Intramolecular [2 + 2] cycloadditions of ketenes

Theoretical Study of the Electronic Effects in the Intramolecular Ketene-Styrene [2+2] Cycloaddition

“…On the other hand, only intermolecular reactions of ketene-styrene cycloaddition, where the substituted styrenes are in para position, have been reported [9][10], whereas the first report on the intramolecular ketene-styrene cycloaddition was recently published in the literature by Bèlanger et al [11]. In this report, two important conclusions can be accounted in the intramolecular ketene-alkene [2+2] cycloaddition: firstly, the regiochemistry of the cycloaddition is determined by the substitution pattern of the double bond [12], in which the more substituted internal alkene carbon will produce bicyclo[n.2.0]alkanones, while substrates in which the terminal alkene carbon is more substituted will give bicyclo[n.1.1]alkanones ( Figure 2). Secondly, the reaction yield will be higher with more nucleophilic alkenes.…”

“…Furthermore, the influence of the electronic character of the substituted alkene in the cycloaddition reaction was modulated by a p-substituted aryl group in which only the bicycle[3.1.1] heptanone was produced as experimen-tal observations with a particular regiochemistry has shown: psubstituted aryl group attached to the bicycle 1, and the phenyl group attached to the bicycle 2 ( Figure 3). The aryl attached to the bicycle is produced in higher yields by electrondonating (ED) groups than by electronwithdrawing (EW) groups [12][13]. Thus it was concluded by means of a Hammett study, that the reaction is accelerated by ED groups because of the determined value of ρ = -1.39 for this reaction in toluene [11].…”

Racemic synthesis of 1,2-secomicrominutinin

“…14 Reaction of 14 with bromoacetic acid produced compound 15 in 88% yield, which, after intramolecular [2+2] cycloaddition reaction, 6,7 furnished the tricyclic benzocyclobutafuranones 16 in 73% yield. The benzofuranofuranone 17 (R = Bn) was obtained in 81% yield through a Baeyer-Villiger oxidation of 16.…”

“…Recently, we described the syntheses and the conformational studies 5 of several model compounds of natural products containing the basic benzofuranofuran skeleton. The intramolecular [2+2] cycloaddition of alkoxyketenes with alkenes, developed by Snider et al 6 and by Brady et al, 7 was used as a useful synthetic method in the preparation of these polycyclic compounds. As part of our interest in the synthesis of a family of compounds bearing the benzofuranofuran moiety, this methodology was also used to synthesize, for the first time, the racemic cinnamic ester derivative 1,2-secomicrominutinin (25).…”

Intramolecular [2+2] Cycloaddition of Homoallylketenes to Bicyclo[2.1.1]hexan-5-ones

“…1) It is known that the tether length between ketenes and alkenes influences the yield of the intramolecular cycloaddition, and many examples of three-atom tethers have been reported. [1][2][3][4] Intramolecular cycloaddition reactions of alkenylketenes with a longer tether have been achieved with reactive ketenes (keteniminium salts 5) and alkoxyketenes 6) ) or ketenes having conformationally restricted tethers. 7) On the other hand, reactions of alkenylketenes with a tether of less than three atoms are very rare.…”