Esters arene alkylation

Synthetic utility of stereoselective alkylations in natural product chemistry is exemplified by the preparation of optically active 2-arylglycine esters (38). Chirally specific a-amino acids with methoxyaryl groups attached to the a-carbon were prepared by reaction of the dimethyl ether of a chiral bis-lactam derivative with methoxy arenes. Using SnCl as the Lewis acid, enantioselectivities ranging from 65 to 95% were obtained. 

Optically active 2-arylalkanoic acid esters have been prepared by the Friedel-Crafts alkylation of arenes with optically active a-sulfonyloxy esters (40). Friedel-Crafts alkylation of ben2ene with (5)-methyl 2-(chlorosulfonyloxy)- or 2-(mesyloxy)propionate proceeded with predorninant inversion of configuration (<97%) to give (5)-methyl 2-phenylpropionate. 

Optically active 2-arylaIkanoic acid esters have been prepared by Eriedel-Crafts alkylation of arenes with optically active esters, such as methyl 3 -2-(chlorosulfonoxy)- or 3 -2-(mesyloxy)propionate, in the presence of aluminum chloride (54,55). 

Different nucleophiles such as methanol, allylsilanes, silyl enol ethers, trimethylsilyl-cyanide, and arenes can be used in this process [62]. When the sulfide itself contains an unsaturated or aromatic fragment and the process is carried out in the absence of a nucleophile, an intramolecular anodic sub-stitution/cyclization might occur [61-63]. Methyl esters of 2-benzothiazolyl-2-alkyl or aryl-acetic acid, oxidized in MeOH/Et4 NCIO4 or H2SO4 in the presence of CUCI2, form 2,2-dimethoxy products (Eq. 7) [64]. 

T1 resin traceless linker [131-134], synthesis of phenols [135], biaryls, alkyl arenes [136, 137], azides [138], aromatic hydrazines, halides [cf. 128, 129, 139], ester, azo compounds, cinno-lines [140], benzotriazoles [141]... 

Cuest-Induced Changes in Membrane Permeability. Calixarene derivatives are also used for sensing systems other than ISEs or optodes. Recently, a systematic investigation on the control of membrane permeability by use of oriented monolayers composed of calixarene esters was carried out. The hosts used were short alkyl chain esters of calix[6]arene [28 (R = Bu )] and calix[4]arene [26 (R = Bu ), 30 both cone conformers]. The permeabilities through the intermo-lecular voids of these monolayers were evaluated by cyclic voltammetry, as described earlier for oriented membranes of nucleobase derivatives. Cationic, anionic, and neutral electroactive compounds were used as the permeability markers. The voltammetric measurements were carried out either for a monolayer... 

A mild and efficient process for the acyl-alkylation of arynes to give ortho-substituted arenes has been reported (Scheme 63)." The transformation results in the formation of two new C-C bonds by the net insertion of an arene unit into the a,/3-single bond of a /3-keto ester. 

All types of electrophiles have been used with 2-lithio-l,3-dithiane derivatives, including alkyl halides, sulfonates, sulfates, allylic alcohols, arene-metal complexes, epoxides, aziridines, carbonyl compounds, imines, Michael-acceptors, carbon dioxide, acyl chlorides, esters and lactones, amides, nitriles, isocyanates, disulfides and chlorotrialkylsilanes or stannanes. The final deprotection of the dithioacetal moiety can be carried out by means of different types of reagents in order to regenerate the carbonyl group by heavy metal coordination, alkylation and oxidation184 or it can be reduced to a methylene group with Raney-nickel, sodium or LiAIII4. 

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