Racemization, degenerate

In initial ICC studies, animals were treated with MDA or MDMA using the protocol described by Ricaurte et al. (1985). Adult Sprague-Dawley rats (150 to 200 g) reeeived subcutaneous injections of racemic MDA or MDMA every 12 hours for 4 days. Each dose was equivalent to 20 mg/kg of the free base. The rats were sacrificed by intracardiac aldehyde perfusion 2 weeks after the final dose. In order to study subacute effects for evidence of degeneration, additional rats received MDA every 12 hours for 2 days and were sacrificed 24 hours after the last injection. Additional experimental details are described elsewhere (O Heam et al. 1986 O Heam et al. 1988). A series of animals treated identically and in parallel were analyzed for changes in 5-HT levels and density of uptake sites using paroxetine binding (Yeh et al. 1986 Battaglia et al. 1987). 

Chirality is due to the fact that the stereogenic center, also called the chiral center, has four different substitutions. These molecules are called asymmetrical and have a Q symmetry. When a chiral compound is synthesized in an achiral environment, the compound is generated as a 50 50 equimolar mixture of the two enantiomers and is called racemic mixture. This is because, in an achiral environment, enantiomers are energetically degenerate and interact in an identical way with the environment. In a similar way, enantiomers can be differentiated from each other only in a chiral environment provided under... 

Glycidol, like all 2,3-epoxy alcohols, is susceptible to the Payne rearrangement when exposed to base. Payne rearrangement of (/ )-or (S)-glycidol is degenerate consequently racemization does not occur. 

Upon heating optically active 49-1-d at 230°C (67) deuterium scrambling isomerization 49-1 -d - 49-5-d and racemization (+)-49 - (-)-49 were observed to take place at approximately the same rate. A reasonable explanation of this result is a degenerate walk rearrangement with inversion at C-7. 

Nucleophilic attack of transition metal centers on a-bound ligands occurs as readily as reaction with the common nucleophiles of organic chemistry. Degenerate exchange of alkyl groups from one metal to another, as exemplified by the reactions in Equations 11.11 and 11.12, is proposed to occur by nucleophilic attack. This exchange of alkyl groups has been proposed as a mechanism to racemize chiral alkylmetal complexes. - ... 

Reactions of degenerate racemization of compounds LX associated with rotation about the S—N bond are considerably accelerated upon addition of the chloride ion. This acceleration can be accounted for by the switching on of a supplementary mechanism of configuration inversion at the sulfur atom, i.e., the reaction of nucleophilic substitution in Eq. (5.18)—see Ref. [173]. 

Also depicted in Figure 6 is the Ai state of 2,4,6-cyclohepatrienylidene, which is a possible transition state for racemization of the conjugated cyclic allene, 1,2,4,6-cycloheptetraene. In the Aj state of 2,4,6-cyclohepatrieny-lidene two electrons occupy the o NBMO, and there are two unfilled n MOs, 2a2 and 3bi that are nearly degenerate. Excitation of one electron from the at o NBMO can thus lead to two triplets, and fi, and two open-shell singlets, A2 and Bi. 

Chirality can also be introduced when one or more chiral carbons are incorporated in the molecules, for example in the hydrocarbon terminal chains [67, 68], within the bent-core [69], or by addition of chiral dopants [6, 70], It was noted during the early research that the handedness of the homochiral structures is very sensitive to chiral dopants [6], or even on chiral surfaces [71]. On the other hand, it was observed that banana-smectics made of enantiomeric chiral molecules form synclinic - antiferroelectric [44] and anticlinic ferroelectric [67] domains. This combination of tilt and polar order implies that the phase is racemic, with a rigid alternation of right- and left-handed chiral layers. This shows that the molecular chirality has no or minor effect on deciding about anticlinic or synclinic packing (which is mainly determined by entropic reasons), but it can bias the otherwise degenerate tilt directions. 

Typically, racemic mixtures of chiral gelators either do not form gels or they form unstable ones that degenerate easily into precipitates or bulk-separated crystals [69a, 70]. However, there have been some interesting exceptions in which a racemate produces stronger gels than their enantio-pure counterparts [70, 71]. 

Girones J, Duran J, Polo A, Real J. Enantioselectivity in the catalytic hydroesterification of acenaphthylene direct evidence of the racemization of Pdll-alkyl species by a degenerate substitution equilibrium with (PdLn)-L-O. Chem. Commun. 2003 1776-1778. 

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