Desymmetrisation of bicyclo[4.4.0]decadienes: A planar-chiral complex proved to be most effective in an asymmetric Heck reaction

The synthesis and desymmetrisation of bicyclo[4.4.0]decadienes is described; the enantioselective Heck reaction using JOSIPHOS as a planar-chiral complex produces a tetracyclic system with three stereogenic centers in up to 84% enantiomeric purity.     

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]     

Synthesis of Si- and N-containing bicyclo[4.2.1]nona-2,4-dienes and bicyclo[4.2.1]nona-2,4,7-trienes

A [6π+2π] cycloaddition of Si- and N-containing alkynes and 1,2-dienes to cyclohepta-1,3,5-trienes in the presence of the two-component catalytic system (acac)₂TiCl₂-Et₂AlCl gives rise to the corresponding bicyclo[4.2.1]nona-2,4-diene and bicyclo[4.2.1]nona-2,4,7-triene derivatives.     

Palladium catalysed [2 + 2 + 1] intramolecular cycloaddition for the preparation of bicyclo[3.3.0]octa-1.5-dien-3-ones from 1,6-diynes

Palladium catalysed [2 + 2 + 1] cycloaddition of 1,6-heptadiynes with CO (1 atm) furnishes bicyclo[3.3.0]octa-1,5-dien-3-ones in 30-74% yield.     

A synthetic approach to bicyclo[6.2.1]undecane ring systems

A synthesis of a functionalized bicyclo[6.2.1]undecane, N-(7-hydroxymethyl-bicyclo[6.2.1]undeca-3,5,9-trien-2-yl)-4-methyl-benzenesulfonamide, is described. Starting with a [6+4] cycloaddition between cyclopentadiene and cycloheptatrienone, the final product was prepared in five steps with an overall 37% yield. The remarkable resistance to hydrolysis of an intermediate lactam was overcome by tosylating the amide and reducing with LiAlH₄.     

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.     

A formal [3+2] cycloaddition for the synthesis of bicyclo[3.2.1]octanes

A new and a direct synthetic method for the construction of the bicyclo[3.2.1]octane system is envisioned by assembling cyclic allylsilanes with electron-deficient double bonds in a single step via a formal diastereoselective Lewis acid-promoted [3+2] cycloaddition.     

Intramolecular [2+2] cycloaddition of homoallylketenes to bicyclo[2.1.1]hexan-5-ones

Intramolecular [2+2] cycloaddition of γ,δ-unsaturated ketenes derived from hex-5-enoyl chloride derivatives gave bicyclo[2.1.1]hexan-5-ones with complete regioselectivity.     

Synthesis of 8-substituted bicyclo[3.2.1]octane-6-carboxylic acids and anti-convulsant properties of the corresponding amides

Novel 8-substituted bicyclo[3.2.1]octane-6-carboxylic acids have been made via [3+2]cycloaddition to alkyne 2. A number of the corresponding amides are anti-convulsant in mice.     

Intramolecular photochemical dioxenone-alkene [2 + 2] cycloadditions as an approach to the bicyclo[2.1.1]hexane moiety of solanoeclepin A.

A synthesis of the bicyclo[2.1.1]hexane substructure of solanoeclepin A (1), the most active natural hatching agent of potato cyst nematodes, was approached via an intramolecular [2 + 2] photocycloaddition. Aldehyde 12 containing the dioxenone chromophore served as a useful starting material, allowing the synthesis of a variety of photocycloaddition substrates via Grignard addition or via a Nozaki-Hiyama-Kishi reaction. Photolysis of the unsubstituted alkene 14 led to the expected crossed cycloadduct bicyclo[2.1.1]hexane 15 according to the so-called rule of five. However, several functionalized alkenes 18, 20, and 31 exhibited a complete reversal of cycloaddition regioselectivity, providing straight cycloadducts bicyclo[2.2.0]hexanes 21-26 and 4, respectively. Their structures were proved by a combination of extensive NMR measurements, X-ray analyses, and subsequent retro-aldol reactions. The latter de Mayo process allowed the formation of spiro-[3.5]nonane 35 and spiro[3.4]octane 36 as well as the cyclobutanes 37 and 38. Finally, the cyclization of the more rigid lactone precursor 28 occurred in high yield in the desired fashion with complete regio- and stereoselectivity to give 3 containing the core bicyclo[2.1.1]hexane skeleton of the natural product.     

