Scheme 19.
Bioorthogonal ligation reactions in aqueous solution. Bottom: HPLC chromatograms of reactions. For each, top panel: starting material used for that reaction, bottom panel: crude HPLC of reaction products. For 116 the amide product was distinguished from the thioester product based on the presence a new amide proton in the NMR spectrum. See the Supporting Information for details. Compound 117 could be the Diels-Alder endo and/or exo diastereomeric products (Diels-Alder stereochemistry not determined). Compound 118 results from isomerization of an initial 1,3-diene product.51a RN = Ac-Thr-Tyr-, RC = -Asn-NH2.