The Chemistry of Coumarin Derivatives .6. Diels-alder Trapping of 3-methylene-2,4-chromandione - A New Entry To Substituted Pyrano[3,2-c]coumarins

3-Methylene-2,4-chromandione (1) can be generated from dicumarol or from paraformaldehyde and 4-hydroxycoumarin. 1 is a versatile substrate for cycloaddition reactions. Depending on the reaction partner, 1 can behave as an ambident heterodiene or as a dienophile. The reaction was investigated with a series of olefins and dienes, including several isoprenoids. Some generalizations could be drawn and rationalized in terms of HOMO-LUMO interactions. The reaction of 1 and certain asymmetrically substituted olefins gives pyrano[3,2-c]coumarins in a highly regio- and chemoselective way. As an application of this reaction to natural products chemistry, a concise (three steps) synthesis of the prenylated coumarin (+/-)-isoferprenin (35a) is reported.