ELECTROPHILE-INITIATED SELECTIVE RING TRANSFORMATIONS OF CYCLOPROPYL KETONES

Article abstract of DOI:10.1016/S0040-4020(01)88598-7

Electrophile-mediated cyclopropane cleavage in tricyclo<3.3.0.0^2,8>octan-3-one (1a) is increasingly directed towards the maximum bond overlap site in the following order of reagents: acetyl methanesulfonate, +Br(1-) or I(1-); t-butyl-dimethylsilyl iodide; t-butyldimethylsilyl trifluoroacetate; trimethylsilyl trifluoroacetate.The latter reagent gives rise to one single regioisomer (->6a).Routine yields of isolated products lie between 78 and 87percent.Increasing regioselectivity is governed by increasing electrophilic power and lowered nucleophilic strength of the reagents.Independent of these two factors, a C(4)-exo substituent in 1 directs the opening modes undirectionally (->2b, 6b).Irrespective of the substitution pattern at C(4) (1a-d), the cyclopropane moiety rearranges smoothly to olefinic ketones (8a-d) when the polymer-supported triflate analog Nafion-TMS is used in toluene at 80 deg C.The reaction proceeds via intramolecular proton (deuterion) abstraction by the transient electron-rich enoxy double bond.This is the first fully proved case of such an intramolecular process.Aro-semibullvalenes (18,22) similarly rearrange to aro-semibarrelenes (e.g. 21, 23) in the presence of Nafion-TMS.The latter rearrangement also takes place at room temperature when 18 or 22 are treated with commercial tetramethylsilane (TMS) and a catalytic amount of trifluoroacetic acid.An unknown impurity in the TMS reacts with the acid to form a powerful electrophilic composition.A cheap and convenient in situ preparation of TMS-triflate is described by mixing trifluoromethanesulfonic acid and TMS at room temperature.