The first synthesis of phosphonoacrolein. Application to Diels-Alder reaction as heterodiene

The first synthesis of phosphonoacrolein 3 was made in quantitative yield by acidic treatment of beta-ethoxy-alpha-(methoxymethyl)vinylphosphonate 2, derived from a beta-ethoxy-alpha-phosphonovinyl anion and MOMCl. The phosphonoacrolein 3 easily underwent a hetero-Diels-Alder reaction with electron-rich alkenes 4a-f or alkynes 9a-c under mild conditions, and phosphono-substituted pyrans 5a-d, 6e,f or pyranopyrans 11a-c were obtained in good to excellent yields. The reaction of 3 with cyclopentadiene and cyclohexadiene led to mixtures of [2 + 4] and [4 + 2] cycloadducts 7a, 8a and 7b, 8b in modest yields. The cycloaddition reaction between 3 and pyranopyran 13 or dibromocarbene and 13 resulted in [4 + 2] or [2 + 1] cycloadducts 14 or 15 in good yields.     

Rhodium N-heterocyclic carbene-catalyzed [4 + 2] and [5 + 2] cycloaddition reactions

[reactions: see text] A rhodium complex of N-heterocyclic carbene (NHC) has been developed for intra- and intermolecular [4 + 2] and intramolecular [5 + 2] cycloaddition reactions. This is the first use of a transition-metal NHC complex in a Diels-Alder-type reaction. For the intramolecular [4 + 2] cycloaddition reactions, all the dienynes studied were converted to their corresponding cycloadducts in 91-99% yields within 10 min. Moreover, up to 1900 turnovers have been obtained for the intramolecular [4 + 2] cycloaddition at 15-20 degrees C. For the intermolecular [4 + 2] cycloadditions, high yields (71-99%) of the corresponding cycloaddition products were obtained. The reaction time and yield were highly dependent upon the diene and the dienophile. For the intramolecular [5 + 2] cycloaddition reactions, all the alkyne vinylcyclopropanes studied were converted to their corresponding cycloadducts in 91-98% yields within 10 min. However, the catalytic system was not effective for an intermolecular [5 + 2] cycloaddition reaction.     

Palladium-catalyzed [2 + 2 + 2] cocyclotrimerization of benzynes with bicyclic alkenes: an efficient route to anellated 9,10-dihydrophenanthrene derivatives and polyaromatic compounds

An efficient method for the cocyclotrimerization of bicyclic alkenes and benzynes catalyzed by palladium phosphine complexes to give the corresponding norbornane anellated 9,10-dihydrophenanthrene derivatives is described. Bicyclic alkenes 1a-i undergo [2 + 2 + 2] cocyclotrimerization with benzynes generated from precursors 2a-d [2-(trimethylsilyl)phenyl triflate (2a), 4,5-dimethyl-2-(trimethylsilyl)phenyl triflate (2b), 6-(trimethylsilyl)-2,3-dihydro-1H-5-indenyl triflate (2c), 4-methyl-2-(trimethylsilyl)phenyl triflate (2d)] in the presence of PdCl(2)(PPh(3))(2) in acetonitrile at ambient temperature to yield anellated 9,10-dihydrophenanthrene products 3a-r in moderate to excellent yields. The [2 + 2 + 2] cocyclotrimerization products from oxa- and azabicyclic alkenes can be applied for the synthesis of polyaromatics, substituted benzo[b]triphenylenes (8a-f), via a simple Lewis acid mediated deoxyaromatization in good yields. In addition the [2 + 2 + 2] products undergo retro Diels-Alder reaction readily, providing a new method for the synthesis of substituted phenanthrenes and for generating isobenzofurans. A plausible mechanism is proposed to account for the catalytic [2 + 2 + 2] cycloaddition reaction.     

