXXVIII) and exo-tricyclo[4.2.1.02 ]nona-3.7-diene (XXXIX) (46). The exo-trans-endo isomer (XXV) decomposes more... 

Enamines (e.g. 377) have been shown to react with conjugated nitroolefins 378 to give mainly dihydro-l,2-oxazine A -oxide derivatives 379 as products of kinetic control (sometimes a cyclobutane ring is formed in these reactions see Section II.B). The stability of these heterocycles is largely dependent on the parent enamine and the type of substituent used on the nitro olefin as has been extensively studied by Valentin and coworkers " . Usually they open into the corresponding nitroalkylated enamines 380 (equation 82), in particular in a solution of methanol or deuteriated chloro-and often an equilibrium between the two forms is established. Stable 1,2-oxazine A -oxides have been obtained in the reaction of 2-nitro-l,3-dienes with cyclic enamines . 

This reaction takes place at 125° and gives a 90% yield. In the absence of the diene, tetrafluoroethylene dimerizes, but this dimerization is slower than reaction with butadiene to form the cyclobutane ring. Unsymmetrically substituted chlorofluoroethylenes cyclize by head-to-head addition 55... 

The catalytic behavior of 1,8-bis-homocubane (20) and its derivatives is quite metal dependent. Ni(0) and Rh(I) complexes promoted the cleavage of a cyclobutane ring to give the dienes of type 21 (5,6,19,20), By contrast, Ag caused the bicyclo[2.2.0]hexane to dicyclopropyl conversion, 20 22 (21,22), The reaction courses with Pd(II) complex... 

Irradiation (s. a. under S/Og) Preferential cycloisomerization Cyclobutane ring from dienes... 

The retrosynthetic disconnection of isocomene leads primarily to the intermediate tertiary carbenium ion 1, which may arise from the intermediate carbenium ion 2 by anionotropic 1,2-alkyl shift. The latter turns out to be the protonation product of the tricycle 3 containing an exocyclic CC-double bond which is generated by a WiT-TiG-methylenation of the tricyclic ketone 4. The concept behind this is formation of the cyclobutane ring in 4 by means of an intramolecular [2-l-2]-photocycloaddition of the 1,6-diene 5. The enone substructure in 5 results from hydrolysis of the enolether and dehydration of the tertiary alcohol function in (65)-l-alkoxy-2,4-dimethyl-3-(2-methyl-l-penten-5-yl)cyclohexene 6. The tertiary alcohol 6 emerges from a nucleophilic alkylation of (65)-3-alkoxy-2,6-dimethyl-2-cyclohexen-l-one 7 with metallated 5-halo-2-methyl-l-pentene obtained by GrigNARD reaction or... 

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