5.9 parts of 7-amino-5-nitrobenzisothiazole, 30 parts of 98% sulfuric acid and 8 parts of 85% phosphoric acid were charged, heated at 60° C. for 30 mins, then cooled to 5° C. 11.4 parts of 40% (w/w) nitrosyl sulfuric acid were added, whilst the temperature was held below 10° C. The diazotization mixture was stirred for a further 2 hrs at 5-10° C. To a separate vessel were charged 6.5 parts of 3-(ethyl-phenyl-amino)-propionic acid vinyl ester, 100 parts of methanol, 1 part sulfamic acid and 100 parts of ice. With stirring, the diazotization mixture was slowly added followed by a further 300 parts of ice. The reaction mixture was stirred over night and the product was isolated by filtration, washed with water and dried to yield 10.1 parts of 3-{ethyl-[4-(5-nitro-benzo[c]isothiazol-3-ylazo)-phenyl]-amino}-propionic acid vinyl ester. λmax=606 nm (DMF).
