Novel transformations of two kinds of chlorinated photo [2 + 2] cycloadducts of 2-pyrone-5-carboxylate

Novel reactions of 7,7-dichloro- and 7,7,8-trichloro-3-oxo-2-oxabicyclo[4.2.0]oct-4-ene-6-carboxylates 5 with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in alcohol gave pyrano[4,3-b]pyran-2,5-diones 8 via (Z)-(2H-pyran-2-on-3-yl)butenoates 7. On the other hand, the same treatment of 7,7,8-trichloro-2-oxo-3-oxabicyclo-[4.2.0]oct-4-ene-5-carboxylate 6b afforded 2-oxo-3-oxabicyclo[4.2.0]oct-4,7-diene-5-carboxylate 14 via cyclobutene formation and S_N2′ displacement by attack of the alkoxy anion.     

Intramolecular ketenimine-ketenimine [2 + 2] and [4 + 2] cycloadditions

Bis(ketenimines), in which the two heterocumulenic functions are placed in close proximity on a carbon skeleton to allow their mutual interaction, show a rich and not easily predictable chemistry. Intramolecular [2 + 2] or [4 + 2] cycloadditions are, respectively, observed when both ketenimine functions are supported on either ortho-benzylic or 2,2'-biphenylenic scaffolds. In addition, nitrogen-to-carbon [1,3] and [1,5] shifts of arylmethyl groups in N-arylmethyl-C,C-diphenyl ketenimines are also disclosed.     

Mechanically facilitated retro [4+2] cycloadditions

Poly(methyl acrylate)s (PMAs) of varying molecular weights were grown from a [4+2] cycloaddition adduct of maleimide with furan containing two polymerization initiators. Subjecting the corresponding PMA (>30 kDa) chains to ultrasound at 0 °C resulted in a retro [4+2] cycloaddition reaction, as observed by gel permeation chromatography (GPC) and UV-vis spectroscopy, as well as labeling of the liberated maleimide and furan moieties with appropriate chromophores featuring complementary functional groups. Similar results were obtained by sonicating analogous polymers that were grown from a thermally robust [4+2] cycloaddition adduct of maleimide with anthracene. The generation of anthracenyl species from these latter adducts allowed for the rate of the corresponding mechanically activated retro [4+2] cycloaddition reaction to be measured. No reduction in the number average molecular weight (M(n)) or liberation of the maleimide, furan, or anthracene moieties was observed (i) for polymers containing the cycloaddition adducts with M(n) < 20 kDa, (ii) for high molecular weight PMAs (M(n) > 60 kDa) featuring terminal cycloaddition adducts, or (iii) when the cycloaddition adducts were not covalently linked to a high molecular weight PMA. Collectively, these results support the notion that the aforementioned retro [4+2] cycloaddition processes were derived from a vectorially opposed mechanical force applied to adducts embedded within the polymer chains.     

Transition metal-catalyzed intermolecular [5+2] and [5+2+1] cycloadditions of allenes and vinylcyclopropanes

Initial examples of the intermolecular Rh(I)-catalyzed [5+2] cycloaddition reaction of bifunctional allenes and vinylcyclopropanes are described. The reactions proceed with facility and in yields of up to 99% with a variety of alkyne-, ester-, styrene-, or cyano-substituents on the allene to afford the corresponding cycloadducts. In the presence of CO, the reaction proceeds to an eight-membered ring cycloadduct and its transannularly closed product, providing the first example of a three-component [5+2+1] cycloaddition with allenes.     

DFT study on [4+2] and [2+2] cycloadditions to [60] fullerene

The functionalisation of C₆₀ fullerene with 2,3-dimethylene-1,4-dioxane (I) and 2,5-dioxabicyclo [4.2.0]octa-1(8),6-diene (II) was investigated by the use of density functional theory calculations in terms of its energetic, structural, field emission, and electronic properties. The functionalisation of C₆₀ with I was previously reported experimentally. The I and II molecules are preferentially attached to a C—C bond shared and located between two hexagons of C₆₀ via [4+2] and [2+2] cycloadditions bearing reaction energies of ?15.9 kcal mol^?1 and ?72.4 kcal mol^?1, respectively. The HOMO-LUMO energy gap and work function of C₆₀ are significantly reduced following completion of the reactions. The field electron emission current of the C₆₀ surface will increase after functionalisation of either the I or II molecule.     

On the diastereoselectivity of ru-catalyzed [5 + 2] cycloadditions

[reaction: see text]. Ru-catalyzed cycloisomerization of cyclopropylenynes proceeds with good to high diastereoselectivities to form hexahydroazulenes.     

Stereoselective [4+2]-Cycloaddition with Chiral Alkenylboranes

A method for the stereoselective [4+2]-cycloaddition of alkenylboranes and dienes is presented. This transformation was accomplished through the introduction of a new strategy that involves the use of chiral N-protonated alkenyl oxazaborolidines as dieneophiles. The reaction leads to the formation of products that can be readily derivatized to more complex structural motifs through stereospecific transformations of the C−B bond such as oxidation and homologation. Detailed computation evaluation of the reaction has uncovered a surprising role of the counterion on stereoselectivity.     

Dual catalysis in enantioselective oxidopyrylium-based [5 + 2] cycloadditions

A new method for effecting catalytic enantioselective intramolecular [5 + 2] cycloadditions based on oxidopyrylium intermediates is reported. The dual catalyst system consists of a chiral primary aminothiourea and a second achiral thiourea. Experimental evidence points to a new type of cooperative catalysis with each species being necessary to generate a reactive pyrylium ion pair that undergoes subsequent cycloaddition with high enantioselectivity.     

