Ecteinascidins: Putative biosynthetic precursors and absolute stereochemistry

New bioactive ecteinascidins (Et's) 597 (1), 583 (2), 594 (3), and 596 (4)-putative biosynthetic precursors of previously described Et's [e.g., Et 743 (5)]-were isolated from the Caribbean tunicate Ecteinascidia turbinata. Structures assigned to these compounds based on spectroscopic data represent a novel series of Et's with L-cysteine or its alpha-oxo analog as unit C. The absolute configuration of the L-Cys unit of 1 was assigned by chiral GC, while a 2D POESY (rotating-frame Overhauser enhancement spectroscopy) spectrum of its acetyl derivative 1a completed the assignment of the stereochemistry of 1 as 1R,2R,3R,4R,11R,13S,21S,1'R.