Studies towards the total synthesis of palau'amine. Formation of 4,5-dihydropyrrole-2-carboxylate intermediates by alkene-enamide ring-closing metathesis

A highly functionalized 4,5-dihydropyrrole-2-carboxylate is assembled by alkene-enamide ring-closing metathesis. Subsequent intramolecular azomethine imine dipolar cycloaddition provides a triazacyclopenta[cd]pentalene intermediate of potential use in a total synthesis of palau'amine.     

Synthesis of functionalized cage propellanes and D3-Trishomocubanes via the ring-closing metathesis and acid-promoted rearrangement

Several functionalized cage propellanes and D₃-trishomocubanes containing spiro linkage have been reported starting with commercially available materials such as 1,4-hydroquinone and dicyclopentadiene. In this regard, Claisen rearrangement, Diels–Alder reaction (DA), ring-closing metathesis (RCM) and acid-promoted rearrangement have been used as key steps. The strategies described here, opens up new opportunities to assemble intricate cage systems that are difficult to construct by conventional methods. Carbocation intermediates generated during the rearrangement process play a prominent role in designing unusual cage systems. One of the rearranged cage compound's structure was unambiguously established by single crystal X-ray diffraction studies.     

An efficient synthesis of nitrogen-containing heterocycles via a tandem carbenoid N-H insertion/ring-closing metathesis sequence

A series of five- to eight-membered nitrogen-containing heterocycles were prepared via a general and efficient one-pot, two-component sequence featuring rhodium-catalyzed insertion of a vinyl-substituted α-diazocarbonyls into the N-H bond of a series of tert-butoxycarbonyl-(Boc)-protected amines, followed by ring-closing metathesis catalyzed by ruthenium benzylidene complexes. This methodology allows easy and convenient access to highly functionalized azacycloalkenes in moderate yields and excellent chemoselectivity in a single transformation.     

Electrochemically reduced tungsten‐based active species as catalysts for cross‐metathesis reactions: cross‐metathesis of erucic acid with 2‐octene

The cross‐metathesis of erucic acid, (CH₃(CH₂)₇CH?CH(CH₂)₁₁COOH), with an excess of 2‐octene in the presence of an electrochemically produced tungsten‐based catalyst has been studied. Cross‐ and self‐hydrocarbon products, viz. 2‐undecene (C₁₁), 6‐dodecene (C₁₂) and 6‐pentadecene (C₁₅), were detected. The influence of several parameters, such as the 2‐octene/erucic acid and 2‐octene/catalyst ratios and the reaction time, on the yield of the cross‐metathesis product, 6‐pentadecene, was studied. The cross‐metathesis of functionalized olefins in the presence of an Al–e^?–WCl₆–CH₂Cl₂ system has not been reported in the literature so far. The cross‐metathesis products in the presence of this catalyst system can be obtained with high yield and high specificity. Copyright © 2004 John Wiley & Sons, Ltd.     

Electrochemically reduced tungsten‐based active species as catalysts for cross‐metathesis reactions: cross‐metathesis of non‐functionalized olefins

The electrochemical reduction of WCl₆ results in the formation of an active olefin (alkene) metathesis catalyst. The application of the WCl₆–e^?–Al–CH₂Cl₂ catalyst system to cross‐metathesis reactions of non‐functionalized acyclic olefins is reported. Undesirable reactions, such as double‐bond shift isomerization and subsequent metathesis, were not observed in these reactions. Cross‐metathesis of 7‐tetradecene with an equimolar amount of 4‐octene generated the desired cross‐product, 4‐undecene, in good yield. The reaction of 7‐tetradecene with 2‐octene, catalyzed by electrochemically reduced tungsten hexachloride, resulted in both self‐ and cross‐metathesis products. The cross‐metathesis products, 2‐nonene and 6‐tridecene, were formed in larger amounts than the self‐metathesis products of 2‐octene. The optimum catalyst/olefin ratio and reaction time were found to be 1 : 60 and 24 h, respectively. The cross‐metathesis of symmetrical olefins with α‐olefins was also studied under the predetermined conditions. Copyright © 2003 John Wiley & Sons, Ltd.     

By-design molecular architectures via alkyne metathesis

Shape-persistent purely organic molecular architectures have attracted tremendous research interest in the past few decades. Dynamic Covalent Chemistry (DCvC), which deals with reversible covalent bond formation reactions, has emerged as an efficient synthetic approach for constructing these well-defined molecular architectures. Among various dynamic linkages, the formation of ethynylene linkages through dynamic alkyne metathesis is of particular interest due to their high chemical stability, linearity, and rigidity. In this review, we focus on the synthetic strategies of discrete molecular architectures (e.g., macrocycles, molecular cages) containing ethynylene linkages using alkyne metathesis as the key step, and their applications. We will introduce the history and challenges in the synthesis of those architectures via alkyne metathesis, the development of alkyne metathesis catalysts, the reported novel macrocycle structures, molecular cage structures, and their applications. In the end, we offer an outlook of this field and remaining challenges.

