Transition Metal Promoted Ring Expansion of Alkynyl- and Propadienylcyclopropanes

Abstract

Transition metal promoted ring expansion of alkynyl- and propadienylcyclopropanes is surveyed with emphasis on the Co₂(CO)₈-mediated reactions developed by the author’s group. A novel rearrangement of 1-(1-alkynyl)cyclopropanols to 2-cyclopenten-1-ones proceeds on complexation of their alkynyl part with Co₂(CO)₈. In the case of the reactions of 1-alkynylcyclopropanols with an alkyl substituent on the cyclopropane, either 4-substituted or 5-substituted 2-cyclopenten-1-ones can be selectively obtained by appropriate choice of stereochemistry and protective group of the substrates. This rearrangement is successfully applied to cyclopentenone annelation reactions onto cycloalkenes. The rearrangement proceeds catalytically on addition of triaryl phosphite as ligand. The same type of rearrangement proceeds when 1-[o-(1-alkynyl)phenyl]cyclopropanols are employed as substrates; these are converted to 2,3-dihydro-1-naphthalenone derivatives on heating their hexacarbonyldicobalt complexes in 2-propanol. Furthermore, a new type of isomerization-cyclization reaction proceeds to give 3a,4-dihydro-3H-cyclopenta[a]inden-2-one derivatives when the same reaction is carried out in the presence of 1,4-diazabicyclo[2.2.2]octane (DABCO). A novel transformation of 1-propadienylcyclopropanols into substituted 1,4-hydroquinones is developed utilizing the interaction of the 1,2-propadienyl group and Co₂(CO)₈. This reaction is applied to the synthesis of vitamin E and K analogs.