Hetero type

Ethyl a-cyano-4-[2-(2-pyridyl)ethenyl]cinnamate (7 OEt) also crystallizes with /3-centrosymmetric-type packing yielding photoreactive crystals and, upon photoirradiation (>410 nm), is converted into the /3-hetero-type dimer (7 OEt-dimer) nearly quantitatively. The 7 OEt-dimer (space group PT, triclinic) has the structure which is predicted from the crystal structure of 7 OEt (space group PT, triclinic). 

The H3 receptor is mainly found as a presynaptic one, both on histaminergic neurons (the auto-type) and on other neuronal systems (the hetero-type). Both the H3 agonist and the H3 antagonist cause important pharmacological effects. Several ligands have become available by now, induding radiolabelled analogues. 

Polymers of monosaccharides may be either of the homo- or hetero-type. When the number of units in a glycosidic chain is in the range of 2 to 10, the resulting compound is an oligosaccharide. More than 10 units are usually considered to constitute a polysac-... 

Where cyclic hydrogen-bond configurations occur, they are referred to as base pairs. They can be of the homo type, if the bases self-associate, or of the hetero type, if different bases interact with each other. The hydrogen-bonded base pairs can form isolated dimers and in many cases this dimerization is extended to two-dimensional ribbon patterns or to three-dimensional networks of hydrogen bonds (see Chap. 16). 

This controlled dye array can be mimicked using thin film preparation techniques. The LB method is the technique most suited to layering various dye molecules in a desired sequence and at desired distances. Figure 6.8 shows hetero-type LB films that can mimic the dye array in the photosynthetic system of a cyanobacterium. Detailed time-resolved fluorescence spectra upon... 

Isoda, S., Y. Hanazato, K. Akiyama, S. Nishikawa, and S. Ueyama (2003). Photoelectrical properties based on photo-induced electron transfer process in flavin-porph5n in hetero-type Langmuir-Blodgett films. Thin Solid Films 441, 277-283. 

Low-valent nitrogen and phosphorus compounds are used to remove hetero atoms from organic compounds. Important examples are the Wolff-Kishner type reduction of ketones to hydrocarbons (R.L. Augustine, 1968 D. Todd, 1948 R.O. Hutchins, 1973B) and Barton s olefin synthesis (p. 35) both using hydrazine derivatives. 

Malpass, 1977). Diels-Alder type [2 + 4]-cycloadditions are possible with certain hetero-"ene components (J.R. Malpass, 1977 S.F. Martin, 1980) or with highly reactive o-quinodimethanes as diene components (W. Oppoizer, I978A). 

In this example both the iso and syndio sequences consist of eight repeat units, with seven triads in each. The repeat unit marked is counted as part of each type of triad, but is itself the center of a hetero triad. 

As apparent from structure [XVIII], a hetero triad occurs at each interface between iso and syndio triads. The total number of hetero triads, therefore, equals the total number of sequences of all other types = SNnj +... 

The first phosphazene polymers containing carbon (79), sulfur (80,81), and even metal atoms (82) in the backbone have been reported. These were all prepared by the ring-opening polymerization of partially or fully chloro-substituted (or fluoro-substituted) trimers containing one hetero atom substituting for a ring-phosphoms atom in a cyclotriphosphazene-type ring. 

Aromatic six-membered heteroeyeles, isoeleetronie with benzene, are widely distributed in nature, and in the world of synthetie ehemistry. Sinee and C are isoeleetronie, the simplest and most direet hetero-analogue of benzene (1) is the pyridinium ion (2). Further azonia substitution of this kind is theoretieally possible, but knowledge of this type of strueture does not extend beyond the disubstituted speeies (3)-(5). 

Cyclocondensation reactions with perfluoroalkyl-subsbtuted CO and CN multiple bond systems can be divided into several subgroups, according to the charge pattern of both reactants On the basis of this simple concept, hetero-l,3-dienes should undergo two types of condensation reactions, classified by the number of skeleton atoms of the diene being incorporated into the ring system (equation 10). 

Reaction type 3 (equation 10), where the complete hetero-l,3-diene skeleton is incorporated into the newly formed ring system, occurs with compounds having both a nucleophilic center and an electrophilic center If these two functionalities are in positions 1 and 2, various types of six-membered ring systems become accessible 4,4-Bis(trifluoromethyl)-I,3-diaza-1,3-butadienes require only room temperature to react with acetyl cyanide to yield l,4,5,6-tetrahydropynmidin-6-ones [96] Likewise, certain open-chain 1,3-diketones (acetylacetone and acetoacetates) and the heterodiene form six-membered nng systems [97] (equation 19)... 

