Polycyclic hydroxy quinones. 28. Synthesis and Diels-Alder reactions of N,N,O-triacyl derivatives of 10-amino-9-hydroxy-1,4-anthraquinones. An efficient, regiospecific synthesis of (.+-.)-5-iminodaunomycinone, (.+-.)-4-demethoxy-5-iminodaunomycinone, and (.+-.)-daunomycinone