Bicyclo[5.3.1]undec-1(10)-ene bridgehead olefin stability and the taxane bridgehead olefin

MM2 calculations have shown how substituent and conformational effects play a role in the stabilities of systems containing the bicyclo[5.3.1]undec-I(10)-ene moiety and that the taxane diterpene skeleton which possesses this structural feature is surprisingly stable.     

A double ring-closing olefin metathesis approach to [3]catenanes

A [3]catenane with peripheral olefinic macrocycles was conveniently synthesized via a double ring-closing olefin metathesis. Highlights of this work include the synthesis of a 65-membered macrocycle featuring two phenanthroline ligands, a Cu(I)-templated synthesis of a [3]pseudorotaxane, and the key double ring-closing olefin metathesis to afford the desired [3]catenane in 71% yield.     

Hydrogenation of [7]paracyclophane. Formation of a hyperstable bridgehead olefin

Catalytic hydrogenation of [7]paracyclophane results in absorption of only two moles of hydrogen to give the olefin 1. Resistance to further hydrogenation and MM2 calculations show that 1 is a member of the class of hyperstable olefins. The structure of 1 is secured by both spectroscopic and chemical means.     

An olefin cross-metathesis approach to vinylphosphonate-linked nucleic acids

[reaction: see text]. The synthesis of vinylphosphonate-linked nucleotide dimers has been achieved using an olefin cross-metathesis (CM) reaction as a key step. The 1,3-dimesityl-4,5-dihydroimidazol-2-ylidine-containing catalyst 5 (Grubbs' second-generation catalyst) was found to be the superior catalyst for this transformation. Both metathesis partners were readily available using known methodology, and the vinylphosphonate-linked dimer was produced with high levels of (E)-selectivity (>20:1) in 58% yield (70% based on recovered starting material).     

A new synthetic approach to phenol derivatives: use of ring-closing olefin metathesis

Phenol derivatives, which are one of the most important classes of aromatic compounds in organic chemistry, were synthesized by ruthenium-catalyzed ring-closing olefin metathesis (RCM) of 1,4,7-trien-3-ones with versatile substitution patterns. The RCM reaction for producing phenol derivatives was also successful with 1,5,7-trien-3-one as another precursor. Most of the phenols prepared here could not be obtained easily by conventional methods.     

A nonelectrocyclic path from the cyclobutene cation radical to the 1,3-butadiene cation radical

The conversion of the cyclobutene cation radical to the 1,3-butadiene cation radical has been studied using MINDO /3 and ab initio SCF MO methods. Not only smooth electrocyclic but also stepwise, non-electrocyclic routes were considered. Both calculational methods agree that the preferred reaction path is a novel nonelectrocyclic one proceeding through an intermediate “cyclopropylcarbinyl cation radical.” The quantitative agreement in the activation parameters calculated by the two methods is excellent. The proposed intermediate also provides an attractive explanation for the mass spectrometric fragmentation patterns of the cyclobutene and butadiene cation radicals.     

A synergistic approach to polycyclics via a strategic utilization of Claisen rearrangement and olefin metathesis

Olefin metathesis promoted by a well-defined metal carbene complex has evolved into an efficient method for the construction of a broad range of carbocyclic and heterocyclic rings of varying size. The synthetic potential of the olefin metathesis has been further increased by combining various other C-C bond forming processes either in tandem or in sequence. Herein, application of Claisen rearrangement and olefin metathesis to prepare various intricate and/or biologically important targets has been described.     

A new approach for controlling birefringent property of cyclic olefin copolymers

This article reports an original method to control the birefringent property of the cyclic olefin copolymer, which has been recently commercialized as a new type of optical resins, via introduction of a third monomer that possesses a negative birefringence into the cyclic olefin copolymer that exhibits an inherently positive birefringence. The mutual compensation between these opposite‐sign birefringences effectively reduced the birefringent magnitude of the corresponding terpolymer. In fact, terpolymerization of norbornene (N), ethylene (E), and styrene (S), in which S exhibits a negative birefringence regarding to the positive birefringence of the NE copolymer was successfully prepared using fluorenylamidodimethyltitanium‐based catalyst, yielding NES terpolymers with controllable birefringent property. Especially, when the S content in the NES terpolymer was controlled at optimum values, it is possible to synthesize a new type of the cyclic olefin copolymer that exhibits an extremely low birefringent magnitude close to zero regardless of high degrees of chain orientation. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 7395–7400, 2008     

High selectivities in electrophilic additions to cyclobutene compounds

Epoxidation of 2, 3, 4 with m-CPBA mainly led to the cis-attack products whereas 1 and 6 led to the other selectivity. The result was reversed, from 4, with Payne's reagent. Bromohydroxylation of 4 involved an intermediate bromonium ion syn to the substituents. Haloselenylations occurred with the syn-selectivity from 1, 2, 3 and 4, to the anti-selectivity from 6, and without selectivity from 5. NOE enhancement measurements and several chemical correlations led to the stereochemical assignments. Formation of the intramolecular reaction products 24 and 25 was also pointed out.     

