The Production of 3-Benzylidene-6-isobutylidene-2,5-dioxopiperazine, 3,6-Dibenzylidene-2,5-dioxopiperazine, 3-Benzyl-6-benzylidene-2,5-dioxopiperazine, and 3,6-Dibenzyl-2,5-dioxopiperazine by a Variant of Streptomyces noursei1