Investigations on the biosynthesis of the coumarins, umbelliprenin ( 1), isopimpinellin ( 2), alloimperatorin methyl ether ( 3) and isoimperatorin ( 4) are described. Extensive degradation studies on these systems have provided the opportunity to evaluate the various centers in these radioactive coumarins obtained from mevalonate incorporation. These results are discussed in the light of previous proposals.