Precursors to the cyclo[n]carbons: From 3,4-dialkynyl-3-cyclobutene-1,2-diones and 3,4-dialkynyl-3-cyclobutene-1,2-diols to cyclobutenodehydroannulenes and higher oxides of carbon

Article abstract of DOI:10.1021/ja00160a047

A series of novel cyclobutenodehydro[n]annulenes (n = 18, 24, 30) have been prepared as precursors in an organic approach to the cyclocarbons C₁₈, C₂₄, and C₃₀. On the way to these macrocycles, synthetic entries to three new classes of enediynes have been developed. Bis(1-propynyl)cyclopropenone was prepared in the reaction of 1-(trimethylsilyl)-1-propyne with trichlorocyclopropenylium tetrachloroaluminate. The 3,4-dialkynyl-3-cyclobutene-1,2-diones were prepared by the reaction of 3,4-dichloro-3-cyclobutene-1,2-dione either with (tri-n-butylstannyl)alkynes in the presence of catalytic amounts of Pd(PPh₃)₄ or with the soluble copper (I) acetylides of (trialkylsilyl)acetylenes. The peculiar downfield resonances of the terminal acetylenic carbon atoms in the ¹³C NMR spectra of the 3,4-dialkynyl-3-cyclobutene-1,2-diones are discussed. The stereospecific and regiospecific reduction of these diones with cerium trichloride-sodium borohydride afforded exclusively the cis-3,4-dialkynyl-3-cyclobutene-1,2-diols. The oxidative Hay coupling of the acetonide of 3,4-diethynyl-3-cyclobutene-1,2-diol or of the bis(ethylene ketal) of 3,4-diethynyl-3-cyclobutene-1,2-dione gave two series of cyclobutenodehydroannulenes with 18π-, 24π-, and 30π-electron perimeters. In both series, the unusually stable octahydro[24]annulene perimeter is formed with the highest yield. Deprotection of the trimer and the tetramer, obtained in the oxidative coupling of the bis(ethylene ketal) of 3,4-diethynyl-3-cyclobutene-1,2-dione, was best effected in concentrated sulfuric acid and yielded quantitatively the (3-cyclobutene-1,2-dione)-annelated dehydroannulenes 4 (C₂₄O₆) and 5 (C₃₂O₈), novel higher oxides of carbon and direct precursors to the cyclocarbons C₁₈ and C₂₄.