Configurational isomers defined

Note that, in situations where a ring fusion produces chiral centres, we can find the number of configurational isomers possible is less than that predicted from the 2" guidelines. This may be the consequence of symmetry, in that an isomer is the same as its mirror image, as we have seen above. However, it can also be the result of restrictions caused by the ring fusion, so that one centre effectively defines the chirality of another, thus reducing the number of combinations. In epoxycyclohexanes, no rran -fused variants can exist. 

Recently Ford and co-workers performed calculations of the structures and relative stabilities of a series of 20 polycyclic nitrenium ions at the HF/6-31G(d) level.Except for symmetrical cases, all ions existed as distinct syn and anti isomers defined by the configuration of the N—H bond with respect to the unsymmetrical aryl group. The energies of configurational isomers differed by between 0.2 and 3.7 kcal/mol. Inversion barriers through a linear transition state were fairly constant in the range from... 

The cases of polysubstituted cyclic systems are obviously more complex due to the existence of a number of possible configurational isomers. A simple cis-trans nomenclature is obviously not sufficient here, so the IUPAC recommends to designate the various diastereoisomers by choosing a reference substituent (the lowest numbered substituent, designated r) and defining its cis or trans (c or t) relationship to all other substituents (see [60]). Thus, the three diastereoisomers of LIII are c-2,c-5-dimethyl-, f-2, f-5-dimethyl and c-2, t-5-dimethyl-r-1 -cyclopentanol. 

Alkaline hydrolysis of the cis- and rra 5 -isomers (defined with respect to R and Ph) of the phosphorinanium salts (130) does not proceed stereospecifically. For both salts, the cis-isomer produces a different ratio of the diastereoisomeric oxides (131) than does the /ran -isomer. For both fran -isomers, the product ratio is the same. However, the respective cw-isomers give markedly different amounts of the diastereoisomeric oxides, there being a greater degree of retention of configuration at... 

The data collected so far (23-28) has shown that the norisoprenoids 8-11 are present in natural substrates in well-defined mixtures, thus enabling inter alia authenticity control of natural flavoring material. (Note The only exception are raspberries, which showed considerable variations in their composition of isomeric theaspiranes depending on their origin). The data also shows that obviously different enzymes are operative in plants, catalyzing the formation of different optical antipodes of Ci3-volatiles. TWs is evident from the fact that, e.g., vanilla beans were found to contain optically pure (2S)-isomers of vitispiranes lOb/d, whereas white-beam (Sorbus aria) leaves contain mainly the (2/f)-configured isomers lOa/c (25). 

The isomer possibilities in DOTA derivatives can increase dramatically. For example, introducing a chiral center by substitution at Ca, of the acetate arms of DOTA can gives rise to six ligand isomers defined by the absolute configuration at the carbon (SSSS), RSSS (SRRR), and the achiral... 

Sometimes the terms erythro and threo are used to specify fee relative configuration of two adjacent stereogenic centers. The terms are derived fom fee sugars erythrose and threose. The terms were originally defined such feat a Fischer projection formula in which two adjacent substituents were on the same side was fee erythro isomer and feat in whidi the substituents were on opposite sides was the threo isomer. 

For (203), models indicated that the isomer containing cis-syn-cis hydrogen atoms on the cyclohexane ring should be able to form clam-type complexes, provided the cyclohexane ring is in the flexible or twist conformation. The models suggested that the cavity defined by the ten oxygen donors would be ideal for K+. However, for the potassium and barium thiocyanate complexes, configurations of type (204) do not occur in the solid state. Instead, two molecules of the bis-crown coordinate simultaneously to two alkali metal ions - both these 2 2 complexes have structures of type (205). 

For the unambiguous description of the various isomers, it is clearly necessary to have formal rules to define the structural configurations. These rules are explained in the following sections. 

Fischer s Convention. Initially, the absolute configurations of optical isomers were unknown to chemists working with optically active compounds. Emil Fischer, the father of carbohydrate chemistry, decided to relate the possible configurations of compounds to that of glyceraldehyde of which the absolute configuration was yet unknown but was defined arbitrarily. 

An early synthesis of A5-palmitoy]-.S -[2,3-bis(palmitoyloxy)propyl]cysteine employed cysteine methyl ester, however, this leads to difficulties in the saponification step of the tri-palmitoylated residue. 96 The optimized procedure, in which the cystine di-fert-butyl ester is used, 90 is outlined in Scheme 6 after N-acylation with palmitoyl chloride, the ester is reduced to the cysteine derivative for S-alkylation with l-bromopropane-2,3-diol to yield chirally defined isomers if optically pure bromo derivatives are used. Esterification of the hydroxy groups is best carried out with a 1.25-fold excess of palmitic acid, DCC, and DMAP. The use of a larger excess of palmitoyl chloride is not recommended due to purification problems. The diastereomeric mixture can be separated by silica gel chromatography using CH2Cl2/EtOAc (20 1) as eluent and the configuration was assigned by comparison with an optically pure sample obtained with 2R)- -bromopropane-2,3-diol. 

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