Cycloaddition of cyclohexa-2,4-dienones with electron deficient 2 pi partners: a novel and stereoselective route to functionalized bicyclo[2.2.2]octenones

A novel route to functionalised bicyclo[2.2.2]octenones via pi 4s + pi 2s cycloaddition of cyclohexa-2,4-dienones with electron deficient 2 pi partners, and crystal structure of 1,7-dimethyl-5-hydroxy-5-chloromethyl-2-carboethoxy bicyclo[2.2.2]oct-7-ene-6-one, is reported.     

Cycloaddition between electron-deficient pi-systems, photochemical and radical-induced reactions: a novel, general, and stereoselective route to polyfunctionalized bridged bicyclo[2.2.2]octanes, bicyclo[3.3.0]octanes, bicyclo[4.2.0]octanes, and tricyclo[4.3.1.0(3,7)]decanes

A novel, general, and stereoselective route to functionalized bridged bicyclo[2.2.2]octanes, bicyclo[3.3.0]octanes, bicyclo[4.2.0]octanes, and tricyclo[4.3.1.0(3,7)]decanes has been described. Various functionalized and substituted bicyclo[2.2.2]octanes endowed with a beta,gamma-enone chromophore were synthesized via cycloaddition of in situ generated cyclohexa-2,4-dienones with electron-deficient 2pi partners and manipulation of the resulting adducts. Triplet sensitized irradiation of bridged bicyclooctenones led to synthesis of bicyclo[3.3.0]octanoids, whereas the direct irradiation furnished bicyclo[4.2.0]octanes in stereoselective fashion as a result of modulation of reactivity in excited states. Further, manipulation of the adducts led to appropriately appended and functionalized bicyclo[2.2.2]octanes that upon radical induced cyclization provided an efficient and stereoselective route to the tricyclo[4.3.1.0(3,7)]decane (isotwistane) framework of pupukeananes.     

Cobalt(I)-catalyzed [6+2] cycloadditions of cyclooctatetra(tri)ene with alkynes

The cobalt-catalyzed [6+2] cycloaddition of cyclooctatetraene 1 with alkynes 3 affords monosubstituted bicyclo[4.2.2]deca-2,4,7,9-tetraenes 4 in fair to good yields. Due to the valence tautomerism, 1,3,5-cyclooctatriene 2, in equilibrium with bicyclo[4.2.0]octa-2,4-diene A, and alkynes 3 are converted to 10 and 11 according to [6+2] and [4+2] cycloadditions, respectively.     

Stereoselective synthesis of bicyclo[3.1.1]heptane derivatives via intramolecular photocycloaddition reaction

Optically active 1,3-bridged cyclobutanes 10 of the bicyclo[3.1.1]heptane ring system and 1,2-bridged cyclobutanes 11 of the bicyclo[3.2.0]heptane ring system were produced by UV irradiation of alpha,beta,gamma,delta-unsaturated esters 9a and 9c-f. The preference of endo-stereochemistry at C-6 bridged head was observed in cross-adducts 10. On the other hand, irradiation of conjugated dienol 9b led via only parallel cycloaddition to 1,2-bridged cyclobutane 11.     

Opening the [4 + 2 + 2] cycloadducts of bicyclo[2.2.1]hepta-2,5-dienes (norbornadienes) to cis-fused bicyclo[5.3.0]decanes

Tetracyclo[5.4.0.0.(2,4)0(3,7)]undec-9-enes, prepared by the transition-metal-catalyzed [4 + 2 + 2] homo-Diels-Alder reactions of norbornadiene and 1,3-butadienes, can be opened using either acid-promoted or Zeise's dimer-mediated cyclopropane ring cleavage, ultimately leading to cis-fused bicyclo[5.3.0]decanes (perhydroazulenes). Stereoselective functionalization of the olefin unit in the tetracycloundecenes to an alcohol or diol prior to ring opening is tolerated by the Zeise's dimer ([Pt(C(2)H(4))Cl(2)](2)) catalyst to yield highly functionalized bicyclo[5.3.0]decanes, which form the core structure of numerous sesquiterpenes.     

Synthesis of bicyclo[3.1.0]hexanes functionalized at the tip of the cyclopropane ring. Application to the synthesis of carbocyclic nucleosides

[reaction: see text] A general synthetic strategy for the preparation of functionalized bicyclo[3.1.0]hexanes is described. The new approach employs a cross metathesis step designed to functionalize the appropriate terminal olefin of the bicyclo[3.1.0]hexane precursor and a carbene-mediated intramolecular cyclopropanation reaction on the corresponding diazo intermediate. This combined methodology allowed the diastereoselective introduction of chemically diverse substituents at the tip of the cyclopropane group, except in cases where the substituents consisted of electron-withdrawing groups where a competing [3 + 2] cycloaddition predominated.     

