Development of useful lanthanide salt-induced cascade reactions and applications to natural products synthesis

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Much interest is presently being directed to reaction using lanthanide reagent in organic synthesis. Especially, samarium(II) iodide (SmI_2) has been effective in carbon-carbon bond forming reactions and considerable attention has been focused on various intramolecular processes. In the present investigation, we developed an effective SmI_2-induced cascade cyclization to form polycyclic compounds. Treatment of (2Z)-2-methyl-7-oxodec-2-ene-1, 10-dioic acid derivatives with SmI_2 in tetrahydrofuran caused cascade reductive cyclization-Dieckmann condensation-lactonization to give 6-methyl-10-oxatricyclo[4.3.2.0^<1,5>]undecane-7, 11-diones corresponding to A/B ring system of antheridiogen-An and gibberellins. And reaction of dimethyl (2Z)-2-methyl-8-oxoundec-2-ene-1, 10-dioate with SmI_2 gave 7-methyl-11-oxatricyclo[5.3.2.0^<1,6>]undecane-8, 12-diones corresponding to A/B ring system of resenolactone. A partial structure of ginkgolides, 7-oxatricyclo[6.3.0.0^<1,5>]undecan-6-one system, was also synthesized by similar method. An application of the cascade cyclization for the synthesis of gibberellins was then investigated. The construction of the C/D ring system involves two crucial steps : stereoselective photo cyclization of 2-cyclohexene derivative with allene to form 8-methylenebicyclo[4.2.0]octan-2-one system and SmI_2-induced pinacol coupling reaction. The SmI_2-induced cascade cyclization of the (2Z)-2-methyl-7-oxods-2-ene1, 10-dioate derivative bearing C/D ring system produced fully functionalized gibbane skeleton in good yield.Thus the authors established an effective and direct one-step process for obtaining bicyclic ring systems with γ-lactone from simple acyclic compounds using one electron transfer reagent SmI_2. By these mild conditions, the use of strongly basic conditions for the Dieckmann condensation is avoided and good to excellent stereocontrol over newly formed ring junctures in the product is possible.