Selective dealkylation with

Chart I. Selective dealkylation with AlCl3-NaI-CH3CN system. 

Treatment of 1,2-. 1,4-, 1,5-, and 1,8-dimethoxyanthra-quinone with BF, etherate in Cf,H (reflux) result.s in difluoroboron chelates, which are hydrolyzed to the corresponding l-hydroxy-2-methoxy-, -4-methoxy-, -5-methoxy-, and -8-methoxyanthraquinones when heated in CH,OH. This selective dealkylation can be used to convert 1,4,5-trimethoxyanthraquinone (1) into either 4-hydroxy-1,5-dimethoxy-anthraquinone (4) or l,4-dihydroxy-5-methoxyanthraquinone (5). 

Dealkylation of phosphonates. Vigorous acid treatment is usually required for this reaction. To circumvent this difficulty, both bromo- and chloro-trimethylsilane have been used with limited success. lodotrimethylsilane is more selective the parent phosphonic acids can be obtained under very mild conditions without modification of a variety of other functional groups. However, monodealkylation is not practical with this reagent. Aryl esters are not cleaved hence selective dealkylation of alkyl aryl esters of phosphonic acids may be possible. 

The protection of alcohols with allyl [71] and related (prenyl, methylallyl, cinnamyl, homoal-lyl) groups is of great importance in carbohydrate synthesis due to their stability under the conditions required for glycoside formation. These groups are moderately stable to acids and bases, and offer the potential for selective dealkylation of differentially protected sites. 

McKenna. C.E., and Schmidhauser, J., functional selectivity in phosphonate ester dealkylation with bromotrimethylsilane. J. Chem. Soc.. Chem. Commun.. 739. 1979. 

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. 

Selective dealkylation of aromatic alkoxylated compounds Selective de-ethylation of 2-ethoxyanisole is achieved by use of KO Bu as the reagent in the presence of 18-crown-6 as the phase-transfer agent (PTA). With addition of ethylene glycol (E.G.), the selectivity is reversed and demethylation occurs (Eq. (61), Table 4.18). Although involvement of microwaves is favorable in both examples, the second reaction was shown to be more strongly accelerated than the first [147]. 

Selective dealkylation of alkyl phenyl ethers (cf Sodium tUoethoxide, 4, 465). This reagent in combination with HMPT cleaves alkyl phenyl ethers under mild conditions. Methoxy groups ortho or para to a formyl group are cleaved selectively. ... 

Inexpensive di-, tri-, and tetramethoxyanthraquinones can be selectively dealkylated to hydroxymethoxyanthraquinones by the formation of difluoroboron chelates with BF3-OEt2 in benzene and subsequent hydrolysis with methanol. These un-symmetrically functionalized anthraquinone derivatives are useful intermediates for the synthesis of adriamycin, an antitumor agent. 2,4,6-Tiimethoxytoluene reacts with cinnamic acid and BF3-OEt2, with selective demethylation, to form a boron heterocycle which can be hydrolyzed to the chalcone aurentiacin (eq 34). 2 ... 

Protection.—Amines have been protected by conversion into their 9-anthrylmethyl carbamates. These carbamates are readily prepared, are stable to acids and bases, and furthermore are easily deblocked by treatment with the sodium salt of methyl mercaptan (Scheme 5). The merits of vinyl chloroformate have been explored for amino protection in peptide synthesis and for the selective dealkylation of tertiary amines. ... 

Selective trimethylation of / -t rf-butylcalix[4]arene (2e R -R = CHs) can be accomplished with (CH3)2S04 in DMF in the presence of BaO and Ba(OH)2, whereas the monoalkyl ethers (2d) can be synthesized directly using CsF in DMF, or indirectly by submitting the easily available 1,3-dialkoxy and the tetralkoxy derivatives to selective dealkylation using, respectively, one or three equivalents of iodotrymethylsilane in CHCI3. 

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. 

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]. 

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>. 

---