Dimer cyclization

Under the same conditions, the hydroamination of acetylene with primary or secondary aromatic amines brings about the formation of dimerization-cyclization products since the generated imines or enamines, respectively, are not stable. 

Zinc-mediated reductive dimerization cyclization of 1,1-dicyano-alkenes occurs to give functionalized cyclopentenes in good yields under saturated aqueous NH4CI-THF solution at room temperature. The trans isomers are the major products (Eq. 10.38).89... 

Another molecule which takes part in the cocyclization [see Eq. (67)] is arylisocyanate. The C=N double bond, rather than the C=0 double bond in phenylisocyanate, reacts with butadiene to give 3-ethylidene-l-phenyl-6-vinyl-2-piperidone (114) in 75% yield. In this reaction, the double bond migration to the conjugated position took place (101). With isoprene, the selective head-to-head dimerization-cyclization took place at 100°C to give 3,6-diisopropenyl-l-phenyl-2-piperidone (115). 

A very short and elegant synthesis of the 16-rtiembered dilactone ( )-pyrenophorin (515) has been accomplished by the dipolar cycloaddition reaction of a trialkylsilyl nitronate (81TL735). Nitromethane was added to 3-buten-2-one and the carbonyl group of the product reduced with sodium borohydride. The nitro alcohol (511) was converted to the acrylate (512) which was then subjected to a dimerization-cyclization reaction by treatment with chlorotrimethylsilane and triethylamine in dry benzene. Hydrogenation of the mixture of isoxazoline products (513) over palladium on charcoal followed by double dehydration of the intermediate bis-/3-hydroxyketone (514) led to ( )- and meso-pyrenophorin (Scheme... 

An interesting transformation which involves dimerization and cyclization was discovered when the y-enone 81 was reductively converted to 82. The mechanism of this dimerization-cyclization is yet to be elucidated. It involves two molecules of reactant, reduction and some aldol type condensation. The process is catalyzed by DMP+ and proceeds with high stereoselectivity yielding a single isomer. 

Dimerization-cyclization of 81 was significant in DMF with 0.1 M (C4H9)4NBF4 at —2.9 V(SCE). However, under these conditions some simple alcohol 83 was also formed and the composition of the electrolysis product was 75% 82 and 20% 83. DMP+ catalyzed this reduction and upon addition of 0.005 M (DMP)BF4 81 could be reduced at —2.70 V(SCE). The DMP+ mediated reduction yielded only 82 (80% isolated pure product). 

Electrochemical reduction of aryl halides in the presence of olefins (94), (equation 54) leads to the formation of arylated products (95). Electroreduction of several aralkyl halides at potentials ranging from -1.24 V to -1.54 V (see) gives products which involve dimerization, cyclization, and reduction to the arylalkanes. Carbanions and/or free radicals were again postulated as intermediates79. Aryl radicals generated from the electrochemical reduction of aryl halides have been added to carbon-carbon double bonds80,81. Electrochemical reduction of aryl halides in the presence of olefins leads to the formation of arylated products78. Preparative scale electrolyses were carried out in solvents such as acetonitrile, DMF and DMSO at constant potential or in liquid ammonia at constant current. The reaction is proposed to involve an S l mechanism. 

The total synthesis of conglobatin has been completed by Seebach and Schre-genberger (100, 101), the main challenges being the enantioselective preparation of the monomeric hydroxy acid 196 and its dimerization-cyclization. The... 

Scheme 87 6-Endo-dig vs. 5-exo-dig cyclization in sequential alkyne dimerization-cyclizations [156]...

Finally, Ling and coworkers [161] have presented an intriguing Pd-catalyzed dimerization-cyclization sequence of alkynones 222 to furnish 3,3 -bifurans 223 with good to high selectivity with respect to the furans 224 in good yields (Scheme 93). 

Scheme 94 Mechanistic rationale of sequential dimerization-cyclization of alkynones [161]...

Indole N-substituted diyne tetracobalt complexes 1467 undergo a Lewis acid-mediated dimerization-cyclization reaction through the indole 3-position to afford indolophanetetrayne cobalt complexes 1468 (Scheme 280) <20030L1003>. 

It was thus of interest to determine whether a transannular addition could occur for the cyclo-octenyl radical. The cyclo-octenyl radical has been prepared from its parent halohydrocarbon in several matrices in the rotating cryostat. At 77°K the radical was stabilized in all of the matrices but when the samples were warmed reaction took place. The e.s.r. spectra showed that the main reaction in matrices of water, benzene or camphane was hydrogen abstraction adjacent to the double bond to give a cyclic allyl radical (9) rather than the bicyclic radical. However, in a matrix of bicyclopentadiene dimer cyclization occurs to give the bicyclic radical and in a matrix of adamantane both allylic and bicyclic radicals were formed. 