Transition metal-catalyzed [4 + 2 + 2] cycloadditions of bicyclo[2.2.1]hepta-2,5-dienes (norbornadienes) and bicyclo[2.2.2]octa-2,5-dienes

The transition-metal-catalyzed [4 + 2 + 2] cycloadditions of norbornadienes, bicyclo[2.2.2]octa-2,5-diene, and benzobarrelene with 1,3-butadienes proceed in excellent yields using cobalt-based catalytic systems. Two key distinctions between these [4 + 2 + 2] cycloadditions and the corresponding transition-metal-catalyzed [2 + 2 +2] reactions of norbornadiene are the requirement of a bimetal catalytic system with a bisphosphine ligand for the former and exclusive regioselectivity in the [4 + 2 + 2] reaction of 2-substituted norbornadienes to produce 1-substituted adducts. These distinctions may indicate two distinct mechanisms for the [4 + 2 + 2] and [2 + 2 + 2] reactions.     

Cycloadditions of 8,8-dicyanoheptafulvene to styrenes: manifestation of dual reactivity modes

A facile cycloaddition reaction of 8,8-dicyanoheptafulvene with styrenes leading to the corresponding [8+2] and [4+2] adducts in excellent yields is described.     

Rhodium-catalyzed [2+2+1+1] cyclocarbonylative coupling of alkynes with carbon monoxide affording tetrasubstituted p-benzoquinones

In this strategy, the tetrasubstituted benzoquinones have been prepared directly by a [2+2+1+1] cyclocarbonylative coupling reaction of internal alkynes with CO in the presence of [RhCl(CO)2]2. The low concentration of CO in the reaction is the crucial point for the chemoselective formation of tetrasubstituted benzoquinones in good to high yields. Functional groups such as chloro, methoxy, cyano, vinyl, fluoro, and carboxylate are tolerated under the reaction conditions.     

Enantioselective Formal [4+2] Cycloadditions to 3‐Nitroindoles by Trienamine Catalysis: Synthesis of Chiral Dihydrocarbazoles

The first enantioselective formal [4+2] cycloadditions of 3‐nitroindoles are presented. By using 3‐nitroindoles in combination with an organocatalyst, chiral dihydrocarbazole scaffolds are formed in moderate to good yields (up to 87 %) and enantioselectivities (up to 97 % ee). The reaction was extended to include enantioselective [4+2] cycloadditions of 3‐nitrobenzothiophene. The reaction proceeds through a [4+2] cycloaddition/elimination cascade under mild reaction conditions. Furthermore, a diastereoselective reduction of an enantioenriched cycloadduct is presented. The mechanism of the reaction is discussed based on experimental and computational studies.     

Rh(I)-catalyzed intramolecular allenic Pauson-Khand reaction: construction of a bicyclo[5.3.0]dec-1,7-dien-9-one skeleton

[reaction: see text] 1-Phenylsulfonylallenes possessing a hexynyl appendage in refluxing toluene in the presence of catalytic amount of rhodium(I) catalyst under a carbon monoxide atmosphere underwent regioselective formal [2 + 2 + 1]-cycloaddition to produce the corresponding bicyclo[5.3.0]dec-1,7-dien-9-one derivatives in acceptable yields.     

Radical [3+2] annulation of N-allyl-N-chlorotosylamide with alkenes via atom-transfer process

[reaction: see text]. A radical [3+2] annulation reaction with an N-centered radical has been developed. The reaction of alkenes with N-allyl-N-chlorotosylamide yields the corresponding pyrrolidine derivatives in good yields in the presence of Et3B as a radical initiator.     

Microwave-assisted iridium-catalyzed [2+2+2] cycloaddition of resin-bound dipropargylamine with alkynes

The [Ir(COD)Cl]₂/dppe system effectively catalyzes the solid-phase [2+2+2] cycloaddition of resin-bound dipropargylamine with alkynes under microwave conditions. The reaction results in high purity of isoindoline derivatives with moderate yields.     

