Unsaturated Macrocyclic Compounds. LI. 1,6-Oxido[10]annulene

9,10-Oxido-1,4,5,8,9,10-hexahydronaphthalene (10) on hydrolysis to 1,4,5,8,9,10-hexahydronaphthalene-trans-9,10-diol (11), followed by successive bromination and dehydrobromination, gives the tribromo oxide 13a and the diene bromo oxide 14 instead of the anticipated 9,10-dihydronaphthalene-trans-9,10-diol (5). The oxide 10 on bromination and subsequent dehydrobromination yields 1,6-oxido[10]annulene (20), as well as 1-benzoxepin (24). In agreement with theory, the ten π-electron system 20 is shown to be an aromatic compound. Various reactions of 20 are described, including its nitration to the 2- and 3-nitro derivatives 27 and 26. 1,6-Oxido[10]annulene (20) is easily converted to naphthalene derivatives, e.g., by lithium aluminum hydride reduction, catalytic hydrogenation, and treatment with acids.