Crude methanol extracts of the ascidian Didemnum granulatum collected in Brazil showed activity in a new screen for G2 cell cycle checkpoint inhibitors. Bioassay-guided fractionation of the extract yielded the known alkaloids didemnimides A (1) and D (2), the new alkaloid didemnimide E (3), and a new G2 checkpoint inhibitor. Two candidate structures for the inhibitor, named granulatimide (4) and isogranulatimide (5), have been prepared via a short and efficient biomimetic synthesis involving the photolysis of didemnimide A (1). The synthesis revealed that the correct structure for the naturally occurring G2 checkpoint inhibitor is isogranulatimide (5). Granulatimide (4), the other candidate structure, was also found to be a G2 checkpoint inhibitor, and it was subsequently detected in chromatographic fractions associated with purification of D. granulatum alkaloids. Granulatimide (4) and isogranulatimide (5) represent the first examples of a new class of G2 specific cell cycle checkpoint inhibitors and the first ones identified through a rational screening program.