Cyclic anhydrides from diacids

The meehanism of the conjugate addition is the same as that in the previous example and the meehanism for ester hydrolysis was covered in Chapter 12, The key step in the dehydration reaction is the formation and eyclization of the mixed anhydride formed from the diacid and acetic anhydride, Both steps have the same mechanism, attack of an acid on an anhydride, but the second step is intramolecular. Like most eyelizations the reaction is entropically favoured as two molecules react to give three—the cyclic anhydride and two molecules of acetic acid... 

Cyclic anhydrides are diacids with one molecule of water eliminated from the condensation of the two acid groups. They can be useful for the synthesis of polyesters. The reaction proceeds in two steps because the free acid formed in the first step (Eq. 2.7) is much less reactive than the original anhydride ... 

Fig. Synthesis of cyclic acid anhydrides from acyclic diacids.

The ability to accept electrons from donors is particularly pronounced in acrylic acid derivatives [85] its alkyl esters [78, 87, 88], acrylonitrile [88], acrylamide, hydroxylacrylates [85], and further in styrenes substituted with an electronegative atom or group m-nitrostyrene, 2,6-dichlorostyrene [86], / -nitrostyrene [89] bicyclobutane-1-carbonitrile [89] lactones /J-propio-Iactone [85], sulfolactone vinyl ketones [87] unsaturated dicarboxylic acids and their derivatives diethyl fumarate, fumaronitrile [90], ROOC—N— N—COOR [86], cyclic anhydrides of diacids [91 ], particularly maleic anhydride [78, 92] ethylenes substituted with electronegative groups [93, 95]... 

Carboxylic acids cannot be readily converted to anhydrides, but dicarboxylic acids can be converted to cyclic anhydrides by heating to high temperatures. This is a dehydration reaction because a water molecule is lost from the diacid. 

As it is known that a cyclic ether (THE) may be formed during degradation and from the 1,4-butanediol by-product, the 1,4-diacid and hydroxyacids noted above could therefore also cyclise, leading to formation of succinic anhydride and y-butyrolactone. Further studies are required to investigate if any of these possible products occur in the thermo-oxidation of PBT. 

Polyanhydrides can be synthesized by melt condensation of trimethylsilyl dicar-boxylates and diacid chlorides to yield polymers with an intrinsic viscosity 0.43 dl/g (Gupta 1988). Direct polycondensation of sebacic acid and adipic acid at a high temperature under vacuum resulted in low molecular weight oligomers (Knobloch et al, 1975). The preparation of adipic acid polyanhydride from cyclic adipic anhydride (oxepane-2,7-dione) was investigated (Albertsson et al., 1990). 

General methods for the synthesis of poly(amide-anhydrides) and poly(amide-esters) based on naturally occurring amino acids were described (Domb et at, 1990). The polymers were synthesized from dicarboxylic acids prepared by amidation of the amino group of an amino acid with a cyclic anhydride, or by the amide coupling of two amino acids with a diacid chloride. Low molecular weight polymers from methylene bis(p-carboxybenzamide) were symthesized by melt condensation (Hartmann et al, 1989). A series of amido containing polyanhydrides based on p-aminobenzoic acid were sy nthesized by melt condensation. The polymers melted at 58 to 177°C and had a molecular weight of 2500 to 12400. 

Synthesis of Derivatives of Carboxylic Acids.—Dicarboxylic acid di-iodides are prepared (in 37 to 94% yield) from the corresponding diacid chlorides and sodium iodide. Carboxylic acid bromides are prepared under neutral conditions by the reaction of the corresponding acid with dibromotriphenylphosphorane. Radicals generated from cyclohexylmercuric acetate and NaBH4 react with maleic anhydride and related compounds to give cyclic derivatives of maleic acid in 55 to 98% yield. The chemistry of acyl cyanides has been reviewed. ... 

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