The well known, extremely reactive [18]annulene 1 and the hypothetical isomer 2 should be capable of being stabilized by CH2- and CHCH-groups as internal and external bridges, respectively. The corresponding hydrocarbons, 3 and 4, are still unknown, although they are of great interest as links between the non-natural annulenes and the porphyrin 5 or the porphycene 6 recently synthesized for the first time. It has now been possible to prepare the tetraoxa analogues of 5 and 6, i.e. the dications 7 and 8. In both cases they could be isolated as the perchlorates and structurally characterized, revealing their relationship with 5 and 6, respectively. In the synthesis of 7 the decisive step was a MacDonald condensation, followed directly by an oxidation, whereas the route to 8 led via a McMurry coupling and subsequent oxidation. Reaction of 2,2′-difurylmethane with formaldehyde afforded tetraoxaporphyrinogen in low yield, which could be oxidized with nitric acid to the tetraoxaporphyrin.
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Vogel et al. (1988) studied this question.