Studies on the Diels–Alder reaction of annulated furans: application to the synthesis of substituted phenanthrenes

In an attempt to utilize the synthetic power of the furan nucleus in the synthesis of complex natural products, we have developed an entry into annulated furan intermediates that involves an electrochemical annulation protocol. An approach to the eunicellin diterpenes based on this methodology required the use of an intermolecular Diels–Alder reaction on an annulated furan. Although well known for simple furans, there is a paucity of related examples on annulated furans. To examine the feasibility of such an approach to these diterpenes, we have studied this key cycloaddition reaction. Our studies indicate that the process can be hampered by a facile retro-Diels–Alder that is highly dependent on the dienophile employed.