Synthesis of 6,6-dimethyltricyclo[5.4.0.02,8]undecane-2,9-diol for (ent-)longipinane-type sesquiterpenoids using two types of radical cyclization reactions

Total Syntheses of (±)-Acetoxymarsupellone and (±)-Marsupellins A, B, and D Enabled by a Reductive Cyclization and Semipinacol Rearrangement Strategy

We report herein the total syntheses of three marsupellin-family sesquiterpenoids, (±)-acetoxymarsupellone and (±)-marsupellins B and D, in 14-19 steps from our known precursor, making marsupellin A also accessible from marsupellin B through a known procedure. The critical tricyclic framework bearing the challenging C7 bridgehead all-carbon quaternary center is strategically constructed through a Ti-mediated reductive cyclization and semipinacol rearrangement sequence. This study provides a general approach to the syntheses of (ent-)longipinane-type molecules.

Synthesis of Succinonitrile Derivatives by Homocoupling from Cyanohydrin Derivatives with a Low-Valent Titanium Reagent

A method is described for synthesizing succinonitrile derivatives bearing alkyl or aryl substituents from cyanohydrin derivatives using low-valent titanium. The active species in this reaction is proposed to be a resonance hybrid of the Ti^IV nitrile enolate and Ti^III alkyl radical.

A new longipinane ketone from Achillea abrotanoides (Vis.) Vis.: chemical transformation of the essential oil enables the identification of a minor constituent

INTRODUCTION

Minor plant constituents are difficult to identify due to the challenging isolation and acquiring of reliable spectral data. Essential oils abound in such minor constituents that might be of high importance for their (e.g. olfactory) properties. The presence of new minor constituents is usually inferred from gas chromatography mass spectrometry (GC-MS) analyses that provide only a mass spectrum and retention data, which are insufficient to allow a positive identification.

OBJECTIVE

To identify a minor unknown constituent of the essential oil of Achillea abrotanoides (Vis.) Vis. (Asteraceae).

METHODOLOGY

The application of chemical transformations (oxidation and reduction) performed directly on crude essential-oil samples, followed by preparative chromatography and detailed spectral analysis, to identify a new longipinane ketone from the mentioned sample.

RESULTS

GC-MS analyses of the essential oil revealed, among other constituents, the presence of a known rare longipinane alcohol (α-longipinen-7β-ol) representing 2.5% of the total GC-peak areas, and a related unidentified oxygenated sesquiterpene (3.8%). Interpretation of their mass spectra led to an assumption that the unidentified one could represent α-longipinen-7-one. Oxidation of the entire essential-oil sample by pyridinium chlorochromate confirmed the assumed relationship among the compounds and gave a simplified enriched mixture containing the ketone (ca. 16%). A straightforward chromatographic separation of the ketone was followed by corroboration of its structure by nuclear magnetic resonance (NMR) (one- and two-dimensional), infrared (IR) and MS.

CONCLUSIONS

The complementing use of chemical transformations of crude essential oils, chromatographic separations, and detailed spectral analysis could have a more general application in the identification of new natural products.

Total Synthesis of (±)-Marsupellins A and B via Acetoxymarsupellone Using an Intramolecular Reductive Cyclization of Epoxycyanohydrin Derivative with Cp2TiI

The first total synthesis of C3- and C9-oxidized ent-longipinane-type sesquiterpenoids containing acetoxymarsupellone, marsupellins A and B, has been accomplished. This unique core common to C3- and C9-oxidized ent-longipinane-type sesquiterpenoids was constructed via a new intramolecular reductive cyclization reaction of an epoxycyanohydrin derivative using Cp₂TiI.

A concise approach to the tricyclic framework of longipinane- and ent-longipinane-type sesquiterpenoids

A general strategy for the synthesis of the common 6,6-dimethyltricyclic[5.4.0.0^2,8]undecane framework of longipinane- and ent-longipinane-type sesquiterpenoids has been successfully developed.