biosynthesis, one notably missing activity is the oxidative generation of carbonate functional groups. This is a
challenging synthetic transformation of which only few examples exist. Here we report a remarkable multifunctional
Baeyer-Villiger monooxygenase CcsB, which catalyzes the formation of an in-line carbonate in the macrocyclic
portion of the potent angiogenesis inhibitor cytochalasin E. We demonstrate that while CcsB inserts the first oxygen
into the ketone-containing substrate via a Baeyer-Villiger mechanism, insertion of the second oxygen to form
carbonate relies on the arrangement of the neighboring vinylogous, 1,5-diketone. Insights from this study not only
expand the repertoire of activities of FMOs, but also provide a possible synthetic strategy for transformation of
ketones into carbonates.