A novel, versatile synthetic approach to linearly fused tricyclopentanoids via photo-thermal olefin metathesis

Fifteen examples of a new, speedy and general approach to linearly fused tricyclopentanoids bearing the tricyclo[6.3.0.0^2,6]undecane (triquinane) frame of high contemporary interest is delineated. The key concept in our synthetic sequence to triquinanes is the novel photo-thermal olefin metathesis of cheap, abundantly available Diels-Alder adducts of 1,3 cyclopentadienes and p-benzoquinones. Thus photolysis of endo-tricyclo[6.2.1.0^2,7] undeca-4,9-dien-3,6-diones (9a–9j, 13a,b) furnished pentacyclo [5.4.0.0^2,6.0^3,10.0^5,9]undecan-8,11-diones (10a–10j, 14a,b), which on thermal fragmentation of the cyclobutane ring gave cis, syn, cis-tricyclo [6.3.0.0^2,6]undeca-4,9-dien-3,11-diones (11a–11j, 15a,b in just three steps and in exceptionally good yields. A few interesting transformations of the readily available parent bis-enone 11a which indicates its wider uses in syntheses, are described. Finally, a smooth thermal isomerisation of cis, syn, cis-bis-enones to cis, anti, cis-bis-enones is reported, which further enhances the scope and versatility of our synthetic theme.     

Stereospecific syn-addition of iodine azide to strained cyclobutene moeity

Addition of tricyclo[4.2.2.0^2.5]deca-3,7-diene and iodine azide was found to be syn on the cyclobutene moiety. Structure determination of the adducts and the formation mechanism are discussed.     

An efficient enantioselective approach to cyclic beta-amino acid derivatives via olefin metathesis reactions

The asymmetric synthesis of polyfunctionalized piperidine- and pyrrolidine-based scaffolds, specifically designed for the preparation of cyclic, conformationally constrained beta-amino acids, is realized combining a biocatalytic access to a versatile chiral building block with a wide range of transformations based on olefin metathesis.     

Complicated goings-on in the metal-manipulated ring-opening of cyclobutene

Most reported examples of transition metal-mediated pericyclic reactions have been shown to proceed by nonpericyclic mechanisms. However, the conversion of Fe(CO)(4)-cyclobutenes to Fe(CO)(3)-butadienes and CO is thought to proceed through a mechanism involving an electrocyclic ring-opening step. We have examined this and possible alternative mechanisms for the parent Fe(CO)(4)-cyclobutene complex using hybrid HF-DFT calculations. In this system there is an inherent preference for a pathway that avoids metal-templated electrocyclic ring-opening, but the pericyclic pathway is predicted to dominate for more elaborate substrates and under certain experimental conditions.     

Ring-closing olefin metathesis of 2,2'-divinylbiphenyls: a novel and general approach to phenanthrenes

[reaction: see text] The ring-closing olefin metathesis (RCM) of 2,2'-divinylbiphenyls, using a second-generation RCM ruthenium-based catalyst, leads to differently substituted phenanthrenes in quantitative yield under very mild reaction conditions, independent of both nature and position of the groups present on the biphenyl moiety.     

An olefin polyanion equivalent: A new olefin synthesis from triflones

α-Trifyl-dimethylsulfone (CF₃SO₂CH₂SO₂CH₃) is a reagent which allows successive polyalkylation of the two carbons with regiocontrol. The polyalkylated trifyl-sulfone then undergoes a Ramberg-Bäcklund reaction with loss of triflinate anion and extrusion of SO₂ to form an olefin. In synthetic terms the net structural change is equivalent to regiospecific alkylation of an olefin polyanion, =CC=.     

The carbene fragmentation-ring expansion route to bridgehead carbocations

[reaction: see text]The products, kinetics, and activation parameters were determined for the fragmentation-ring expansions of 1-norbornylmethyloxychlorocarbene (11) and 3-noradamantylmethyloxychlorocarbene (17). Products from 11 (in dichloroethane) included 1-chlorobicyclo[2.2.2]octane (9, 56%) and 1-chlorobicyclo[3.2.1]octane (10, 37%); from 17, we obtained 1-chloroadamantane (15, 68%) and protoadamantyl chloride (16, 28%).     

Intramolecular oxidative cyclizations in heteroarylthioureas: A versatile pathway to bridgehead heterocyclic systems

Intramolecular oxidations in N-alkyl-N'-heteroarylthioureas represent a facile and versatile synthetic pathway to fused heterocyclic systems including the bridgehead ones. This kind of heterocycles are the main feature in commom biologically active compounds.     

Diversity-oriented approach to macrocyclic cyclophane derivatives by Suzuki-Miyaura cross-coupling and olefin metathesis as key steps

Palladium-catalyzed Suzuki-Miyaura (SM) cross-coupling reaction with allylboronic acid pinacol ester and titanium assisted cross-metathesis (CM)/ring-closing metathesis (RCM) cascade has been used to synthesize macrocyclic cyclophane derivatives.