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

A series of novel cyclobutenodehydro(n)annulenes (n = 18, 24, 30) have been prepared as precursors in an organic approach to the cyclocarbons C{sub 18}, C{sub 24}, and C{sub 30}. 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-dionines 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(PPh{sub 3}){sub 4} or with the soluble copper (I) acetylides of (trialkylsilyl)acetylenes. The peculiar downfield resonances of the terminal acetylenic carbon atoms in the {sup 13}C NMR spectra of the 3,4-dialkynyl-3-cyclobutene-1,2-diones are discussed. 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 18{pi}-, 24{pi}-, and 30{pi}-electron perimeters.