Utility of the tandem Pauson-Khand reaction in the construction of tetracycles

Total Synthesis of the Hexacyclic Sesterterpenoid Niduterpenoid B via Structural Reorganization Strategy

To date, it remains challenging to precisely and efficiently construct structurally intriguing polycarbocycles with densely packed stereocenters in organic synthesis. Niduterpenoid B, a naturally occurring ERα inhibitor, exemplifies this complexity with its intricate polycyclic network comprising 5 cyclopentane and 1 cyclopropane rings, featuring 13 contiguous stereocenters, including 4 all-carbon quaternary centers. In this work, we describe the first total synthesis of niduterpenoid B using a structural reorganization strategy. Key features include the following: (1) an efficient methoxy-controlled cascade reaction that precisely forges a highly functionalized tetraquinane (A-D rings) bearing sterically hindered contiguous quaternary stereocenters; (2) a rhodium-catalyzed [1 + 2] cycloaddition that facilitates the construction of a strained 3/5 bicycle (E-F rings) angularly fused with ring D.

Transition-Metal-Mediated Chemo- and Stereoselective Total Synthesis of (-)-Galanthamine

An asymmetric synthetic route to (-)-galanthamine (1), a pharmacologically active Amaryllidaceae alkaloid used for the symptomatic treatment of early onset Alzheimer's disease, was successfully established with very high levels of stereocontrol. The key to achieving high chemo- and stereo-selectivity in this approach was the use of transition-metal-mediated reactions, namely, enyne ring-closing metathesis, Heck coupling, and titanium-based asymmetric allylation.

A pH-controlled recyclable indolinooxazolidine tagged N-heterocyclic carbene Ru catalyst for olefin metathesis

A pH-controlled recyclable indolinooxazolidine tagged N-heterocyclic carbene Ru catalyst for olefin metathesis has been developed.

Highly efficient nitrogen chelated ruthenium carbene metathesis catalysts

Highly efficient nitrogen chelated ruthenium carbene metathesis catalysts containing an N-heterocyclic carbene (NHC) and a carbonyl group have been developed.

Synthesis of an advanced intermediate en route to the mitomycin natural products

[Structure: see text] An advanced intermediate in our planned synthesis of mitomycin C has been acquired in nine steps from tert-butyl glyoxylate. The aziridinyl pyrrolidine and quinone subunits are coupled regioselectively to arrive at an enamine that is prepared for C10 homologation.

Synthesis of dicyclopenta[a,e]pentalenes via a molybdenum carbonyl mediated tandem allenic Pauson-Khand reaction and the X-ray crystal structure of a planar dicyclopenta[a,e]pentalene

Two 14 pi cross-linked annulenes which belong to the family of dicyclopenta[a,e]pentalenes have been synthesized, 14 pi bis enol triflate ester 27 and the 3,7-diisopropylsilyl substituted 14 pi dicyclopenta[a,e]pentalene 30. The new allenic tandem Pauson-Khand reaction mediated by Mo(CO)(6) was employed as the key process to construct the core of the tetracycles. The two linear dicyclopenta[a,e]pentalenes 27 and 30 underwent significant electronic delocalization, perhaps even aromaticity, as revealed by the X-ray structure of 27. The tetracyclic rings in 27 assumed a flat geometry (Figure 4); the bond lengths of the tetracycle in 27 also fit well into the criteria for aromatic compounds. A comparison of the NMR and UV spectra of both 27 and 30 demonstrated that they both exhibited similar electronic properties, therefore, the purple colored 14 pi cross linked annulene 30 is planar as well as delocalized.

The synthesis of a dicyclopenta[a,e]pentalene via a molybdenum hexacarbonyl-mediated tandem allenic Pauson-Khand reaction

An approach to the 14pi annulene has been developed, by employing the molybdenum hexacarbonyl-mediated tandem allenic Pauson-Khand reaction as a key step. A di-triisopropylsilyl substituted dicyclopenta[a,e]pentalene was synthesized. This 14pi annulene showed some electronic delocalization, which was not observed in previous cases.

Endo-selective intramolecular Pauson-Khand reactions of gamma-oxygenated-alpha,beta-unsaturated phenylsulfones

A wide variety of 1,6-enynes and 1,7-enynes incorporating gamma-oxygenated-alpha,beta-unsaturated phenylsulfone moieties have readily been prepared by piperidine-promoted condensation of the corresponding alkynyl aldehyde with phenylsulfonyl-(p-tolylsulfinyl)methane and further protection of the hydroxyl group. Despite the enduring claim concerning the unsuitability of electronically deficient olefins in Pauson-Khand reactions, we report that these 1-sulfonylated enynes are excellent substrates in intramolecular Pauson-Khand reactions under both thermal and amine N-oxide-promoted conditions. Moreover, in contrast with the usual exo-selective Pauson-Khand cyclization of allylic substituted enynes, the reactions of these 1-sulfonylated-3-oxygenated enynes occur with a moderate or high endo selectivity. The evaluation of the chemical and stereochemical scope of the process in comparison with the Pauson-Khand cyclization of non-sulfonylated enynes, its application to the stereoselective preparation of optically pure C6-substituted bicyclo[3.3.0]oct-1-en-3-ones, and the interpretation of the stereochemical outcome are also discussed.