A Quantitative Evaluation of Methods for Coupling Asparagine

Article abstract of DOI:10.1021/jo01292a001

A quantitative procedure was developed to evaluate methods for coupling asparagine.Boc-asparagine was coupled to glycyloxymethylphenoxymethyl-copoly(styrene-divinylbenzene) resin under a variety of conditions, and the product, Asn-Gly, and any byproducts were cleaved from the resin with 50percent trifluoroacetic acid in methylene chloride.The mixture was then separated on a sulfonated ion exchange column and quantitated by the ninhydrin reaction.Coupling by the dicyclohexylcarbodiimide (DCC) or symmetrical anhydride methods gave large amounts of the dehydration product, β-cyanoalanylglycine, and smaller amounts of β-aspartamidinoacetic acid, α-aspartylglycine, and β-aspartylglycine in addition to the desired asparaginylglycine.Activation of Boc-Asn by DCC plus hydroxybenzotriazole or by the nitrophenyl ester gave 98 to 99percent of Asn-Gly, but very low levels of the byproducts were detectable with the sensitive chromatographic method.Protection of the amide function of Boc-Asn with the 4,4'-dimethoxybenzhydryl group avoided completely the formation of the nitrile during DCC coupling.Pure Ala(CN)-Gly was quantitatively reconverted to Asn-Gly by HF.The rehydration of nitrile also occurred in 50percent trifluoroacetic acid in dichloromethane, but much more slowly.