Cyclic allenes, cycloaddition

Beyond dimerization and oligomerization, [2 + 2]- and [4+ 2]-cycloadditions with conjugated dienes and styrenes and the addition of nucleophiles are typical reactions of strained cyclic allenes. These transformations have been studied most thoroughly with 1,2-cyclohexadiene (6) and its derivatives [1, 2]. Concerning the cycloadditions, a theoretical study had the surprising result that even the [4+ 2]-cycloadditions should proceed in two steps via a diradical intermediate [9]. In the case of nucleophiles, the sites of attack at several 1,2-cyclohexadiene derivatives having an... 

In addition to 534, further [4+2]-cycloadducts of 5 were prepared by using 1,3-dienes, some of which are well known as trapping reagents of short-lived cyclic allenes and cycloalkynes. Further, cycloadditions could be achieved with tropone and several 2-substituted tropones, 8,8-dicyanoheptafulvene, 1,3,5-cycloheptatriene and a few of its 7-substituted derivatives. The products of these reactions are represented in Scheme 6.108. 

Six- to eight-membered cyclic allenes are too strained to be isolated. Spontaneous [2 + 2]-cycloaddition occurs with these allenes to give the dimer as soon as they are generated in solution [5, 6]. 

Cyclic allenes have improved reactivity due to ring strain. The cycloaddition of 1,2,4-cyclohexatriene (28) with styrene (29), for example, afforded exclusively cyclobutane 31 (equation 9)18. Semi-empirical calculations (AMI) determined the diradical intermediate 30 to be at an energy minimum19. 

In addition to the concerted [2 + 2] cycloadditions of cyclic allenes reported by Elliot and colleagues, Kimura and coworkers22 reported [2 + 2] cycloadditions of several 4-ethenylidene-l,3-oxazolidin-2-ones 35 with alkenes and alkynes (equation 11). The... 

Strained alkenes also react by [2 + 2] cycloadditions (see Section 1.3.2.1.1.). A recent study on the preparation of cyclohexa-l,2,4-triene (isobenzene, 44) and the corresponding 4,5-benzo derivative 45 as short-lived transients reported the cycloaddition of these cyclic allenes.29... 

The intramolecular 2 + 2-photo-cycloadditions of optically active allenesilanes (5) with enones and enoates produce silyl-substituted exo-methylenecyclobutanes (6) in high enantiometric excess. Photo-desilation leads to the parent unsaturated exo-methylenecyclobutanes (7) (Scheme 3).19 The cycloaddition of naphthoquinone to allyltrimethylsilane in the presence of Me2 A1C1 yields the expected 2 + 2-cycloadduct that slowly rearranges to the 2 + 3-adduct.20 hi the presence of bases, Cephalosporin triflates (8) undergo 2 + 2- and 4 + 2-cycloaddition with alkenes, alkynes, and dienes via an intermediate six-membered cyclic allene (9) (Scheme 4).21... 

C(2)-C(3) fused polycyclic cephalosporins have received considerable attention as new candidates for /3-lactam antibiotics. An access to tricyclic cephalosporins based on metal-promoted alkenylation of 3-trifloxy-A3-cephem and subsequent Diels-Alder reaction has been published <1996TL5967>. Alternatively, the reaction of a cephalosporin triflate with silyl enol ethers and silylketene acetals has been described to afford tri- and tetracyclic cephalosporins <1996TL7549>. A related process is the formation of fused polycyclic cephalosporins 27 and 28 bearing a wide range of functionalities from the reaction of cephalosporin triflates 26 with unsaturated compounds (alkenes and alkynes) and a base (Scheme 5) <1997JOC4998>. These studies have suggested that the reaction proceeds via the intermediacy of a six-membered cyclic allene which undergoes concerted nZs + K2a cycloaddition with alkenes and acetylenes. 

The diyne 66 undergoes an intramolecular dehydro Diels-Alder reaction in toluene under reflux to afford naphtho[2,3-c]chromene derivatives in reasonable yield (Scheme 20). The reaction is presumed to proceed via a [4+2]-cycloaddition of the alkyne to the aryl alkyne group to form the cyclic allene intermediate 67, which then isomerizes to the aromatic product <2003SL1524>. 

Heating the diselenoallene 140 leads to an intramolecular cycloaddition affording the fused selenin 141 (Scheme 15) <2001JOG1787>. The reaction appears to take place by an initial intramolecular hydrogen abstraction by the sp-hybridized carbon of the allene affording a diradical intermediate. Radical coupling then leads to the selenin. The cyclic allene precursor could be prepared by dilithiation of the benzylacetylene followed by treatment of the bis-selenocyanate. 