Three skeleton atoms of a hetero-l,3-diene are incorporated into a five-membered ring system on reaction with 1,2 [n,n] compounds (e.g., hydrazines, reaction type 4, equation 10). On further heating in the presence of azobisiso-butyronitrile (AIBN), the bis(trifluoromethyl)-substituted 1,2,4-triazolines are transformed into 5-tnfluoromethyl-l,2,4-triazoles in high yield [t02 (equation 22). 

Perfluoroalkyl groups adjacent to multiple bond systems lower the frontier molecular orbitals (FMOs) Therefore, cycloaddition reactions preferentially occur with electron-rich multiple-bond systems The preference of bis(trifluoromethyl)-substituted hetero-l,3-dienes for polar reacuons makes them excellent model compounds for developing new types of diene reactions deviating from the well documented Diels-Alder scheme (pathway 1) A systematic study of the reactions of diene (1 =2-3=4)-dienophile (5=6) combinations reveals new synthetic possibilities that have not yet been fully exploited as tools for preparative organic cherrustry (equation 25)... 

This new reaction type should be transferable to nonfluorinated hetero-1,3-di-enes that are capable of stepwise cycloaddition reactions... 

The terms polypeptide and protein are used interchangeably in discussing single polypeptide chains. The term protein broadly defines molecules composed of one or more polypeptide chains. Proteins having only one polypeptide chain are monomeric proteins. Proteins composed of more than one polypeptide chain are multimeric proteins. Multimeric proteins may contain only one kind of polypeptide, in which case they are homomultimeric, or they may be composed of several different kinds of polypeptide chains, in which instance they are heteromultimeric. Greek letters and subscripts are used to denote the polypeptide composition of multimeric proteins. Thus, an ag type protein is a dimer of identical polypeptide subunits, or a homodimer. Hemoglobin (Table 5.1) consists of four polypeptides of two different kinds it is an hetero-multimer. 

Heteroatom fullerene-type clusters — The possibility of incorporation of hetero atoms into C clusters has excited the attention of both theoreticians and experimentalists since the earliest days of fullerene chemistry, particularly in view of the known stability and ubiquity of organic heterocycles. The structural relationship between Ceo and /3-rhombohedral boron has already been alluded to (p. 142). 

Formation of a quinonoid carboline-type anhydro-base requires loss of resonance stabilization of the indole moiety. In the carboline anhydro-bases this is counterbalanced by the preservation of a 677 system in the hetero ring. No such balancing factor is present in the case of 3,4-dihydro-j3-carboline derivatives. Formation of the exocyclic anhydro-base in the latter case preserves benzenoid resonance. It is noteworthy that in the two cases where formation of a carboUne-type anhydro-base was reported in dihydro derivatives additional aromatic conjugation is present. 

Finally, the Hammett equation has been applied in a few instances to heterocycles of the indole-benzofuran type. The double p method of Eq. (3) was first designed for this type of system and was here applied. When this approach was originally proposed, the only truly hetero-cychc system to which it was apphed was the substituted phthahds 14, and pertinent data on the hydrolysis of these compounds are included in Table IX. 

In recent years, several interesting developments in the chemistry of pseudooxazolones have been reported. The chemical properties of this system depend to some extent on the type of substitution at the 2-position of the hetero ring. The discussion, therefore, is divided into two parts. 

For azapentatriafulvalenium salts of type 35 the charge distribution was made evident by a comparison of the NMR spectra of these heteroful-valenes and of 3-benzhydrylidene-3H-indolium salts and their common indole precursors (68TL5541).The results suggested that the best description involves extensive delocalization of charge throughout the cyclopropene and indole rings. 

Hetero Diels-Alder reactions can also he classified into two types of cy-... 

The final class of reactions to be considered will be the [4 + 2]-cycloaddition reaction of nitroalkenes with alkenes which in principle can be considered as an inverse electron-demand hetero-Diels-Alder reaction. Domingo et al. have studied the influence of reactant polarity on the reaction course of this type of reactions using DFT calculation in order to understand the regio- and stereoselectivity for the reaction, and the role of Lewis acid catalysis [29]. The reaction of e.g. ni-troethene 15 with an electron-rich alkene 16 can take place in four different ways and the four different transition-state structures are depicted in Fig. 8.16. 

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