Construction Of The Tricyclic Core Of Steenkrotin- Type Diterpenoids And Research Of Cationic Type [5+2] Cycloaddition Premoved By Hypervalent Iodine

This article mainly talks about the two aspects, the first part introduces the background of natural terpenoids product Steenkrotin A and its skeleton research; the second part reviews a series of progresses in the new reactions catalyzed by iodine reagent in recent years.On the basis of the research, we developed a intramolecular [5+2] cycloaddition reactioncatalyzedby trivalent iodine（Ⅲ）and its application in the total synthesis of natural products.1. （±）-Steenkrotin A is a novel diterpenoid which was isolated by Hussein and co-workersfrom the ethanol extract of leaves of Croton steenkampianus （Euphorbiaceae） in 2008. Steenkrotin A possesses anintricate[3,5,5,6,7] pentacycliccarbon frameworkconsisting of a stericallycongestedhydroxytetrahydrofuran subunit.The molecule contains eight stereogenic centers, of which six are contiguous including one all-carbon quaternary center. Preliminary antiplasmodial testrevealed that steenkrotin A has mild activity against the chloroquine-resistant strain Dd2 (IC₅₀=5.8 μg/mL) of Plasmodium falciparum.The unprecedentedstructures as well as the intriguing biological activities rendered steenkrotins A as an attractive target for synthetic studies. In Charpter 2, we synthesized the [6,7] bicycle of steekrotins A via an intramolecular nitrile oxide/alkene [3+2] cycloaddition strategy. However, attempts to construct the [5,6,7] tricyclic structure of steekrotins A via Cope rearrangement met with failure.2. Various hypervalent iodine reagents have received particular attentionowing to their applications in catalysis. Varioussynthetic transformations have been successfully achievedusing both iodine （Ⅲ） or iodine（Ⅴ） species as catalytic intermediates, such as oxidation, cyclizations, α - functionalizations of carbonyl compounds and oxidative rearrangements. Most of all, phenolic compounds with internal nucleophilescould undergo an intramolecular cyclization using hypervalentiodine reagentsvia the formationof C - O, C-N, and C -C bonds has been a efficient approachfor the synthesis pharmacologically active scaffolds of many natual products.We designed a reaction which the vinyl phenol substrate was oxidated to aromatization by hypervalent iodine reagents, and intramolecular [5+2] cycloaddition occurred to generate a tetracyclic compound, but we got a product of the acyl migration accidently. This reaction of constracting tetracyclic system by one step can be applied to the skeleton synthesis of natural products Pharicin A. In addition, the reaction of vinyl phenol has wide applications, whether electronic substituents on the aromatic ring is rich or poor, or side chain is replaced by heteroatomic, it can all be smoothly occur ed at a moderate yield.