The recent synthesis of novel shape-persistent 2D and 3D molecular architectures via alkyne metathesis is reviewed and the critical role of catalysts is also highlighted.     

A convergent total synthesis of a new antiepileptic ceramide and its triacetyl derivative using olefin cross metathesis

A convergent total synthesis of a new antiepileptic ceramide 1b and its triacetyl derivative 1b′ was completed by using two important C-C bond forming reactions, Wittig methylenation and olefin cross metathesis as the key steps. The easily available 3,4,6-tri-O-benzyl-d-galactal was used as a chiral pool for the synthesis of highly functionalized amide 3 and the commercially available 1,12-dodecanediol for the long chain olefin counterpart 4. The long hydrocarbon chain of the new ceramide 1b was installed by using olefin cross metathesis between amide 3 and long chain terminal olefin 4 followed by hydrogenation.     

Influence of α-methylation in constructing stapled peptides with olefin metathesis

Ring-closing metathesis is commonly utilized in peptide macro-cyclization. The influence of α-methylation of the amino acids bearing the olefin moieties has never been systematically studied. In this report, controlled reactions unambiguously indicate that α-methylation at the N-terminus of the metathesis sites is crucial for this reaction to occur. Also, we first elucidated that the E-isomers of stapled peptides are significantly more helical than the Z-isomers.     

Ring-closing metathesis for the synthesis of heteroaromatics: evaluating routes to pyridines and pyridazines

Ring-closing olefin metathesis (RCM) has been applied to the efficient synthesis of densely and diversely substituted pyridine and pyridazine frameworks. Routes to suitable metathesis precursors have been investigated and the scope of the metathesis step has been probed. The metathesis products function as precursors to the target heteroaromatic structures via elimination of a suitable leaving group, which also facilitates earlier steps by serving as a protecting group at nitrogen. Further functionalisation of the metathesis products is possible both prior to and after aromatisation. The net result is a powerful strategy for the de novo synthesis of highly substituted heteroaromatic scaffolds.     

Domino intramolecular enyne metathesis/cross metathesis approach to the xanthanolides. Enantioselective synthesis of (+)-8-epi-xanthatin

The first total synthesis of (+)-8-epi-xanthatin (1) has been achieved in 14 steps starting from the commercially available ester 24, which was converted into aldehyde 23 in six steps. An enantioselective aldol reaction of 23 gave 30, which was transformed into triflate 22 in four steps, setting the stage for a palladium-catalyzed carbonylation reaction to form acrylate 34. Compound 34 was then subjected to a deprotection/lactonization sequence to furnish enyne 21, which underwent a domino enyne ring-closing metathesis/cross metathesis process to form a seven-membered carbocycle and (E)-conjugated dienone, thereby completing the synthesis of 1. This domino ruthenium-catalyzed metathesis reaction thus serves as an efficient method to construct the core of xanthanolide and other sesquiterpene lactones.     

In situ generation of highly active olefin metathesis initiators

A ruthenium based benzylidene complex bearing an O,N-bidentate Schiff base ligand exhibits poor olefin metathesis activity but generates a highly active catalyst system when treated with acidic cocatalysts. Various Lewis acids were able to boost the ring opening metathesis polymerization (ROMP) of cycloocta-1,5-diene (COD). The best results were obtained with trichlorosilane (HSiCl₃), which was also found to improve metathesis activity of the Grubbs second generation catalyst [RuCl₂(CHPh)(H₂IMes)(PCy₃)] in the ROMP of COD and the ring closing metathesis (RCM) of diethyl diallylmalonate.     

Total synthesis of haminol A: an analysis of vinylpyridine metathesis reactivity

The total synthesis of haminol A has been completed featuring a masked-alkene metathesis reaction followed by bis-acyloxysulfone elimination to install the 1,3,8-triene subunit. During the course of our synthesis, the metathesis reactivity of 3-vinylpyridine was evaluated and our data suggest the rapid formation of a ruthenium pyridylalkylidene that no longer participates in productive metathesis. A chemotaxis assay using Caenorhabditis elegans demonstrated that haminol A produced an avoidance response from this organism.     

Synthesis of coumarins by ring-closing metathesis using Grubbs’ catalyst

A novel generally applicable synthesis of coumarins from phenolic substrates utilizing ring-closing metathesis is described. This sequence involves O-allylation of phenols followed by ortho-Claisen rearrangement, subsequent based-induced isomerization affording 2-(1-propenyl)phenols, acylation with acryloyl chloride, and finally ring-closing metathesis (RCM) with Grubbs’ second generation catalyst.     

