Cycloadditions vinylallenes

Stereoselective construction of tetrasubstituted exocyclic alkenes from the [4 + 2] cycloaddition of vinylallenes [155]... 

Murakami and co-workers have shown that phenyl- and vinyl-substituted vinylallenes react in a palladium-catalyzed intermolecular [4+ 4]-cycloaddition in the presence of a palladium complex to give the cyclooctadiene cycloadducts in moderate to good yields (Scheme 29).103 In a method reported by Lee and Lee, bicyclo[6.4.0]-dodecatrienes are prepared in good overall yields via a two-step, one-flask procedure that involves a serial palladium-catalyzed cross-coupling/[4 + 4]-cycloaddition followed by [4 + 2]-cycloaddition (Scheme 30). Overall, this two-step process impressively brings together five simple components to form relatively complex bicyclic products.1... 

Moving on to vinylallene (2) and its derivatives as substrates, the situation becomes rapidly more complex - and more interesting. The possibility of using vinylallenes as diene components in Diels-Alder additions was recognized many years ago [5]. As shown in Scheme 5.44, the [2+ 4] cycloaddition of a generalized dienophile 283 to 2 yields 3-methylencyclohexene adducts 293. 

Vinylallenes and bisallenes participate in the Diels-Alder-type cycloaddition as the diene component, providing a powerful tool for the construction of complex ring systems. They also undergo thermal electrocydic ring closure to form methylenecy-clobutene derivatives. 

If the vinylallene possesses a substituent at the vinylic terminal position, an endo adduct is preferentially obtained owing to the secondary orbital overlap. Only the E-isomer of propenylallene underwent the regio- and stereoselective cycloaddition with methyl vinyl ketone to afford the endo-isomer as the major product. The Z-iso-mer was unreactive because it preferred the transoid conformation [165]. 

Cycloaddition of vinylallene 201 having methyl and w-butyl substituents at the terminal allenic carbon gave a moderate ratio of geometric isomers owing to the small steric difference between the two substituents [167]. 

The [4 + 2]-cycloaddition reaction of dienylallene 204 with TCNE took place at 70 °C to give the adduct 205 in good yield [173], The dienylallene behaved not as butadiene but vinylallene, partly owing to the thermodynamic stability of the adduct. 

Intramolecular [4 + 2]-cycloadditions of vinylallenes have been utilized in the synthesis of complex molecules including natural products such as compactin. Heating the vinylallene 208 at 140 °C gave hexahydronaphthalene 209. The crude mixture was immediately reduced with LiB(sBu)3H because of the instability of /3,y-unsatu-rated ketones [175]. 

In the cycloaddition of vinylallenes 217 and 218, both Lewis acid catalysis and thermal conditions gave analogous results in terms of stereoselectivities [178],... 

An Rh-catalyzed [4+1 -cycloaddition reaction of vinylallenes with carbon monoxide provies an efficient synthetic method for cross-conjugated cydopentenones (Scheme 16.40) [39, 40]. 

Scheme 16.40 Rh-catalyzed [4 + l]-cycloaddition of vinylallenes with carbon monoxide.

An Rh-catalyzed asymmetric [4 +l]-cycloaddition of vinylallenes with carbon monoxide was realized for the first time to furnish chiral 5-substituted 2-alkylidene-3-cydopentenones (Scheme 16.42) [42],... 

A geometric isomer of the vinylallene mentioned above also undergoes [4 + 21-cycloaddition with butadiene to furnish a cydohexene derivative in 90% yield with excellent diastereomeric purity (>99 1). The preferred formation of a jt-allylpalla-dium complex from the axially oriented complex accounts for the trans selectivity (Scheme 16.79) [89]. 

Intermolecular [4+2]-cycloaddition of vinylallenes with alkynes is efficiently mediated by means of an electronically tuned rhodium catalyst (Scheme 16.81) [91]. A five-membered rhodacycle is formed from the vinylallene. Coordination followed by insertion of an alkyne to the rhodacycle generates a seven-membered rhodacycle, from which rhodium(I) is eliminated reductively to produce a cyclohexatriene, leading to the aromatic compound. 

The use of vinylallenes as the diene component in Diels-Alder reactions is very common, thus resulting in their ubiquitous use in natural product synthesis. A vinylal-lene has even been proposed by Schreiber and Kiessling [10] as a biogenetic intermediate in the synthesis of the skeleton of esperamicin A (32 —> 33). Their synthetic approach to esperamicin A (34) was modeled after this biogenetic proposal in which a Type II intramolecular Diels-Alder cycloaddition was used to gain access to the highly unsaturated bicyclic core of 34 (Scheme 19.8) [10]. 

In a study Rh-catalyzed cycloaddition of unactivated substrates, specifically, vinylallenes and acetylenes, is described.656 Appropriate catalysts were selected on the basis of a study of the bonding interactions of the reactants and transition metals. For this reaction a complex prepared in situ from [Rh(COD)2]OTf and P[OCH(CF3)2]3, one of the most strongly electron-accepting ligands available, was used. Under the conditions applied, even ethylene, one of the most sluggish... 

Asymmetric [4+1] cycloaddition of vinylallenes and carbon monoxide is promoted by a cationic Rh complex formed in situ from [Rh(cod)2]PF6 and chiral diphosphine ligand, (R.R)-Me-DuPHOS to afford 2-alkylidene-3-cyclopentenones with high asymmetric induction [72] (Eq. 8A.48). 

The rhodium-catalysed asymmetric 4 + 1-cycloaddition of vinylallenes (147) with CO furnishes 5-substituted 2-alkylidenecyclopent-3-enones (148) with up to 95% enantioselectivities (Scheme 56).276... 

Murakami and Ito have highlighted the utility of cationic Me-DuPhos-Rh catalysts for novel asymmetric (4 + 1) cycloaddition reactions between vinylallenes and carbon monoxide.68 Complex cyclopentenone derivatives such as 64 have been constructed in a single step and with enantiose-lectivities up to 95% in this process (Scheme 13.22). 

Dangyan, Y.M., Panosyan, G.A., Voskanyan, M.G., and Badanyan, S.O., Reactions of unsaturated compounds. Part 88. Vinylallenic phosphonates in cycloaddition reactions with unsymmetric dienophiles, Zh. Obshch. Khim., 53, 61, 1983 J. Gen. Chem. USSR (Engl. Transl.), 53, 47, 1983. 

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