A isocyclic

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. 

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. 

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. 

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. 

The position of the entry for any compound is determined by that fragment of its structure which is classified latest in the Beilstein System. Thus, for example, a compound containing a heterocyclic ring, a carbocyclic ring and an aliphatic chain is classified as a heterocycle, irrespective of the other structural elements, since all heterocycles are ordered after acyclic and isocyclic compounds in the Beilstein System. This feature of the systematic classification of compounds is called the Principle of Latest Systematic Entry. 

Another heterocyclic compound obtainable in considerable quantities-quin-oline-nitrates in the isocyclic ring. Indole Mid carbazole behave in a similar way. Nitro derivatives of these compounds have not yet found practical application. 

In the presence of Me2AlCl, isocyclic allylsilane 28 adds to 3-butyn-2-one to afford an ct-enone bearing an allylsilane moiety (Equation (38)).1S1,1Sla In contrast, the ZnI2-promoted ene reaction of an acyclic /3-substituted allylsilane with an a-ynone forms a vinylsilane product exclusively.152... 

We neglect all interaction integrals pa. between atoms not bound to each other and we put the other p5s in molecules where the conjugated system consists of one kind of atom, such as the isocyclic hydrocarbons, polyenes etc., equal to each other. We also put the ok equal for all the atoms in this case. This is actually strictly correct for a molecule such as benzene, but this is no longer obvious even for naphthalene. In this approximation we also put the % always zero. 

The basic structure of Chls is the tetrapyrrolic porphyrin macrocycle with an attached isocyclic ring (Fig. 1). This structure has been termed phytoporphyrin by the lUPAC commission (2). The four nitrogens bind a magnesium ion (Mg +) as central... 

The best source for Chi a is the cyanobacterium Spirulina platensis, which is available commercially. Chi a/b mixtures can be obtained from all green plants. All other Chls are less readily accessible, which limits their applications. Several Chi derivatives are used as dyes for food colorants (Cu-chlorophyllin) and cosmetics. The chlorophyll used for the latter is a complex mixture of degradation products. More recently, (B)Chl derivatives have gained increasing interest as photosensitizers in photodynamic therapy of cancer, these compounds include pigments in which the isocyclic ring is opened and/or the central metal has been removed or replaced (e.g., by Pd" " ) to increase phototoxcicity (12, 13). 

As a result of this first report, a large number of compounds containing an isocyclic or a heterocyclic ring fused tc the steroid skeleton were prepared and tested for androgenic and anabolic activity. Included are steroidal isoxazoles [205], thiazoles [94], pyrroles [206], pyrimidines [207,208], oxazines [209], thiazenone [92], quinoline [107], pyridine [122], and furazane [121], all prepared by the fusion of the heterocyclic ring to ring A. Several bis heterocyclic steroids were also prepared by... 

Only fusion of heterocyclic rings to the 2,3-position proved to be a useful extension of the steroidal nucleus to produce enhanced activity or better dissociation of androgenic-anabolic activity. Isocyclic ring extensions and fusion of a heterocyclic ring to other positions, such as 1,2 (D-98) or 16,17 (E-27,E-31), gave products with decreased or abolished activities. 

In addition to the skeletal modifications accomplished by replacement of one carbon atom, several attempts have been made to improve hormonal activity by replacement of a whole isocyclic ring by a heterocyclic ring. The isocyclic ring A was replaced by a pyrazole [211], isoxazoline [212], pyrimidine [213], or thiopyrimidine [214] ring however, all these types of compounds were inactive. In view of the fact that fusion of the same heterocyclic rings in addition to the steroid nucleus led to highly active compounds, this finding emphasizes the importance of the steroid skeleton. 

---