Ring dehydrative

Paterson, L, Acena, J.L., Bach, J., Chen, D.Y.-K., and Coster, M.J. (2003) Synthesis and biological evaluation of spongistatin/altohyrtin analogues E-ring dehydration and C46 side-chain truncation. Chem. Commun., 462-463. 

In his cephalosporin synthesis methyl levulinate was condensed with cysteine in acidic medium to give a bicyclic thiazolidine. One may rationalize the regioselective formation of this bicycle with the assumption that in the acidic reaction mixture the tMoI group is the only nucleophile present, which can add to the ketone. Intramolecular amide formation from the methyl ester and acid-catalyzed dehydration would then lead to the thiazolidine and y-lactam rings. The stereochemistry at the carboxylic acid a-... 

Intermediates. 3,4-Dihydro-2H-pyran [110-87-2] is prepared by a ring-expanding dehydration of tetrahydrofurfuryl alcohol. It is used as a protecting agent for hydroxyl compounds and as an intermediate. 2-Methylfuran is a chemical intermediate for 5-methylfurfural [620-02-0] (151) and... 

Methylphenol. This phenol, commonly known as o-cresol, is produced synthetically by the gas phase alkylation of phenol with methanol using modified alumina catalysis or it may be recovered from naturally occurring petroleum streams and coal tars. Most is produced synthetically. Reaction of phenol with methanol using modified zeoHte catalysts is a concerted dehydration of the methanol and alkylation of the aromatic ring. 2-Methylphenol [95-48-7] is available in 55-gal dmms (208-L) and in bulk quantities in tank wagons and railcars. 

Further deprotonation, dehydration, and polymerization of monomers and dimers may yield ringlike stmctures of hydroxy—aluminum complexes (10). Coalescence of ring compounds into layers by further growth results in the formation of crystalline aluminum hydroxide at pH 6, the point of minimum aqueous solubiUty. 

Primary cycloaUphatic amines react with phosgene to form isocyanates. Reaction of isocyanates with primary and secondary amines forms ureas. Dehydration of ureas or dehydrosulfuri2ation of thioureas results in carhodiimides. The nucleophilicity that deterrnines rapid amine reactivity with acid chlorides and isocyanates also promotes epoxide ring opening to form hydroxyalkyl- and dihydroxyalkylaniines. Michael addition to acrylonitrile yields stable cyanoethylcycloalkylarnines. 

The reaction is very slow in neutral solution, but the equiUbrium shifts toward the lactam rather than glutamic acid. Under strongly acidic or alkaline conditions, the ring-opening reaction requires a very short time (10). Therefore, neutralization of L-glutamic acid should be performed cautiously because intramolecular dehydration is noticeable even below 190°C. 

Heat. When heated, succinic acid loses water and forms an internal anhydride with a stable ring stmcture. Dehydration starts at 170°C and becomes rapid at 190—210°C (25). Further heating of succinic anhydride causes decarboxylation and the formation of the dilactone of gamma ketopimelic acid (26) (eq. 1). The same reaction takes place at lower temperatures in the presence of alkaU. 

Under acidic conditions, dehydration to an anhydrotetracycline [20154-34-1] (8), C22H22N20y, occurs under basic ones, ring C opens to an isotetracycline [3811-31-2] (9), C22H24N20g. The anhydrotetracyclines, such as (8), appear to exhibit a mode of antibacterial action, but it is unlike that of tetracycline (24). Epimerization (23,25,26) at C-4 occurs in a variety of solvents within the pH range 2—6, particularly in acetic acid (25). A number of anions (27) facihtate this reaction. The reverse process, from 4-epitetracycline [79-85-6] C22H24N20g, to tetracycline, is promoted by chelation with ions such as calcium and magnesium (28). 

Trinidad asphalt has a relatively uniform composition of 29% water and gas, 39% bitumen soluble in carbon disulfide, 27% mineral matter on ignition, and 5% bitumen that remains adsorbed on the mineral matter. Refining is essentially a process of dehydration by heating the cmde asphalt to ca 165°C. The refined product averages 36% mineral ash with a penetration at 25°C of about 2 (0.2 mm), a softening point (ring and ball method) of 99°C, a flash point (Cleveland open cup) of 254°C, a sulfur content of 3.3%, and a saponification value of 45 mg KOH/g. The mineral matter typically contains... 

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