Construction of the stemodane nucleus by a hydroxyl-directed intramolecular ene reaction. Total synthesis of (±)-2-desoxystemodinone

Article abstract of DOI:10.1021/ja00101a012

A synthesis of the diterpene 2-desoxystemodinone was completed from the known tricyclic ketone 9 in eight steps and 35% overall yield. The key step involved a thermal intramolecular ene reaction of α-hydroxy aldehyde 21 which led to 24 in 94% yield. By contrast, a Lewis acid-catalyzed ene reaction of 21 gave oxetane 25 as the major product. The pivotal role of the hydroxyl substituent of 21 in facilitating the ene reaction was demonstrated and is rationalized by a hydrogen bond which orients the carbonyl in a favorable conformation for rearrangement.