Enantioselective synthesis of the tricyclic core of (+)-strigol

Synthetic Studies toward Leucosceptroid Family of Natural Products

Leucosceptroids are sesterterpenoids with potent antifeedant and antifungal activities. In this paper, efforts on two synthetic strategies toward stereoselective total synthesis of the leucosceptroid family of natural products are reported. Intramolecular addition cyclization strategy could lead to a stereochemically mismatched core structure, while intermolecular addition/ring-closing metathesis cyclization strategy successfully furnished an advanced common intermediate bearing eight contiguous stereogenic centers, including three tetra-substituted ones, which fully matches all the stereochemistry on the tricyclic framework in leucosceptroid H. Late-stage transformation of this intermediate to leucosceptroid H encountered difficulty in oxidizing the secondary hydroxyl group to a carbonyl group in the target. Instead of the desired oxidation, an interesting tricyclic spiral product originating from a C-C bond cleavage was observed.

Enantiomeric impurities in chiral catalysts, auxiliaries, and synthons used in enantioselective syntheses. Part 5

The enantiomeric excess of chiral starting materials is one of the important factors determining the enantiopurity of products in asymmetric synthesis. Fifty-one commercially available chiral reagents used as building blocks, catalysts, and auxiliaries in various enantioselective syntheses were assayed for their enantiomeric purity. The test results were classified within five impurities level (ie, <0.01%, 0.01%-0.1%, 0.1%-1%, 1%-10%, >10%). Previously from 1998 to 2013, several reports have been published on the enantiomeric composition of more than 300 chiral reagents. This series of papers is necessitated by the fact that new reagents are forthcoming and that the enantiomeric purity of the same reagent can vary from batch to batch and/or from supplier to supplier. This report presents chiral liquid chromatography (LC) and gas chromatography (GC) methods to separate enantiomers of chiral compounds and evaluate their enantiomeric purities. The accurate and efficient LC analysis was done using newly introduced superficially porous particle (SPP 2.7 μm) based chiral stationary phases (TeicoShell, VancoShell, LarihcShell-P, and NicoShell).

Total synthesis of avenaol

Avenaol, isolated from the allelopathic plant black oat, was the first C₂₀ germination stimulant related to strigolactones. Structurally, it consisted of a bicyclo[4.1.0]heptanone skeleton containing a cyclopropane ring bearing three main chains projecting in the same direction (i.e. all-cis-substituted cyclopropane). Herein, we report the total synthesis of avenaol using a robust strategy involving the formation of an all-cis-substituted cyclopropane via an alkylidenecyclopropane. The key factors in the success of this total synthesis include the Rh-catalysed intramolecular cyclopropanation of an allene, an Ir-catalysed stereoselective double-bond isomerisation, and the differentiation of two hydroxymethyl groups via the regioselective formation and oxidation of a tetrahydropyran based on the reactivity of a cyclopropyl group. This strategy effectively avoids the undesired ring opening of the cyclopropane ring and the formation of a caged structure. Furthermore, this study confirms the proposed structure of avenaol, including its unique all-cis-substituted cyclopropane moiety.Avenaol is a potent germination stimulant that can be extracted from black oat. Here, the authors report the total synthesis of avenaol by developing a strategy to access all-cis-substituted cyclopropanes.