Selective dealkylation

The N-dealkylation selectivities follow the order benzyl allyl t.butyl s.alkyl > n.alkyl piperidine scission. 

However, vinyl chloroformate largely has been replaced for N-dealkylation by 1-chloroethyl chloroformate which shows similar dealkylation selectivities and equally easy replacement of N-alkyl by the carbamate group (see section 3-3). 

The authors used a 16 channel reactor at a pressure of 25 bar and a temperature of 400°C and studied zeolite catalysts with different ring sizes and pore connectivity. A clear correlation between the dealkylation selectivity for ethylbenzene and the zeolite pore volume was observed. Dealkylation selectivity increased with decreasing pore volume. In addition, toluene, which is formed by a bimolecular reaction as a primary product, is also favoured by specific pore systems, with that of zeolite beta and ITQ-23 being the most favourable ones. This study showed, that the combination of high-throughput reactors with properly designed experiments can accelerate the understanding of the catalytic action, in this case of zeolites. 

The feedstock is usually extracted toluene, but some reformers are operated under sufftciendy severe conditions or with selected feedstocks to provide toluene pure enough to be fed directiy to the dealkylation unit without extraction. In addition to toluene, xylenes can also be fed to a dealkylation unit to produce benzene. Table 20 Hsts the producers and their capacities for manufacture of benzene by hydrodealkylation of toluene. Additional information on hydrodealkylation is available in References 50 and 52. 

Deall lation. Chloroformates such as vinyl chloroformates (40) are used to dealkylate tertiary amines. Chloroformates are superior to the typical Von Braun reagent, cyanogen bromide, because of increased selectivity producing cleaner products. Other chloroformates such as aHyl, methyl, phenyl, and trichloroethyl have also been used in dealkylation reactions. Although the dealkylation reaction using chloroformates is mostiy carried out on tertiary amines, dealkylation of oxygen or sulfur centers, ie, ethers or thioethers, can also be achieved. a-Chloroethyl chloroformate [50893-53-3] (ACE-Cl) (41,42) is superior to all previously used chloroformates for the dealkylation reaction. ACE-Cl has the advantage that the conditions requked for ACE... 

Fenoldopam (76) is an antihypertensive renal vasodilator apparently operating through the dopamine system. It is conceptually similar to trepipam. Fenoldopam is superior to dopamine itself because of its oral activity and selectivity for dopamine D-1 receptors (D-2 receptors are as.sociated with emesis). It is synthesized by reduction of 3,4-dimethoxyphenylacetonitrile (70) to dimethoxyphenethylamine (71). Attack of diis last on 4-methoxystyrene oxide (72) leads to the product of attack on the epoxide on the less hindered side (73). Ring closure with strong acid leads to substituted benzazepine 74. O-Dealkylation is accomplished with boron tribromide and the catechol moiety is oxidized to the ortho-quinone 75. Treatment with 9NHC1 results in conjugate (1,6) chloride addition and the formation of fenoldopam (76) [20,21]. 

A new and interesting chioroformate reagent, a-chloroethyl chioroformate, was recently introduced for selective N-dealkylation of tertiary amines (106). When it was applied to the demethylation of 3a-acetoxytropane (2), the corresponding nor salt (198) was obtained in 97% yield (Scheme 20). 

Atrazine, used as a selective pre- and post-emergence herbicide to control annual weeds in several crops, is the most representative compound of this group. It is also used as a non-selective herbicide in non-crop areas. After absorption, the compound is metabolized to dealkylated and deisopropy-lated derivatives. The unchanged compound and its metabolites are excreted in urine, where they can be detected by chromatography or enzyme-linked immunosorbent assay (Lucas et al., 1993). A mercapturic acid conjugate of atrazine has also been found in urine samples of workers spraying this herbicide (Lucas et al., 1993) (Table 6). 

On ferrierite, ZSM-22 and EU-1 zeolite catalysts, 10MR monodimensional zeolite structures (ID), the main reaction is the isomerization of ethylbenzene (figure la). ZSM-5, 10MR three-dimensional structure (3D) zeolite is very selective in dealkylation (90%) (figure lb) and no deactivation was observed within 8 hours of reaction. This particular selectivity of the zeolite ZSM-5 can be partly explained by the presence of strong acid sites and its porous structure that on one hand promotes the containment of molecules in the pores (presence of 8-9A cages at the intersection of channels) and on the other hand prevents the formation of coke and therefore pore blockage. 

As previously observed [4] on EU-1 catalyst deactivation leads to isomerization selectivity improvement (table 2) whereas dealkylation and disproportionation selectivity decreases. The same effect is observed for ferrierite and ZSM-22 catalysts to a lesser extent. Isomerization selectivity reach more than 70% for these catalysts after 8... 

Alkoxy-l,3,4-thiadiazoles can be dealkylated under acidic conditions to give l,3,4-thiadiazol-2(3//)-ones. The selective and clean dealkylation of the ethoxy group in thiadiazole 109 was achieved with HBr in refluxing ethanol to give the thiadiazolone 110 (90%) (Equation 33) <1999H(51)2739>. 

Reaction (F) represents one of the uses of methanol (reaction (C)), and is also an example in which reaction selectivity is an important issue. The reaction cannot be allowed to go to ultimate completion, since the complete oxidation of CH3OH would lead to C02 and HzO as products. Similarly, in reaction (D), benzene and other (unwanted) products are produced by dealkylation reactions. 

Smith and Rosazza have suggested that microbial transformation experiments could best be carried out by using a series of perhaps 10 metabolitically prodigious microorganisms as microbial models. Microorganisms for such work may be selected on the basis of considerable literature precedence for their abilities to catalyze the desired biotransformation reaction (i.e., O-dealkylation, N-dealkylation, aromatic hydroxylation, and reductions). The alkaloid substrate... 

As mentioned earlier, at higher temperature the selective conversion of ethylbenzene is further enhanced by opening an additional pathway, i.e., dealkylation, that yields increased amounts of benzene of high purity ... 

We have shown that the high selectivity of ZSM-5 in xylene isomerization relative to larger pore acid catalysts is a result of its pore size. It is large enough to admit the three xylenes and to allow their interconversion to an equilibrium mixture it also catalyzes the transalkylation and dealkylation of ethylbenzene (EB), a necessary requirement for commercial feed but it selectively retards transalkylation of xylenes, an undesired side reaction. 

Phenothiazines The phenothiazines (PTZs) undergo extensive metabolism. Metabolic routes include S-oxidation, aromatic hydroxylation, N-dealkylation, N-oxidation, and a combination of these processes. Chlorpromazine, for example, possesses 168 possible metabolites, a large proportion of which are pharmacologically active compounds. The development of an HPLC assay capable of resolving a large number of these metabolites is virtually impossible and assays that permit the simultaneous determination of the parent compound and a selected number of active metabolites must suffice. The PTZ group of compounds includes chlorpromazine, thioridazine, fluphenazine, and perphenazine. 

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