Stereocontrolled Total Synthesis of Hemibrevetoxin B

The stereocontrolled total synthesis of hemibrevetoxin B (1) has been achieved in 56 steps and 0.75% overall yield from D-mannose. The intramolecular reaction of γ-alkoxyallylstannane with aldehyde is a key step for the present total synthesis. Thus, the BF₃·OEt₂-mediated reaction of 24 gave 6 as a sole product. We encountered difficulty in the synthesis of γ-alkoxyallylstannane 30 from the corresponding allyl ether 29 in which the γ-alkoxy substituent became sterically quite bulky. This problem was solved by developing the acetal cleavage method for the synthesis of γ-alkoxyallylstannanes. The cyclization of 38 proceeded smoothly to give the key intermediate 5 in a highly stereoselective manner. Construction of the α-vinyl aldehyde and (Z)-diene moieties were performed using Nicolaou's protocol.