Donor–acceptor cyclopropanes as ortho-quinone methide equivalents in formal (4 + 2)-cycloaddition to alkenes

Abstract

A new type of donor-acceptor cyclopropane reactivity towards alkenes was revealed for 2-arylcyclopropane-1,1-diesters that contain an OH-group in the ortho-position of the aryl substituent. In this case, the initial cyclopropanes participate in formal (4 + 2)-cycloaddition as synthetic equivalents of ortho-quinone methides which are potential intermediates generated under mild conditions in the presence of a Lewis acid.

Protic Ionic Liquid as Reagent, Catalyst, and Solvent: 1‐Methylimidazolium Thiocyanate

“…To achieve further proof of concept for the triple role of PILs,w ee mployed 1-methylimidazolium thiocyanate to perform the annulation of isothiocyanic acid with 2-hydroxyphenyl-derived cyclopropane 1ah, [32] serving as the equivalent of the corresponding ortho-quinone methide. [33] Indeed, under heating in HMimNCS at 70 8 8C, cyclopropane 1ah was smoothly transformed into [1,3]benzoxazine-2-thione 7 in reasonable yield. We believe that this reaction proceeds by as imilar stepwise mechanism, including the nucleophilic opening of at hree-membered ring with thiocyanate ion followed by an attack of the ortho-hydroxy group on the formed isothiocyanate moiety (Scheme 6).…”

Protic Ionic Liquid as Reagent, Catalyst, and Solvent: 1‐Methylimidazolium Thiocyanate

“…We believe that this reaction proceeds by a similar stepwise mechanism, including the nucleophilic opening of a three‐membered ring with thiocyanate ion followed by an attack of the ortho ‐hydroxy group on the formed isothiocyanate moiety (Scheme 6). This process, together with the reported transformations of 2‐hydroxyphenyl‐containing D‐A cyclopropanes, [33, 34] could be responsible for the diminished yields of pyrrolidine‐2‐thiones 2 i and 2 j in the reactions of the corresponding cyclopropane‐1,1‐diesters 1 i,j (Scheme 3).…”

Convenient Synthesis of Functionalized Cyclopropa[c]coumarin-1a-carboxylates

“…All reactions were carried out using freshly distilled and dry solvents. Dimethyl 2-(2-hydroxyaryl)cyclopropane-1,1-dicarboxylate was synthesized by the published procedure [36,37]. Commercial reagents employed in the synthesis were analytical grade, obtained from Aldrich (St. Louis, MI, USA) or Alfa Aesar (Ward Hill, MO, USA).…”

“…Yield 75 mg (72%); colorless solid; mp = 98–100 °C (lit. 100–102 °C [34,35]; oil [37]). Spectral data are consistent with the reported ones [34,35].…”

“…Therefore, the development of efficient procedure for the synthesis of 1-unsubstituted cyclopropa[ c ]coumarins remains a challenging task for organic chemists. Herein, we describe a new general approach to cyclopropa[ c ]coumarins based on the use of 2-(2-hydroxyphenyl)cyclopropane-1,1-dicarboxylates (Scheme 1) which can be easily obtained from the corresponding salicylic aldehydes [36,37].…”