He directed there the premiere of "Judith" by Siegfried Matthus. In 1988, he staged at the Bayreuth Festival Wagner's "Der Ring des Nibelungen". "The Los Angeles Times" called the production "high-tech kitsch … a bizarre stylistic jumble"; "The Guardian" remarked on a "Götterdämmerung" "full of paltry gimmickry"; and "The Times" commented that "the production gets progressively lazier [and] peters out in the clichés of the day before yesteryear, which Kupfer doubtless thinks the last word in modernity". Kupfer premiered several operas, including Udo Zimmermann's "" at the Staatstheater Dresden in 1973, conducted by Siegfried Kurz. He staged the GDR premiere of Schönberg's "Moses und Aron" there, also conducted by Kurz in 1975. In 1979, he directed there the world premiere of Zimmermann's "Der Schuhu und die fliegende Prinzessin", conducted by Max Pommer, also the premiere of Georg Katzer's "Antigone oder die Stadt" at the Komische Oper Berlin in 1991, conducted by Jörg-Peter Weigle, the musical "Mozart" by librettist Michael Kunze and composer Sylvester Levay at the Theater an der Wien in 1999, conducted by , and in 2000 Reimann's "Bernarda Albas Haus", at the Bavarian State Opera, conducted by Zubin Mehta. Kupfer co-wrote the libretto with composer Krzysztof Penderecki of Penderecki's opera "Die schwarze Maske". He directed the 1986 world premiere production in Salzburg and the US premiere production at the Santa Fe Opera in 1988. Kupfer and his wife, the music teacher and soprano , had a daughter, , who is an actress. He died on 30 December 2019 in Berlin. Kupfer worked in the tradition of realistic directing, as developed by Walter Felsenstein and practised especially at the Komische Oper Berlin. The works are interpreted with a focus on implications drawn from them; actions on stage, conflicts and the development of drama are related to the score and to the logic of relationships between the characters. Kupfer always worked individually with the singers, including the choir members, requesting talent for acting and rendering credibility to the actions.