Ruthenium-catalyzed homo Diels-Alder [2 + 2 + 2] cycloadditions of alkynyl phosphonates with bicyclo[2.2.1]hepta-2,5-diene

Ruthenium-catalyzed homo Diels-Alder [2 + 2 + 2] cycloadditions between alkynyl phosphonates and bicyclo[2.2.1]hepta-2,5-diene were studied. The observed reactivity was found to be dependent on the presence of the phosphonate moiety. The Ru-catalyzed cycloaddition was compatible with a variety of aromatic and aliphatic substituted alkynyl phosphonates, providing the corresponding phosphonate substituted deltacyclenes in low to good yields (up to 88%).     

[6+2]Cycloadditions catalyzed by titanium complexes

The Ziegler catalyst TiCl₄-Et₂AlCl and the arenetitanium(II) complex (η6-C₆H₆)Ti(II)(AlCl₄)₂ induce [6 + 2]cycloaddition reactions of cycloheptatriene with dienes and acetylenes. Addition to 1,3-butadiene affords 7 - endo - vinyl - bicyclo[4.2.1]nona - 2,4 - diene (main product) and bicyclo[4.4.1]- undeca - 2,4,8 - triene, a product of [6+4]cycloaddition. Isoprene reacts similarly, yielding mainly 7- endo - isopropenyl - bicyclo[4.2.1]nona - 2,4 - diene. 2,3 - Dimethyl - 1,3 - butadiene gives 8,9dimethylbicyclo [4.4.1]undeca - 2,4,8 - triene, a product of [6 + 4]cycloaddition, while [6 + 2]cross-adducts are minor products. The reaction of cycloheptatriene with norbornadiene gives mainly hexacyclo[6.5.1.0^2,7.0^3,12.^6,10.0^9,13]tetradec - 4 - ene via [6+2]cycloaddition followed by intramolecular Diels-Alder reaction. As a by-product, pentacyclo[7.5.0.0^2,7.0^3,5.0⁴⁸]tetradeca - 10,12 - diene is formed by a [2+2+2]mechanism. Addition of cycloheptatriene to diphenylacetylene and bis - (tri- methylsilyl)acetylene furnishes sustituted bicyclo[4.2.1]nona - 2,4,7 - trienes. Alkenes, E,E-2,4 - hexadiene and 1,3 - cyclooctadiene are unreactive. The [6+2]cycloaddition is made possible by coordination of cycloheptatriene to titanium, which changes the symmetry of the frontier orbitals in the triene. The reactivity of the trienophile is also enhanced by coordination to the catalyst.     

endo-6-(Hydroxymethyl)bicyclo[3.1.0]hept-3-en-2-one esters and the photochemical challenge: [2+2] cycloaddition versus skeletal rearrangement

The photochemical reactivity of enantiomerically pure bicyclo[3.1.0]hept-3-en-2-ones has been examined to determine whether a [2+2] cycloaddition with an alkene could be achieved without interference from a photochemically induced skeletal rearrangement. The formation of a tricyclo[5.2.0.0^2,5]nonan-6-one was indeed the major pathway, exclusively providing the cis-anti-cis isomer. However, the formation of norbornenones was also observed, as was stereochemical scrambling at C6 in the starting material.     

Carbon-13 chemical shifts of bicyclo[3.2.1]octane and bicyclo[2.2.1]heptane derivatives

¹³C chemical shifts of more than fifty bicyclo[3.2.1]octane and bicyclo[2.2.1]heptane derivatives (hydrocarbons, alcohols, ketones and esters) have been determined. The usefulness of ethyl derivatives for the assignment of close ¹³C chemical shifts in bicyclic methyl derivatives is shown both for the bicyclo[3.2.1]octane and bicyclo[2.2.1]heptane series. Comparison of substituent effects on α-, β-, γ- and δ-carbons in both series of compounds shows remarkable differences in steric interactions. In contrast to the rigid bicyclo[2.2.1]heptane system, both chair and boat conformations can be predominant in the bicyclo[3.2.1]octane series with the conformationally flexible 6-membered ring.