Carbenoid character in transition structures for reactions of ketenes with alkenes

Catalytic Enantioselective Allenoate–Alkene [2 + 2] Cycloadditions

“…This stands in sharp contrast to the vast majority of reported catalytic enantioselective methods (vida supra). (2) In general, higher yields are observed with more strained alkenes. For example, the more strained cyclooctene undergoes cycloaddition with higher yield than a reaction with less strained cyclohexene (compare yields of products 18 and 20).…”

“…The majority of methods for catalytic enantioselective [2 + 2] cycloadditions involve the formal cycloaddition between highly polarized electron-rich and electron-poor alkenes. 2,3 Catalytic enantioselective photochemical [2 + 2] cycloadditions have also recently been reported. 4 Several Au-catalyzed cycloadditions of electronrich allenes and styrenes have been disclosed.…”

A Theoretical Study of the Properties and Reactivities of Ketene, Thioketene, and Selenoketene

“…[1,40,41] Two modes of addition are possible: CC addition leads to a cyclic ketone (16) while CO addition gives a cyclic ether (17). Both reactions are symmetry allowed, which involve the interaction of the frontier orbitals of ketene and ethene in two different fashions.…”

“…Wang and Houk described this type of reaction as a [2ϩ2ϩ2] reaction. [41] Likewise, the transition structure for CO addition (19) involves ethene interacting with C α and X, has a significantly shorter CC bond than the CO bond. Upon substitution on the ketene, the HOMO becomes higher-lying, while the LUMO becomes more low-lying.…”

Utilização de α,α-Diclorociclobutanonas em Síntese de Produtos Naturais

“…Os resultados obtidos pelos cálculos realizados, independentemente, por Houk e Wang 22 e Pericàs e Moyano 23 parecem ser bastante consistentes com os dados experimentais. Eles Alguns antibióticos nucleosídicos exibem elevadas atividades antiviral e antitumoral 27a-c .…”