Thermal chemistry of bicyclo[4.2.0]oct-2-enes

At 300 degrees C, bicyclo[4.2.0]oct-2-ene (1) isomerizes to bicyclo[2.2.2]oct-2-ene (2) via a formal [1,3] sigmatropic carbon migration. Deuterium labels at C7 and C8 were employed to probe for two-centered stereomutation resulting from C1-C6 cleavage and for one-centered stereomutation resulting from C1-C8 cleavage, respectively. In addition, deuterium labeling allowed for the elucidation of the stereochemical preference of the [1,3] migration of 1 to 2. The two possible [1,3] carbon shift outcomes reflect a slight preference for migration with inversion rather than retention of stereochemistry; the si/sr product ratio is approximately 1.4. One-centered stereomutation is the dominant process in the thermal manifold of 1, with lesser amounts of fragmentation and [1,3] carbon migration processes being observed. All of these observations are consistent with a long-lived, conformationally promiscuous diradical intermediate.     

Titanium(IV) chloride-mediated intramolecular ring enlargement of methylenecyclopropanes with propargylic esters: a concise synthesis of bicyclo[4.2.0]oct-5-ene derivatives

Titanium(IV) chloride-mediated intramolecular ring enlargement of methylenecyclopropanes with propargylic esters has been described in this context, affording the corresponding chlorinated bicyclo[4.2.0]oct-5-ene derivatives in moderate to good yields under mild conditions. The E- and Z-methylenecyclopropanes could all be converted to the corresponding bicyclo[4.2.0]oct-5-enes with moderate to high diastereoselectivities.     

Thermal intramolecular cycloaddition of 4-alkenylfulvene; highly regio- and diastereoselective formation of [4+2] adduct

4-Alkenylfulvenes were prepared by the annulation of 1,4-ynediones and allylidenetriphenylphosphorane and subjected to a thermal reaction. Highly regio- and stereoselective [4+2] cycloaddition is accomplished with 4-((R)-3-benzyloxypent-4-en-1-yl)fulvene and the resulting adduct is transformed into bicyclo[3.3.0]octene derivative.     

Microwave-assisted intramolecular [2 + 2] Allenic cycloaddition reaction for the rapid assembly of bicyclo[4.2.0]octa-1,6-dienes and bicyclo[5.2.0]nona-1,7-dienes

Microwave irradiation of alkynyl allenes affords an intramolecular [2 + 2] cycloaddition reaction. This cycloaddition provides an efficient route to bicyclomethylenecyclobutenes. The reaction occurs with complete regioselectivity for the distal double bond of the allene for the selective formation of a variety of hetero- and carbocyclic substrates. Bicyclo[4.2.0]octadienes and bicyclo[5.2.0]nonadienes have been prepared in high yield. [reaction: see text]     

Deuterium kinetic isotope effects and mechanism of the thermal isomerization of bicyclo[4.2.0]oct-7-ene to 1,3-cyclooctadiene

The thermal conversion of cis-bicyclo[4.2.0]oct-7-ene to cis,cis-1,3-cyclooctadiene might involve a direct disrotatory ring opening, or it might possibly take place by way of cis,trans-1,3-cyclooctadiene. This cis,trans-diene might possibly form the more stable cis,cis isomer through a [1,5] hydrogen shift or a trans-to-cis isomerization about the trans double bond. Deuterium kinetic isotope effect determinations for the isomerizations of 2,2,5,5-d(4)-bicyclo[4.2.0]oct-7-ene and 7,8-d(2)-bicyclo[4.2.0]oct-7-ene rule out these two alternatives because the observed effects are much smaller than would be anticipated for these mechanisms: k(H)/k(D)(d(4)) at 250 degrees C is 1.17 (1.04 per D), and k(H)/k(D)(d(2)) at 238 degrees C is 1.20 (1.10 per D). The direct disrotatory ring opening route remains the preferred mechanism.     

Synthesis of 1-acetyl-2-silyoxycycloheptane derivatives via highly stereoselective formal [5+2] cycloaddition reaction

A stereoselective [5+2] cycloaddition reaction using a new five-carbon unit, that has a dicobalt acetylene complex moiety and an enol silyl ether moiety, was developed. In the presence of a Lewis acid, the five-carbon unit reacted with an enol triisopropylsilyl ether to give a 1-acetyl-2-silyoxycycloheptane derivative, in which the three contiguous substituents on the seven-membered ring arrange cis to each other.     

Syntheses of optically active bicyclo[3.3.0]oct-2-ene derivatives based on asymmetric elimination

Bicyclo[3.3.0]oct-2-ene derivatives with high optical purities (87–99% e.e) were obtained by asymmetric elimination of the trifluoromethanesulfonates (10, 11, 12) by chiral N,N-dimethyl-1-phenethylamine.     

Gold-catalyzed tandem 1,3-migration/[2 + 2] cycloaddition of 1,7-enyne benzoates to azabicyclo[4.2.0]oct-5-enes

A synthetic method that relies on gold(I)-catalyzed tandem 1,3-migration/[2 + 2] cycloaddition of 1,7-enyne benzoates to prepare azabicyclo[4.2.0]oct-5-enes is described.     

