Cyclodehydration, diacid

Mamyama et al.25 have obtained high-molecular-weight poly(benzoxazole)s by the low-temperature solution polycondensation of A,A 0,0 -tetrais(trimethyl-silyl)-substituted 2,2-bis(3-amino-4-hydroxyphenyl)-l,l,l,3,3,3-hexafluoro-propane (25) with aromatic diacids and subsequent thermal cyclodehydration of the resulting poly(o-hydroxy amide)s in vacuo. In this method, aromatic diamines with low nucleophilicity are activated more positively through the conversion to the /V-silylated diamines, and the nucleophilicity of the fluorine-containing bis(o-aminophenol) can be improved by silylation. 

Analogously stable 4//-pyran-2,6-dicarboxylic acids 32 were available by cyclodehydration of a, a -diketo diacids 31 with sulfuric acid.71 73... 

While the condensation of enamine 37 with methyl OY7 s-2-butenoate, followed by acid hydrolysis and sodium borohydride reduction affords lactone 38 with reasonable efficiency, the cyclodehydrative ring contraction of this intermediate with PPA gives a mixture of bicyclo[3.3.0]octenones in abysmal (< 5 %) yield.66 To circumvent this difficulty and enable the large scale production of 39,2-carbo-methoxy-4,4-dimethylcyclohexanone was initially transformed to tram diacid 40 under Favoiskii conditions (Scheme 14). Conversion to the diacid chloride and condensation with lithium dimethylcuprate resulted in formation of the diacetyl derivative. In basic solution, the latter is reported to experience epimerization and aldol cyclization with dehydration in 82 % yield. With hydrogen and palladium on charcoal, the essentially quantitative production of 39 was achieved.66 ... 

A wide range of polyoxadiazole polymers with hexafluoropropylidene linkages have been synthesized in Cambridge (3. As illustrated in Scheme 5, aromatic oxadiazole polymers 18 and 20 have been synthesized by a one pot reaction of the diacid 15 with tiie dicarboxylic hydrazide 16 and hydrazine sulphate 17 respectively at 100 °C in the presence of P2O5/CH3SO3H which serves both as solvent and cyclodehydration agent. 

Aromatic poly(benzoxazole)s are commonly prepared by one of two methods. The first is very similar to the low temperature, two-step synthesis of polyimides. Bis(o-aminophenol)s are reacted with aromatic diacid chlorides to form soluble poly(o-hydroxy amide)s which are processed. Thermal cyclodehydration then converts the backbone groups into benzoxazoles. ... 

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