No abstract

No abstract

A new effective olefin cyclization reagent, mercury(II) triflate/AvV-dimethylaniline complex (1), has been developed. By use of 1, (EJSJS) -geranylgeranyl p-nitrobenzoate (4a) has been efficiently cyclized to give spongian (5) and labdane (6 and 7) type products. When acetate 4b is employed as the substrate, a unique hydroxylative cyclization takes place, affording C-13 hydroxylated products 14 and its stereoisomers. One of the stereoisomers is derived to 17, which is identified with a marine natural diterpenoid, isoaplysin-20, and the reported formula is corrected. On the cyclization of 4b with 1 in the presence of water (or methanol), intermediates in each stage can be trapped to give mono-, bi-, and tricyclic tertiary alcohols 19ab, 20ab, and 14. The structures of bicyclic alcohols 20a and 20b have been definitely established by completion of the total synthesis of (±)-(13E)-13labdene-8,15-diol ( 21) and (±)-aplysin-20 (22b), respectively. These results provide clear experimental evidence on the mechanism of a biomimetic olefin cyclization which takes place in a stepwise manner via conformationally flexible cationic intermediates such as 27, 28 and 29.

No abstract