Treatment of the porphyrin-α-dione 7 with 3-chloroperoxybenzoic acid gives the ring-expanded porphyrin anhydride, 7,8a-dioxo-5,10,15,20-tetraphenyl-7H,8aH-8-oxa-8a-homoporphyrin 8. Hydrolysis of 8 affords the porphyrin lactone, 8-oxo-5,10,15,20-tetraphenyl-8H-7-oxaporphyrin 11. Methanolysis of 8 gives (5RS,6SR)-5,6-dihydro-6-(methoxycarbonyl)-8-oxo-5,10,15,20-tetraphenyl-8H-7- oxaporphyrin12 , the structure of which was determined by X-ray diffraction methods and refined to a residual of 0.059 for 1 841 independent observed reflections. The crystals of 12 are monoclinic, P2 1 ,a = 13.030(3), b = 12.132(4), c = 13.632(2) a, β 113.27(2) o , Z = 2. Compound 12 has a non-aromatic macrocyclic ring and is the only example of a stable 5,6-dihydroporphyrin. Compound 12 is an isomer of 2,3-dihydroporphyrins (chlorins) 2, 5,22-dihydroporphyrins (phlorins) 3 and 5,15-dihydroporphyrins (porphodimethenes) 4 but is restricted from tautomerizing to these well-known ring systems by the fact that the 6-position is tetrasubstituted. The distortion caused by thesp 3 -hybridized C-5 and C-6 atoms in compound 12 is accommodated in the remainder of the macrocyclic ring by the formation of two nearly planar dipyrromethene-like units (one formed by rings B and C and the other by rings A and D), which are hinged about the meso carbon (C-15) opposite the sp 3 atoms. A mechanistic rationale for the formation of compound12 is presented.Le traitement de la porphyrin-α-dione 7 par l'acide 3-chloroperbenzoique conduit a la 7,8a-dioxo-5,10,15,20-tetraphenyl-7H, 8aH-8-oxa-8a-homoporphyrine 8. L'hydrolyse de 8 fournit la lactone 8-oxo-5,10,15,20-tetraphenyl-8H-7-oxaporphyrine 11. La methanolyse de 8.conduit a la (5RS,6SR)-5,6-dihydro-6-(methoxycarbonyl)-8-oxo-5,10,15,20-tetraphenyl-8H-7- oxaporphyrine 12 dont la structure a ete determinee par les methodes de diffraction aux RX et affinee jusqu'a 0.059 pour 1 841 reflexions independantes observees. Les cristaux de 12 sont monocliniques, P2 1 , a = 13.030(3), b= 12.132(4), c = 13.632(2) a, β = 113.27(2) o , Z = 2. Le compose 12 possede un macrocycle non aromatique et represente le seul exemple d'une 5,6-dihydroporphyrine stable. Le derive 12 est un isomere de 2,3-dihydroporphyrines (chlorines) 2, 5,22-dihydroporphyrines (phlorines) 3 et 5,15-dihydroporphyrines 4, mais ne peut realiser de tautomerisation pour conduire a ces cycles bien connus car la position 6 est tetrasubstitutee. Dans le compose 12 la distorsion provoquee par les carbones C5 et C6 hybrides en sp 3 est compensee par la formation de deux proches unites dipyrromethene planes (une formee par les cycles B et C et l'autre par les cycles A et D) qui sont sur le carbone meso (C15) oppose aux atomes sp 3 . Un mecanisme pour la formation de 12 est presente.