Cross-substitution principle

This is the cross-substitution principle, which allows the same stoichiometry to be kept, and possibly also the same structure type, if other parameters, such as a compatible size of the substituting cations, have been taken into account. This parallel drawn with oxides, the corresponding structures of which are well-known, has led to a comprehensive structural study of nitride-type compounds (see for example Marchand et al. 1991a). 

Rings with more than two differently substituted carbons can be dealt with on similar principles. In some cases, it is not easy to tell the number of isomers by inspection. The best method for the student is to count the number n of differently substituted carbons (these will usually be asymmetric, but not always, e.g., in 68) and then to draw 2" structures, crossing out those that can be superimposed on others (usually the easiest method is to look for a plane of symmetry). By this means, it can be determined that for 1,2,3-cyclohexanetriol there are two meso compounds and a dl pair and for 1,2,3,4,5,6-hexachlorocyclohexane there are seven meso compounds and a dl pair. The drawing of these structures is left as an exercise for the student. 

In principle, three basically different types of reaction modes are applied for cross-coupling reactions of allenes. First, cross-couplings of allenes with suitable halogen or metal substituents at one of the sp2-hybridized carbons furnish products still bearing the intact cumulene it-system. On this basis, numerous reactions for conversions of precursor 1 or 3 into substituted allenes 2 have been developed (Schemes 14.1 and 14.2). 

Clindamycin is a chlorine-substituted derivative of lincomycin. However it is more potent and is better absorbed from the gastrointestinal tract and has therefore replaced lincomycin in most situations. Clindamycin is in principle a bacteriostatic agent. Its indications are mainly limited to mixed anaerobic infections. As mentioned above it has a similar mechanism of action as erythromycin. It selectively inhibits bacterial protein synthesis by binding to the same 50s ribosomal subunits. Erythromycin and clindamycin can interfere with each other by competing for this receptor. Also cross-resistance with erythromycin frequently occurs. Resistance is rather chromosomal rather than plasmid mediated and is especially found in cocci and Clostridium difficile. 

Using this principle, Kibayashi and coworkers [147] have introduced a sequential cyclic carbopalladation-Stille vinylation of enyne compounds. Upon treating the enyne 196 and vinyl tributylstannane with catalytic amounts of Pd2(dba)3 CHCI3 in the presence of AcOH the allyl-substituted methylene cyclopentane 197 was formed in 53% yield (Scheme 80). The subsequent cross-coupling occurs with complete suppression of /M I-climinalion and the Alder-ene product 198 was not detected. Likewise, this sequence was extended to heteroatom-linked enynes and further vinyl tin compounds to provide the heterocyclic analogs 199 in moderate to excellent yields (Scheme 81). 

Several approaches have been used to calculate the effect of isotopic substitution on the absorption cross section of N2O. The zero point energy (ZPE) model [83] as described in Section 4, 2D and 3D wavepacket dynamics [110,118], a semi-empirical model [85,86] and an extended reflection principle model [84] have been used to explain the differences in the absorption that stem from isotopic substitution. In the following the emphasis lies on the enhanced understanding of the isotopic differences in the photodissociation reactions, which arises from employing wavepacket propagation calculations for the various isotopologues. 

The photochemistry of the carboxyl group has attracted only limited attention because it absorbs only at wavelengths well below 250 nm. The principle primary photoprocess is known to be either homolytic cleavage in the case of undissociated acids [for example, the dissociation energy (Dc c) in CH3—COOH is 385 kJ mol ] or heterolytic cleavage in the case of carboxylate anions, followed by a photodecarboxylation step (Scheme 6.143).322,1025-1027 The latter process may involve the formation of a radical cation precursor. The photoreactions of aliphatic derivatives usually proceed via an excited singlet state (the intersystem crossing efficiency in substituted acetic acids is low) and they are inefficient ([Pg.331]

The synthesis of alkene-substituted arenes by cross-coupling can, in principle, be achieved either by the reaction of alkenyhnetals with aryl halides and related electrophiles or by that of arylmetals with alkenyl electrophiles. For the sake of simplicity, the former reaction is termed the alkenyl-aryl coupling, while the latter is termed the aryl-alkenyl coupling in this Handbook. Similar terms may be devised by linking the carbon groups of the organometal and organic electrophile with a dash in this order. 

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