Synthesis of 1,6-diethynylcyclohepta-1,3,5-triene and its oxidative coupling to dimethano-bridged octadehydro[20]annulene and trimethano-bridged dodecadehydro[30]annulene

1,6-Diethynylcyclohepta-1,3,5-triene 27 was synthesized from 1,6-diacetylcyclohepta-1,3,5-triene 24. Oxidative coupling of the diacetylene 27 with copper(II) acetate in pyridine (Eglinton conditions) led to the cyclic ‘dimer’, a dimethano-bridged octadehydro[20]annulene 28, in 18% yield and the cyclic ‘trimer’, a trimethano-bridged dodecadehydro[30]annulene 29, in 19% yield. Both compounds 28 and 29 consisted of an equilibrating mixture of two conformers which differed in the relative disposition of the methano bridges. Oxidative coupling of compound 27 with oxygen, copper(I) chloride and ammonium chloride (Glaser conditions) afforded 2,3,4,5-tetrachloro-1,6-methano[10]annulene 32 in 3.5% yield together with compounds 28 and 29.