Reactivation CarbE

Aldol reactions using a carbocation as an organocatalyst An organocatalytic aldol reaction based on a different concept was developed by the Chen group. The chiral triarylcarbenium ion 34 was used as a novel non-metallic Lewis acid catalyst in a Mukaiyama-type aldol reaction which led to enantiomerically enriched aldol products (Scheme 6.17) [67]. Although non-chiral trityl salt-mediated catalytic aldol reactions had previously been reported by Mukaiyama and co-workers [68], the construction of a suitable chiral carbenium ion remained a challenge. Optically active salts of type 34 were synthesized as Lewis acids based on a reactive carbe-... 

Thiophene-2-carbaldehyde, 3-bromo-synthesis, 4, 81 Thiophenecarbaldehydes benzothiophene synthesis from, 4, 906 reactions, 4, 807 synthesis, 4, 148 Wittig reactions, 4, 807 Thiophene-2-carb aldehydes bromination, 4, 753 conformation, 4, 33 halogenation, 4, 753 reactions, 4, 72-73 reactivity, 4, 72-73 reduction, 4, 776 Thiophene-3-carb aldehydes conformation, 4, 33 reactivity, 4, 72... 

The relaPve reactivity of 1,2,2-trifluoroethylidenecarbene insertion into carb on-hydrogen bonds was studied for 11 substrates [70] (equation 11)... 

Utilizing more reactive discrete palladium-N-heterocyclic carbene (NHC) complexes (for example, Pd(carb)2) or in situ generated palladium/imidazolium salt complexes (1 mol% ligand A), Caddick and coworkers were able to extend the rapid amination protocols described above to electron-rich aryl chlorides (Scheme 6.61) [128],... 

The estimation of the conditions, suppressing the silyl-mediated catalysis and preferable for the carbenium-promoting catalysis, is of significant importance to introduce the chiral information in the product. Since it was observed that a carbe-nium salt promoted the reaction and thus provided the enantioselectivity in the outcome, the rigid conformation and the enhanced reactivity of the carbocation may be the key requirement for the productive enantioselective carbenium catalysis in the aldol-type additions. 

Monosubstitution is obtained in the photoinitiated reaction of different carb-anions, such as CH(COMe)2, CH(C02Et)2, CH(COMe)C02Me, CMe(COMe) C02Me and CEt(C02Et)2 (60-92%), with electrophilic substrates. Investigations have shown that 4-iodo-l,l,2,2,9,9,10,10-octafluoro[2.2]paracydophane (4) exhibits excellent SRN1 reactivity with some of these stabilized enolates (Scheme 10.12) [24]. 

Figure 13 Correlation of the reactivities of alkenes and dienes toward the carbe-nium ion AnPhCH+ (CH2CI2, -70° C) with the free energies of activation for ethanolysis of the addition products P-Cl. (Reprinted with permission from Ref. 82. Copyright 1990 American Chemical Society.) Letters refer to compound symbols in Ref. 82.

As discussed in previous sections, the reactivity of carbenium ions depends on both steric and electronic factors. The normal growing carbe-... 

Interaction of CarbE with nerve agents follows a kinetic of first order characterized by inhibition of CarbE at the active site serine residue described by a bimolecular rate constant, ki (Maxwell and Brecht, 2001). For noncharged nerve agents (e.g. sarin and soman) the ki of rat serum CarbE was found to be >10 M min whereas cationic substrates (e.g. VX) are converted with poor reactivity (ki < 10" M min ). This specificity is explained by the electrostatic characteristics of the large active site containing only a few cation-II bonding and anionic residues (Maxwell and Brecht, 2001 Satoh and Hosokawa, 2006). 

Sarin is metabohzed to IMPA, which slowly undergoes further hydrolysis to the very stable MPA. IMPA also forms in the course of spontaneous reactivation of sarin-inhibited CarbEs and ChEs. IMPA has low oral toxicity in rats and mice, but it produces mild skin irritation in rabbits. 

The mechanism of interaction of A- and B-esterases with OP is similar. B-esterases initially form Michaelis complex with an OP inhibitor producing phosphorylated or inhibited enzyme that either reactivates very slowly or does not reactivate at all (see Figure 69.1 in Chapter 65). However, after formation of Michaelis complex with OP A-esterases perform hydrolysis of OP and their catalytic activity and turnover rate are very high. It was aheady shown that CarbE, as a typical B-esterase, can hydrolyze carboxylic esters that serve as functional groups in OP such as mala-thion thus performing detoxification of the compound (WHO, 1986 Fukuto, 1990). 

Standard therapy of OP poisoning consists of the administration of a combination of atropine, oxime, and diazepam with other supportive measures when necessary. However, the possibility of addition of purified enzymes such as AChE, ChE, CarbE, and A-esterases to this therapeutic scheme has been considered and preliminary experiments in animals have shown much better protective effect after addition of exogenous enzymes. In this respect, protective effects of AChE, ChE, and CarbE are based on formation of covalent conjugates or phosphory-lated enzymes in the stoichiometric ratio 1 1. Capacity for binding of these enzymes is limited by the number of active sites on the enzyme to which OP molecules can be bound. This means that more enzymes have to be administered in order to achieve better detoxification of OPs which may not always be possible due to adverse effects. This can also be infiuenced by differences in the extent of spontaneous reactivation of these enzymes inhibited by OP. 

Another advantage of CarbE is the much greater size of its active site compared to AChE (lOx difference) andChE (6x difference) (Saxena etal.,l999). The large active site volume of CarbE minimizes steric hindrance effects at the active site and maximizes the potential for reactivation. In a study investigating the sfructural specificity of AChE, ChE, and... 

Similar sensitivity to DAM reactivation is reported for various soman-inhibited CarbEs. Following in vitro inhibition by soman, the CarbE isoenzymes with low pi in plasma of both rat and guinea pig are partially reactivated (50-60%) by DAM within 5 min and are not further reactivated during the next 30 min (Sterri and Foimum, 1987 Sterri, 1989). The two isoenzymes in rat plasma cannot be discriminated based on the DAM reactivatability (Sterri, 1989), whereas the one with high pi (6.1) in guinea pig plasma is about half as sensitive to DAM reactivation as the other two (Sterri and Fonnum, 1987). Also, two out of three CarbE isoenzymes in rat small intestine display similar reactivatability as the plasma CarbEs (Sterri, 1989), whereas none of the three CarbE isoenzymes in guinea pig liver can be reactivated by this oxime after soman inhibition (Sterri and Foimum, 1987). The latter results correspond well with the poor effect by DAM on soman-inhibited commercial CarbEs from porcine liver (Fonnum et al., 1985). 

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