Unusual [8 + 2] Annelation in the Reactions of Allenic Ester/Ketone-Derived 1,3-Dipoles with Tropone

Abstract

All-carbon 1,3-dipoles derived from allenic esters/ketones, by catalytic interaction with triphenylphosphine, undergo an unusual [8 + 2] annelation with tropone, leading to 8-oxa-9-(ethoxycarbonyl/acylalkylidene)bicyclo[5.3.0]deca-1,3,5-trienes. Dipoles derived from allenic ketones as well as an alpha-methyl-substituted allenic ester display high reactivity and selectivity.

Lewis Base Catalysis in Organic Synthesis

“…This result is noteworthy because other cycloaddition pathways are generally favored over the [6+3] cycloaddition pathway with tropone. [146] It is interesting to note that all the Lewis base catalyzed reactions discussed in the preceding section employed phosphanes. In contrast to the Morita-Baylis-Hillman reaction, which proceeds equally well with nitrogen and phosphorus Lewis base catalysts, these reactions appear to be uniquely susceptible to phosphane catalysis.…”

Studies on the asymmetric, phosphine-promoted [3+2] annulations of allenic esters with 2-aryl-1,1-dicyanoalkenes

“…6 Other classes of electron-deficient olefins were shown to be competent substrates for [3+2] annulations with allenoates, however, asymmetric variants of these reactions have not been reported so far. Examples include annulations on a,b-unsaturated a-aminoacid derivatives 7 and amides, 8 tropone, 9 chromones, 10 and acrylonitrile. 11 In this context, the aim of enlarging the scope of enantioselective cyclizations on allenoates prompted us to investigate the use of chiral phosphines as catalysts for [3+2] annulations on a special class of electron-deficient a,b-unsaturated olefins, that is, arylidene malononitriles.…”

Phosphine catalysis of allenes with electrophiles

“…36 Notably, the [8+2] cycloadducts 85 were obtained as the only products in excellent yields; the normal electron-deficient allene–olefin [3+2]/[4+2] annulation products were not observed. Mechanistically, γ-addition of the phosphonium dienolate at the C2 atom of tropone formed the intermediate 84 ; subsequent intramolecular cyclization and elimination of phosphine provided the desired product 85 .…”