Imidazolone and Imidazolidinone Artifacts of a Pivotal Imidazolthione, Zyzzin, from the poecilosclerid sponge Zyzza massalis from the coral sea. The first thermochromic systems of marine origin

Dry acetone extracts of the freeze‐dried, deep‐red powder of the poecilosclerid sponge Zyzza massalis (D ENDY ) from the Coral Sea gave the orange alkaloid zyzzin (= 4‐(1 H ‐indol‐3‐yl)‐1,5‐dihydro‐5‐thioxo‐1 H ‐imidazol‐2‐one 14 ), which underwent exchange of the 5‐thioxo for the 5‐oxo group during aqueous chromatographic workup, giving yellow 13 . Both 13 and 14 added hydroxylic solvents at C(4)=N giving colourless, racemic products, 3 and 8 , respectively, by incorporation of MeOH and 10 and 11 , respectively, by incorporation of H 2 O. On warming 3 or 10 in DMSO, 13 was recovered, thus furnishing a novel thermochromic system. Optically active (+)‐ 12 , which may be viewed to derive from enzymatic reduction of 14 at C(4)=N followed by S→O exchange, was also isolated from this sponge, along with the linear amides 16 and 19 . Compound 3 proved to have antibacterial and antifungal activities.