Substituted cis-3,4-epoxyocta-7-en-l-ynes are converted into tricyclic 5,6-dihydropyran-2-ones 36 and 37 on treatment with Co2(CO)8 in benzene a Co-stabilised cyclic allene intermediate is proposed. The cyclopentanone-fused products 36 arise from tandem [5+1] -[2+2+1] cycloadditions, while the cyclobutane-fused derivatives 37 are produced through a tandem cyclocarbonylation and [2+2] cycloaddition <07JOC567>. 

Dihydrobenzo[c]furans can also be obtained via an intramolecular Diels-Alder cycloaddition via strained cyclic allenes as intermediates. An example is shown below <05SL2062>. The same group also reported an Ir-catalyzed enantioselective [2+2+2] cycloaddition of an octayne with a monoalkyne to form a chiral noviaryl in high chemical and optical yields <05CC6017>. 

The intermediacy of the cyclic allene was also demonstrated by a different trapping experiment. Pyrolysis of the dienyne in the presence of styrene gave, among many other products, the two 2 -h 2 cycloaddition product of the cyclic allene." ... 

A one-step Lewis acid-catalysed intermolecular 4- -3-cycloaddition of aromatic a,)3-unsaturated aldehyde and ketones (105) with epoxides (106) formed seven-membered oxacycles (107) under mild conditions (Scheme 34).The effect of oxygen-, sulfur-, and halogen-substituents on the reactivity of nitrogen-stabilized oxyallyl cations in 4- -3-cycloaddition reactions has been extensively investigated. Aza-oxyallyl cationic intermediates react with cyclopentadiene and furan via an aza-4 -I- 3-cycloaddition reaction to form bicyclic cycloadducts in moderate yields. The intramolecular formal 4- -4-cycloaddition of conjugated enynes with an e-deflcient cyclobutene (108) yielded a strained six-membered cyclic allene (109) that isomerized to a 1,3-cyclohexadiene (110). This intermediate underwent a thermal or acid-promoted six-electron electrocyclic ring opening to yield a 2,4,6-cyclooctatrienone (111) (Scheme 35). ... 

Cyclonona-1,2-diene undergoes principally 1,4-photocycloaddition to benzene to give (162) a minor product (163) results from 1,3-cycloaddition. The allene forms eomplexes with bis(tricyclohexylphosphine)nickel and related ligands from which the allene may be displaced unchanged, whereas non-cyclic allenes give dimeric products. ... 

The ability of conjugated enynes to serve as a four-carbon unit in thermal or Lewis acid-mediated cycloaddition reaction with alkynes to form aromatic product was first recognized by Danheiser et al. [1] (Scheme 14.1). Extensive experimental and theoretical mechanistic studies suggest intermediacy of strained cyclic allene in this transformation [2]. Although this enyne-yne cycloaddition reaction represents a powerful method for the synthesis of bicyclic aromatic compounds [1,3], it is limited to intramolecular processes. 

Because the fluorine substituents both inductively and hyperconjugatively withdraw electron density from the C(2)-C(3) tt bond, the LUMO is located there, and Diels-Alder reactions take place exclusively with this bond [25] 1,1 -Difluoro allene and fluoroallene reaet readily with a large selection of cyclic and acyclic dienes, and acyclic dienes, [2+2] cycloadditions compete with the Diels-Alder processes As shown in the example in equation 79, a significantly different regiochemistry is observed for the [2+4] cycloaddition compared with the [2+2]... 

The thermal [2 + 2] cycloaddition is limited to certain activated alkenes. For instance tetrafluoroethylene, tetrachloroethylene, allenes e.g. 17, ketenes and ena-mines can form cyclic dimers or react with other alkenes ... 

The reaction is versatile and proceeds with a variety of cyclic and acylic alkenes substituted with alkyl, aryl, vinyl and heteroatom substituents. Allene derivatives also undergo cycloaddition with nitrones ". A variety of cyclic and acyclic aliphatic nitrones bearing aliphatic and aromatic substituents has been tested. The reaction is, however, relatively sensitive to steric constraints and proceeds easily only for mono- and disubstituted alkenes. Steric requirements for a nitrone molecule are similar and, although several reactions with R, R2 are known, good yield has been achieved only with R = H (equation 105). 

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