Isocycles

Division II. Acyclic compounds Isocyclic stem nuclei). The carbon... 

Chlorophyll a (L.P. Vernon, 1966) contains an unsymmetrical porphyrin chromophore with two special features the double bond between C-17 and C-18 is hydrogenated and carhon atoms 13 and 15 hear a carboxylated, isocyclic cyclopentanone ting E. 

As expected, heterocyclic enols and potential enols (i.e, compounds existing mainly in the CH form) behave toward diazomethane similarly to the open chain and isocyclic enols, i.e. they form enol methyl ethers by reactions of the SnI type (cf. footnote 29). Examples of this behavior are barbituric acid, picrolonic acid, dchydroacetic acid (64), 3-methyl-l-phenylpyrazolin-5-one, 1-phenylpyrazoli-dine-3,5-dione, 1,2-diphenylpyrazolidine-3,5-dionc, 3-hydroxy-... 

An alternative route to the reaction sequence applied by Woodward for the formation of the isocyclic ring E of chlorophyll a in his synthesis has been developed.40 55,56 It was shown that... 

The typical isocyclic ring E present in chlorophylls is susceptible to a number of different modifications such as epimerization, which produces stereoisomers by inversion of the configuration at C-13 of their parent pigments. These 13 -epichlorophylls, known as chlorophylls a and b, are minor pigments. They are considered artifacts produced in the course of handling plant extracts and sometimes are also found in small amounts in heated and deep-frozen vegetables, hi the old Fischer systan of nomenclature that can still be found in some literature, these epimers were named 10-epichlorophylls. 

Esterification of the propionic acid side chain at C-13 (ring C) with a methyl group catalyzed by S-adenosyl-L-methionine-magnesium protoporphyrin 0-meth-yltransferase yields protoporphyrin IX monomethyl ester (MPE), which originates protochlorophyllide by a P-oxidation and cyclization of the methylated propionic side chain. This molecule contains a fifth isocyclic ring (ring E), the cyclopentanone ring that characterizes aU chlorophylls. 

The acyclic division, the isocyclic division and all hetero-classes are further divided into 28 main classes, the first of which consists of the stem nuclei (i.e., hydrocarbons in the acychc and isocyclic divisions), whilst the others depend upon the functional group present. The most important main classes and functional groups are collected below. 

Isocyclic and heterocyclic compounds are very often synthesized by cycloaddition reactions. The most widespread reactions are the Diels-Alder reaction and the 1,3-dipolar cycloaddition. 

In this paper, we present the detailed spectral information obtained for individual NH-tautomers and straightforward experimental arguments which permit us to relate the real position of the H-H axis to the molecular oscillator axes determined by substituents of different types (isocycle or alkyl groups) in two NH-tautomers. Then we used this structural information as a basis for analyzing the inversion of electronic Qx(0,0) and Qy(0,0) band intensities using the four-orbital model. 

Synthesis, purification and identification of cyclopentanporphyrins and their chemical dimers covalently linked via isocycles have been described in previous papers (17,19-23). The porphyrins with the cyclopentanone ring have been prepared by familiar methods (24-26). 

When analyzing In detail the spectroscopic observations of the Investigated compounds and symmetrical porphyrins some spectral pecularltles should be emphasized, which are characteristic only of porphyrins with non-symmetrical substitution. Figures 2-9 give some experimental results obtained for porphyrins with different side and Isocycle substituents. 

The reactions in Methods A, B and C, which all start from pyrylium salts are analogous to the well-known conversions of 2.4.6-substituted pyrylium salts 27 with ammonia, primary amines, hydrogen sulfide or the anions of CH activated compounds to the corresponding heterocyclic or isocyclic aromatic systems The first step involves addition of the basic phosphine at C-2 (or C-6) to form 2S. Ring-opening, ring-closure and elimination of water are likely steps in the formation of the product 2. 

This family of w-excessive heterocycles whose members are isocyclic with cyclooctatetraene has been studied in considerable detail and is currently known in parent as well as variously benzoannulated forms. 

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