General approach to halogenated tetrahydrofuran natural products from red algae of the genus Laurencia. Total synthesis of (.+-.)-kumausallene and (.+-.)-1-epi-kumausallene
摘要:
An efficient, stereocontrolled entry to the dioxabicyclo[3.3.0]octane ring system of (+/-)-kumausallene has been developed. This synthesis builds upon our general strategy for preparing C-15 halogenated tetrahydrofuranoid marine lipids using the ring-enlarging tetrahydrofuran annulation of cyclic, allylic diols as the central step. The key intermediate, hydrobenzofuranone 8, is available with complete stereocontrol from the Lewis acid-catalyzed condensation of 1-vinylcyclopentanediol (4) and alpha-(benzoyloxy)acetaldehyde(Scheme II). Elaboration of 8 into the bicyclic ring system of 13 requires only four steps and proceeds with excellent stereocontrol in 41% overall yield. The total synthesis of (+/-)-kumausallene from 8 is accomplished in 17 steps and 2% overall yield. As a corollary, the total synthesis of (+/-)-1-epi-kumausallene confirms the stereochemical assignment for the bromoallene moiety of the natural product.