A highly efficient synthesis of bicyclo[n.3.1] ring systems by allene intramolecular cycloaddition: Tandem intramolecular [2+2] cycloaddition-[3,3]-sigmatropic rearrangement

A novel one-step construction of bicyclo[5.3.1]undecane skeleton via intramolecular cycloaddition of allenyl ethers is described.     

Highly regio- and chemoselective [2 + 2 + 2] cycloaddition of 1,6-heptadiynes with allenes catalyzed by cobalt complexes

The CoI2(PPh3)2/Zn system effectively catalyzes the [2 + 2 + 2] ene-diyne cycloaddition of 1,6-heptadiynes with allenes in a highly regio- and chemoselective fashion to yield substituted benzene derivatives in good to excellent yields.     

Conformational analysis in the reversible intramolecular [2+2] photocycloaddition of diphenylbicyclo[4.2.0]oct-3-ene-2,5-diones

An irradiation of diphenylbicyclo[4.2.0]oct-3-ene-2,5-diones bearing variously substituted Me groups resulted in the reversible intramolecular [2+2] cycloaddition between the excited enedione CC double bond and the facing endo-phenyl ring to exclusively give pentacyclotetradeca-10,12-diene-2,7-diones. The equilibrated product ratios were much dependent on the substitution pattern of the Me-groups as well as the irradiated wavelength. The regiochemistry of these photoadditions was elucidated on the basis of the restricted conformation of the starting enediones.     

Catalytic multicomponent cycloaddition assembling three different substances to form highly substituted bicyclo[4.2.0]octanes

A catalytic multicomponent (4+2)-(2+2) cascade cycloaddition process assembling three different substances has been developed. The process is able to rapidly provide a highly substituted bicyclo[4.2.0]octane skeleton from a 2-siloxydiene and two molecules of α,β-unsaturated carbonyl partners. The MCR process is accompanied by stereoselective formation of four carbon-carbon bonds and four stereogenic centers in a single operation.     

Novel 2-phosphabicyclo[2.2.2]oct-5-ene derivatives and their use in phosphinylations

The [4 + 2] cycloaddition of the double-bond isomers ( A and B ) of dihydrophosphinine oxide 1 afforded novel phosphabicyclo[2.2.2]oct-5-ene derivates ( 2–4 ), formation of which was justified by PM3 semiempirical calculations. The compounds of dimer type ( 2–4 ) were utilized in the UV light-mediated fragmentation-related phosphinylation of nucleophiles, especially in that of alcohols. To explore the role of structural modifications on the fragmentation ability, disulfide 5 , phosphabicyclooctane 7 obtained by the hydrogenation of 2 , and the adduct of dihydrophosphinine oxide 1 with benzoquinone ( 7 ) were also synthesized and tested in fragmentation. © 2004 Wiley Periodicals, Inc. Heteroatom Chem 15:97–106, 2004; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.10221     

Studies on the pauson-khand reaction. Exclusive formation of angularly fused triquinanes from bicyclo[3.3.0]oct-2-ene and propargyl derivatives

Reaction of bicyclo[3.3.0]oct-2-ene with different hexacarbonyl dicobalt complexes of propargyl derivatives leads, under a great variety of experimental conditions, to the exclusive formation of angularly fused triquinanes, the corresponding reduced products being formed if the reaction is run at high temperature. The intermediacy of a π-allyl complex is suggested in order to account for these results, as well as for the presence of minor amounts of Diels-Alder adducts in the reaction mixture.     

A sulfinyl-directed asymmetric [5C + 2C] intramolecular acetoxypyranone-alkene cycloaddition

[reaction: see text] Introduction of a homochiral p-tolylsulfinyl group at the trans terminal position of an alkene induces a total diastereodifferentiation in the intramolecular cycloaddition of the latter to 3-oxidopyrylium ylide precursors. The cycloadducts can be readily desulfinylated to afford enantiomerically pure oxa-bridged bicyclo[5.3.0]decane ring systems. Theoretical calculations confirm that diastereoselectivity stems from the conformational preferences of the alkenylsulfoxide unit in the transition state of the reaction.     

Thermal [2+2] cycloaddition of allenynes: easy construction of bicyclo[6.2.0]deca-1,8-dienes, bicyclo[5.2.0]nona-1,7-dienes, and bicyclo[4.2.0]octa-1,6-dienes

The simple refluxing of allenynes, having a phenylsulfonyl functionality on the allenyl group, in xylene (or mesitylene) without microwave irradiation resulted in the efficient formation of bicyclo[5.2.0]nona-1,7-dienes and bicyclo[4.2.0]octa-1,6-dienes in high yields. This method was shown to be successfully applicable to the first construction of bicyclo[6.2.0]deca-1,8-dienes. Construction of the corresponding oxa- and azabicyclo[m.2.0] frameworks could also be attained. This thermal ring-closing reaction involves the formal [2+2] cycloaddition in which the distal double bond of an allenyl moiety exclusively served as one of the pi-components regardless of the position of the phenysulfonyl functionality on the allenyl moiety.