Indan formation is favored by the relatively high equilibrium monomer concentration of the corresponding a-methylstyrene ([ML 1 mol/L at ambient temperature). For example, a-methylstyrene itself is dimerized rather than polymerized at [M]0 < 1 mol/L, and the resulting unsaturated dimers cyclize efficiently to provide dimeric indans, especially at elevated temperatures. 

Lund and co workers [51] reported interesting stereochemical results in the reductive dimerization-cyclization of benzylidenemalononitrile, but the reaction pathway was not stereochemically clarified. Recently, it has been clearly demonstrated by some workers [51-54] that the reductive cyclodimerization of activated olefins in aprotic media affords the 1,2-disubstituted cyclobutanes as shown in Eq. (4). 

Properties and Reactions of Ylidenemalononitriles, F. Freeman (1980). Dicyanomethylene compounds, their molecular structure and spectral properties, toxicity and analyses are reviewed. Various reactions like hydrolysis, oxidation and reduction, dimerizations, cyclizations, photochemistry and thermolysis are discussed. The review, which contains 380 references, also deals with unsaturated compounds, oxygen, nitrogen, phosphorous and sulfur compounds. 

We have demonstrated the first example of oxidative dimeric cyclization between two different classes of allenes, i.e., 2,3-allenoic acids and 1,2-allenyl ketones. The product is a dumb-bell-type bicyclic product 4-(3 -furanyl)-butenolide71 (Scheme 33) [24]. 

Dimerization/cyclization of open chain N-Cbz-dipeptidoyl benzotria-zobdes (141), derived from 138, 139, and 140, led to formation of both C2 symmetrical and unsymmetrical cycbc tetrapeptides (142) by Pd-assisted tandem deprotection/cycbzation reactions (Scheme 36) (2014CEJ4874). 

As with oleate and linoleate, some volatile decomposition compounds are formed from linolenate hydroperoxides that cannot be explained by the classical A and B cleavage mechanisms, including acetaldehyde, butanal, 2-butyl furan, methyl heptanoate, 4,5-epoxyhepta-2-enal, methyl nonanoate, methyl 8-oxooctanoate, and methyl lO-oxo-8-decenoate. Some of these minor volatile oxidation products can be attributed to further oxidation of unsaturated aldehydes. Other factors contribute to the complexity of volatile products formed from hydroperoxides, including temperature of oxidation, metal catalysts, stability of volatile products and competing secondary reactions including dimerization, cyclization, epoxidation and dihydroperoxidation (Section E). 

Amino-3-benzylpyridine 294 reacts with triphosgene to give 2-isocyanato-3-henzylpyridine 295. Dimerization/cyclization of this isocyanate intermediate gave 5-benzyl-3-(3-benzyl-2-pyridyl)pyrido[l,2-a][l,3,5]triazine-l,3-dione 296 [191]. 

Numerous biocatalytic arene dimerizations mediated by laccases are also described. 3-Hydroxyanthranilic acid 148 is a natural substrate for oxidative dimerization by fungal laccases to give cinnabarinic acid 149, and the applicability of this transformation to other nonnatural substrates has been demonstrated (e.g., 150 to 151) [79, 80]. Crossed dimers arising from the oxidative coupling of two different hydroxyaniline substrates have also been reported [81]. Some more unusual examples of laccase-mediated oligomerizations include the oxidative dimerization/ cyclization of tyrosol 152 to give 153 [82] and the oxidative trimerization of indole 154 to give... 

Acetylene compounds having a reactive hydrogen react with palladium compounds to afford the (j-compounds by substitution reaction as shown in eq. (20.40). But acetylene compounds not having a reactive hydrogen form 7c-complexes as shown in eq. (20.8) [146]. On the other hand, as described below, under some reaction coonditions, their dimeric cyclizations also proceed as shown in eq. (20.52) [147— 154],... 

Dotz reaction, methylenation of ketones and alkene metathesis (dimerization, cyclization). 

SCHEME2.191 Plausible dimerization, cyclization, and oligomerization of N-vinylpyrroles during their acid-catalyzed hydrolysis. 

Lastly, another Cytophaga species collected on a biofilm in the North Sea (strains BI0137 and BI0138) produced a series of cyclic polysulfides, the biological roles of which are still unknown. All of these unusual compounds most likely originate via the dimerization-cyclization of 2-meth-ylpropane-1,2-thiol, which is also present in trace amounts (Sobik et al, 2007). 

In a final example, again with a transition metal, rhenium (VII) catalysts were found to be effective for a tandem Prins dimerization-cyclization that provided a novel route to tetrahydropyran (THP)-containing macrocycles (152). Intramolecular Prins reactions previously have been employed for natural product total synthesis, hut this provides a route that is also amenable to de novo macrocycles. The difficult nature of this transformation is highlighted by the fact that the optimized conditions developed for the formation of 152 gave only a 28% yield of dimer product starting from the... 

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