The photochemical [4+4] cyclodimerization of 2-pyrazinone in the solid state

Irradiation of 1-methyl-5,6-diphenyl-2-pyrazinone (Ia) in the solid state gave the [4+4] anti dimer (II) in 100% yield, while (Ia) in a solution phase was inert upon irradiation.     

2 + 4] and [4 + 2] cycloadditions of o-thioquinones with 1,3-dienes: a computational study

Cycloadditions of o-thioquinones (o-TQs) with 1,3-dienes could proceed via either a [2 + 4] or a [4 + 2] mechanism. Under kinetic control and with acyclic dienes the reaction affords the spiro cycloadducts 5deriving from the [2 + 4] path as the main products. Under thermodynamic control, or with cyclic dienes, the o-TQs behave as heterodienes to give the benzoxathiin derivatives 4, in most cases with complete regioselectivity. In the present computational study, DFT calculations were performed in order to achieve a deep understanding of both [2 + 4] and [4 + 2] paths. The reactions of three o-TQs with six 1,3-dienes were thoroughly investigated at the B3LYP/TZVP//B3LYP6-31G level, and the two reaction mechanisms were then compared, evidencing that [2 + 4] cycloadditions are kinetically favored, strongly asynchronous, or even unconcerted, while [4 + 2] reactions are thermodynamically favored, quite asynchronous, but undoubtedly concerted. Moreover, the observed regioselectivity was rationalized by mean of the FMO theory and by comparison of the activation energies for different pathways.     

Photoinduced [4 + 4] cycloadditions of o-quinones with oxazoles

[reaction: see text] Photocycloadditions of 9,10-phenanthraquinone (PQ) with oxazoles (1a and 1b) gave [4 + 4] products 2 with the O=C-C=O functionality in PQ and the 2-azadiene moiety in oxazole as 4pi addends. Photoreactions of 1-acetylisatin (IS) with 1a, 1c, and 1d gave [4 + 4] product A, which underwent further [2 + 2] reactions with another (3)IS to furnish 5. These regioselective and diastereoselective [4 + 4] photocycloadditions are rationalized by the intervening of the key conformers for ISC and bond formation of the most stable 1,6-diradical intermediates.     

NHC-catalyzed enantioselective [2 + 2] and [2 + 2 + 2] cycloadditions of ketenes with isothiocyanates

The enantioselective N-heterocyclic carbene-catalyzed formal [2 + 2] and [2 + 2 + 2] cycloaddition of ketenes and isothiocyanates were developed. Reaction with N-aryl isothiocyanates at room temperature favors the [2 + 2] cycloaddition, while reaction with N-benzoyl isothiocyanates at -40 °C favors the [2 + 2 + 2] cycloaddition.     

Rhodium-catalyzed intramolecular [4 + 2] cycloadditions of alkynyl halides

[reaction: see text] Cationic rhodium(I)-catalyzed intramolecular [4 + 2] cycloadditions of diene-tethered alkynyl halides were found to occur in good yields (70-87%). The halide moiety is compatible with the cycloaddition reactions, and no oxidative insertion to the alkynyl halide was observed. The halogen-containing cycloadducts could be transformed into a variety of products that are difficult or impossible to obtain via direct cycloaddition.     

Intramolecular [2 + 2]cycloadditions of dialkylketenes with alkenes. Regiochemistry of intramolecular [2+2]cycloadditions of ketenes with alkenes

Unsaturated dialkylketenes 7a, 7b and 7c undergo intramolecular [2 + 2]cycloadditions to give 8a (45%), 9b (23%) and 9c (45%). Intramolecular cycloadditions of dialkylketenes give higher yields than intramolecular cycloadditions of monoalkylketenes, even though dialkylketenes are less reactive than monoalkylketenes. An intramolecular competition experiment with ketene 17 establishes that trans-alkenes are approximately 33 times more reactive than cis-alkenes in intramolecular cycloadditions. Ketene 36 furnishes 22% of the expected bicyclo[3.2.0]heptanone 37 and 28% of bicyclo[3.1.1]heptanone 38.     

Guest-induced conformation shift of p-sulphonatothiacalix[4]arene in the solid state and solution manipulated by [Zn(dipy)3]2+

The ¹H NMR and X-ray analysis data reveal the guest-induced cone → partial cone and cone → 1,2-alternate conformational shift in aqueous methanol solutions and solid state for a p-sulphonatothiacalix[4]arene-[Zn(dipy)₃]²⁺ host–guest system. The experimental data, together with DFT calculations, show that the guest-induced conformational shift of p-sulphonatothiacalix[4]arene is facilitated by the deprotonation of phenolic groups.     

Ruthenium-catalyzed [2 + 2] cycloadditions of ynamides

Ruthenium-catalyzed [2 + 2] cycloadditions between norbornene and ynamides were investigated. The ynamide moiety was found to be compatible with the ruthenium-catalyzed cycloaddition conditions, giving the corresponding cyclobutene cycloadducts in moderate to good yields (up to 97%). [reaction: see text]     

Synthetic studies and mechanistic insight in nickel-catalyzed [4+2+1] cycloadditions

A new nickel-catalyzed procedure for the [4+2+1] cycloaddition of (trimethylsilyl)diazomethane with alkynes tethered to dienes has been developed. A broad range of unsaturated substrates participate in the sequence, and stereoselectivities are generally excellent. Stereochemical studies provided evidence for a mechanism that involves the [3,3] sigmatropic rearrangement of divinylcyclopropanes.