Cyclization, radicals 6 + 4 -cycloaddition

This section is devoted to cyclizations and cycloadditions of ion-radicals. It is common knowledge that cyclization is an intramolecular reaction in which one new bond is generated. Cycloaddition consists of the generation of two new bonds and can proceed either intra- or intermolecularly. For instance, the transformation of 1,5-hexadiene cation-radical into 1,4-cyclohexadienyl cation-radical (Guo et al. 1988) is a cyclization reaction, whereas Diels-Alder reaction is a cycloaddition reaction. In line with the consideration within this book, ring closure reactions are divided according to their cation- or anion-radical mechanisms. 

One of the important methods in the construction of five-membered rings is [3-f2] radical cycloaddition. This reaction is initiated by radical-induced opening of a three-membered ring followed by addition of ethylene to create a new radical and subsequent cyclization onto a double bond (Scheme 20). Addition of the initial ring-opened radical to the double bond can be carried out either inter- or intra-molecularly [27],... 

Keywords Cyclization Dipole cycloaddition Cross-coupling Axially biaryls Radical-mediated coupling Planar chirality... 

Intramolecular enone-alkene cycloadditions are also possible. In the case of (3-(5-pentenyl) substituents, there is a general preference for exo-type cyclization to form a five-membered ring.195 This is consistent with the general pattern for radical cyclizations and implies initial bonding at the (3-carbon of the enone. 

Thermolysis of 44 produced products derived from the Myers-Saito cyclization reaction. However, when 43 having a trimethylsilyl substituent at the acetylenic terminus was subjected to heating in the presence of 1,4-CHD at 70 °C for 3 h, the 1H-cyclobut[a]indene 46 was produced. A reaction mechanism involving an initial Schmittel cyclization to generate the benzofulvene biradical 45 followed by an intramolecular radical-radical coupling was proposed to account for the formation of the formal [2 + 2]-cycloaddition product 46. 

Treatment of the propargylic alcohol 144, readily prepared from condensation between benzophenone (143) and the lithium acetylide 101, with thionyl chloride promoted a sequence of reactions with an initial formation of the chlorosulfite 145 followed by an SNi reaction to produce in situ the chlorinated and the benzannulated enyne-allene 146 (Scheme 20.30) [62], A spontaneous Schmittel cyclization then generated the biradical 147, which in turn underwent a radical-radical coupling to form the formal [4+ 2]-cycloaddition product 148 and subsequently, after a prototropic rearrangement, 149. The chloride 149 is prone to hydrolysis to give the corresponding 11 H-bcnzo h fluoren-ll-ol 150 in 85% overall yield from 144. Several other llff-benzo[fc]fluoren-ll-ols were likewise synthesized from benzophenone derivatives. 

The present volume contains 13 chapters written by experts from 11 countries, and treats topics that were not covered, or that are complementary to topics covered in Volume 1. They include chapters on mass spectra and NMR, two chapters on photochemistry complementing an earlier chapter on synthetic application of the photochemistry of dienes and polyenes. Two chapters deal with intermolecular cyclization and with cycloadditions, and complement a chapter in Volume 1 on intramolecular cyclization, while the chapter on reactions of dienes in water and hydrogen-bonding environments deals partially with cycloaddition in unusual media and complements the earlier chapter on reactions under pressure. The chapters on nucleophiliic and electrophilic additions complements the earlier chapter on radical addition. The chapter on reduction complements the earlier ones on oxidation. Chapters on organometallic complexes, synthetic applications and rearrangement of dienes and polyenes are additional topics discussed. 

Similar oxidative cyclization reactions involving the direct oxidation of acyclic 1,3-dicarbonyl compounds have not been reported. However, the generation of radical intermediates by the direct oxidation of cyclic 1,3-dicarbonyl compounds at an anode surface has been reported. Yoshida and coworkers have shown that the anodic oxidation of cyclic 1,3-dicarbonyl compounds in the presence of olefin trapping groups gives rise to a net cycloaddition reaction (Scheme 10) [23]. These cycloaddition reactions proceeded by initial oxidation of the 1,3-dicarbonyl compound at the anode followed by a radical addition to the second olefin. Following a second oxidation reaction, the material then... 

Scheme 29 describes a plausible mechanism for the formation of the products which fit the observed coulometric (n 0.45 F/mol) and preparative results. The intramolecular cyclization process involves a dimerization between a radical cation 52a and the ketene imine 52 to form the intermediate radical cation 52b which then cyclizes to the radical 52c which can abstract a hydrogen atom leading to 54 or can be further oxidized and transformed through a cyclization and deprotonation reaction to 53 which involves 1 F/mol. However, it seems that the [2 -1- 3]-cycloaddition between the parent compound 52 and the cation 52d giving rise to 55 is the fastest reaction as compared with the intramolecular cyclization of 52d to 53. This can also explain the low consumption of electricity. 

Recently it has been shown that radical anionic cyclization of olefinic enones effectively compete with intramolecular [2 -I- 2]-cycloaddition to form spirocy-clic compounds [205, 206], 3-Alkenyloxy- and 3-alkenyl-2-cyclohexenones 235 are irradiated in the presence of triethylamine. As depicted in Scheme 46 two reaction pathways may operate. Both involve electron transfer steps, either to the starting material (resulting in a direct cyclization) or to the preformed cyclobutane derivative 239, which undergoes reductive cleavage. The second... 

Enamine carbaldehyde 76 (Scheme 20) by sequential photochemical cycloaddition and iminium ion-propargylsilane cyclimtion furnishes allenes 77a-c in good yield and with high diastereoselectivity (92X2081). (Gas chromatographic fR values and thin-layer chromatographic Rp values have been reported.) Radical cyclization of ca-iodoalkyl isoquinolone 78a under... 

The intramolecular cyclization of oximes with alkene substituents to dihydropyrroles in the presence of radical initiator or by heating was also describedThus, oxime 83 underwent a tandem 1,2-prototropy-cycloaddition sequence and gave an unstable cycloadduct 84, which on treatment with NaOH afforded indolizine 85 (equation 36). ... 

Reviews including aspects of P-lactam chemistry are ketene-imine cycloaddition reactions <98CHE1222>, radical cyclization processes <98MI169>, combinatorial synthesis <98AJC875>, electrophilic cyclization of unsaturated amides <98T13681> and theoretical studies on the synthesis of P-lactams <98MI245>. 

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