DDQ) it can also be converted into the corresponding tetraketone (ceric ammonium nitrate), into the monosulfone (peracids) as well as into a still unidentified red ionic species (electrochemically). By way of contrast, tctraoxaporphyrinogen 6 has thus far always afforded the dication 1 with all reagents which effect oxidation with retention of the ring skeleton. The electrochemical oxidation of 6 also gave 1 (private communication from Prof. E. Sfeckhan, Universitlt Bonn (FRG)).[I41 C,,H,,CI,O,S,, decomp. at 240 "C, crystals from HCIO,; triclinic, space group P i , 2 = 1; a = 5.850(1), 6 = 8.994(1), c = 10.547(2) A, u = 78.90(2), p = 81.03(2), y = 82.19(2)"; e..,.,, = 1.800 measuring conditions as for 7; refinement (C,CI, 0,s anisotropic, H isotropic) taking into account 1791 reflections with F, > 4u(F,); R = 0.045, R , = 0.053 1201.[15] In the case of 2 a there are no indications of bonding between sulfur atoms that lie opposite to each other in the porphyrin core-as has been assumed and is theoretically SubStanlidled in the case of tetraphenyl-21,23-dithiaporphyrin on the basis of the extremely short S . . . S distances of 3.05 8, [4c, d] Isophlorin 1, a still elusive dihydroporphyrin first referred to by Woodwardsome 30 years ago in connection with the synthesis of chlorophyll,"' is a molecule equally interesting from the standpoint of porphyrin and annulene chemistry. It seems justified to regard 1 as a "cyclobutadiene homologue" since it constitutes an essentially planar [20]annulene associated with a degenerate, presumably very fast x-bond shift. As a consequence of its x-electron structure, 1 is expected to exhibit a pronounced tendency to undergo a two-electron oxidation with formation of resonance-stabilized diprotonated porphyrin, and hence to be a dihydroporphyrin of only fleeting existence.The anticipated ready oxidizability of 1 would explain why N,N-dihydroporphycene 2, the analogue of 1 in the detection. Other than in the case of 1, there is considerable steric hindrance of the NH-hydrogen atoms in 2. For this reason 2 is in all probability not planar but has the pyrrolophanediene structure 2 a.f3, 41 We have now found that N,N'-dihydroporphycenes are in certain cases isolable, are in fact pyrrolophanedienes, and, what was unforeseen, form redox systems with the corresponding porphycenes.Examination of the stereochemistry of porphycene 3 reveals that peripheral alkyl groups, particularly in the 9, 10, 19, 20 as well as in the 3, 6, 13, 16 positions are capable of modulating the geometry of the tetrapyrrolic macrocycle relatively strongly through non-bonding interactions. A representative example of this is the 9,10,19,20-tetrapropylporphycene 6, which is obtainable analogously to the synthesis of 3 by reductive coupling of the bipyrrole derivative 4 with titanium tetrachloride and zinc.f5] According to an X-ray structure analysis, 6, like 3, has a planar ring skeleton, but due to the repulsion of the propyl groups there is a convergence of the oppositely disposed nitrogen atoms, so that the N, cor...