Isophlorins: Molecules at the Crossroads of Porphyrin and Annulene Chemistry

The isophlorins already discussed by Woodward in connection with chlorophyll synthesis are Janus-faced—on one hand porphyrins (N, N′-dihydroporphyrins) and on the other true annulenes ([20] annulenes not conforming to the Hückel rule)—and as such are structurally particularly interesting. Whereas isophlorins not substituted at NH have evaded discovery and synthesis, the N, N′, N″, N″′-tetramethyloctaethylisophlorin (1, RCH3) has been prepared by two-electron reduction of the known N, N′, N″, N″′-tetramethyloctaethylporphyrin dication.