Enantioselective synthesis of (+)-anatoxin-a via enyne metathesis

A concise synthesis of the potent nAChR agonist (+)-anatoxin-a (1) has been completed by a series of nine chemical operations and in 27% overall yield from commercially available d-methyl pyroglutamate (12). The strategy featured the application of a new protocol for the diastereoselective synthesis of cis-2,5-disubstituted pyrrolidines bearing unsaturated side chains and an intramolecular enyne metathesis to provide the bridged bicyclic framework of 1. Thus, d-methyl pyroglutamate (12) was converted in five steps to 32, which underwent facile enyne metathesis to deliver the bicyclic diene 33. Selective oxidative cleavage of the less substituted carbon-carbon double bond in 33 followed by deprotection furnished (+)-anatoxin-a (1).     

Lewis acid-modified mesoporous alumina: A new catalyst carrier for methyltrioxorhenium in metathesis of olefins bearing functional groups

Lewis acid-modified mesoporous alumina was found to be an efficient carrier as well as an activator for methyltrioxorhenium (MeReO₃) in olefin metathesis reactions. Especially, MeReO₃ doped on zinc chloride-modified mesoporous alumina catalyzed the metathesis of olefins with functional groups such as acetoxy, alkoxycarbonyl, acyl, chlorine, and bromine groups under mild conditions. The novel heterogeneous catalytic system promoted the metathesis of not only such functionalized olefins but also simple olefins without double bond migration that was often encountered on strong solid acids. We here present a new methodology for activation of a metal complex with Lewis acidic mesoporous materials in the metathesis reactions. This novel heterogeneous catalyst would be advantageous over conventional one from the viewpoint of environmental and economical organic synthesis.     

Latent ruthenium olefin metathesis catalysts featuring a phosphine or an N-heterocyclic carbene ligand

The synthesis and characterization of latent 18-electron ruthenium benzylidene complexes (PCy₃)((^κN,O)-picolinate)₂RuCHPh (5) and (H₂IMes)((^κN,O)-picolinate)₂RuCHPh (6) are described. Both complexes appear as two isomers. The ratio between the isomers is dependent on l-type ligand. The complexes are inactive in ring-closing metathesis and ring-opening metathesis polymerization reactions even at elevated temperatures in the absence of stimuli. Upon addition of HCl, complexes 5 and 6 become highly active in olefin metathesis reactions. The advantage of the latent catalysts is demonstrated in the ring-opening metathesis polymerization of dicyclopentadiene, where the latency of 6 assures adequate mixing of catalyst and monomer before initiation. Trapping experiments suggests that the acid converts the 18-electron complexes into their corresponding highly olefin metathesis active 14-electron benzylidenes.     

Synthesis of cyclic sulfamoyl carbamates and ureas via ring-closing metathesis

Synthetic routes to a diverse set of cyclic sulfamoyl carbamates and ureas are reported. These routes utilize 3-component coupling, Mitsunobu alkylation, and ring-closing metathesis using the second-generation Grubbs catalyst to achieve the synthesis of the target S-heterocyclic compounds. Cyclic S-heterocycles ranging from 9- to 11-membered rings have been obtained.     

Synthesis of cyclitols via ring-closing metathesis

A convenient synthesis of enantiomerically pure and differentially protected l-chiro- and myo-inositols as well as conduritols B and F from 2,3,4-tri-O-benzyl-d-xylopyranose via ring-closing metathesis is reported. The facile synthesis of conduritol B constitutes a short formal synthesis of (−)-cyclophellitol.     

A convenient approach to cyclic enol phosphates via ring-closing metathesis

A strategy employing the second generation Grubbs catalyst in order to facilitate the generation of a variety of cyclic enol phosphates, including 2H-chromen-4-yl, 1,2-dihydroquinolin-4-yl, 2H-thiochromen-4-yl, 2H-thiochromen-4-yl 1,1-dioxide, and benzofuran-2-yl enol phosphate scaffolds is described. This work represents the first case of an olefin metathesis reaction in which one of the groups participating in the metathesis event is an enol phosphate moiety.     

Synthetic approach to cis and trans-decalins via Diels—Alder reaction and ring-closing metathesis as key steps: further extension to dioxapropellane derivative by ring-closing metathesis

9,10-Substituted cis and trans-decalins were synthesized by a simple route using the Diels-Alder reaction and ring-closing metathesis (RCM) as key steps. Later, the cis-decalin system has been extended to 3,8-dioxa[8.4.4]propellane derivative by RCM sequence as a key step.