Phosphine-catalyzed [4+2] cycloaddition of sulfamate-derived cyclic imines with allenoates: synthesis of sulfamate-fused tetrahydropyridines

Using n-PrPPh₂ as the nucleophilic catalyst, the [4+2] cycloaddition reaction of the sulfamate-derived cyclic imines with allenoates works efficiently to yield various sulfamate-fused tetrahydropyridines in high yields with excellent diastereoselectivities. Using amino acid-based bifunctional phosphine as chiral catalyst, an asymmetric [4+2] cycloaddition reaction was achieved, giving chiral sulfamate-fused tetrahydropyridines in high yields with good enantiomeric excesses.     

Transition metal-catalyzed hetero-[5 + 2] cycloadditions of cyclopropyl imines and alkynes: dihydroazepines from simple,readily available starting materials

The first example of a transition metal-catalyzed hetero-[5 + 2] cycloaddition reaction is described. Use of cyclopropyl imines as five-atom components, an alkyne as a two-carbon component, and a Rh(I) catalyst enables a new route to dihydroazepines. This new hetero-[5 + 2] cycloaddition works well with aldimines, ketimines, and with substituted cyclopropanes and affords the desired dihydroazepines in excellent yields as single regioisomers. Use of serial imine formation/aza-[5 + 2] cycloaddition generates the desired dihydroazepines in one operation from three commercially available starting materials. The reaction has been scaled to give gram quantities of dihydroazepine.     

A novel photorearrangement of a cyclohexadiene derivative of C60

[reaction: see text] Photorearrangement of tetraalkoxycarbonyl-substituted cyclohexadiene derivatives of C(60) yields not only well-known bis(fulleroid) but also bis(methano)fullerene. Existence of a labile and structurally new intermediate is observed in the reaction mixture. The discovery of the compound suggests the existence of another possible pathway giving those two products other than the widely accepted [4 + 4]/[2 + 2 + 2] mechanism.     

Cycloaddition to a C3-ethynylated chlorophyll derivative and self-aggregation of zinc chlorin–pyrazole/triazole conjugates

Several kinds of cycloaddition reactions were applied to C3-ethynylated pyropheophorbide-a methyl ester to develop C3-functionalized chlorophyll derivatives. Its [2+2] cycloaddition with tetracyanoethylene, [2+3] cycloaddition with diazomethane, [2+3] Huisgen reaction with trimethylsilyl azide, and [2+4] Diels–Alder reaction with tetraphenylcyclopentadienone gave the expected products in moderate to high yields. Zinc complex of the resulting 3-pyrazolyl-chlorin was found to show self-aggregation in a less polar solvent, in an aqueous micelle, and in the solid thin films more readily than the corresponding zinc 3-triazolyl-chlorin.     

Enantioselective synthesis of chiral 4H-pyran derivatives through [3+3] tandem reaction over a squaramide catalyst

An efficient [3+3] tandem reaction between 1-phenyl-3-methyl-5-pyrazolones and 2-(1-alkynyl)-2-alken-1-ones over a tertiary amine–squaramide catalyst is described. The pyran fused pyrazolone derivatives were successfully obtained in 53–88% yields and with 62–84% ee by this synthetic methodology. This methodology involves easily accessible starting material, mild condition, satisfied yields and ee values. Additionally, when enynes were used as the substrates for the [3+3] cascade reaction, isomerized target products could be obtained directly.     

A novel approach to 2-arylated quinolines: electrocyclization of alkynyl imines via vinylidene complexes

[reaction: see text] Alkynyl imines underwent [4 + 2] electrocyclization in the presence of 20 mol % W(CO)(5)(THF) to give 2-arylated quinolines in good yields. A deuterium labeling study suggests that the reaction proceeds via a tungsten vinylidene complex.     

Nucleophilic phosphine-catalyzed [3+2] cycloaddition of allenes with N-(thio)phosphoryl imines and acidic methanolysis of adducts N-(thio)phosphoryl 3-pyrrolines: a facile synthesis of free amine 3-pyrrolines

In this report, the dipolarophile imines with easily removable activating group O,O-diethyl(thio)phosphoryl have been investigated in the nucleophilic phosphine-catalyzed [3+2] cycloaddition reaction of electron-deficient allenes. Under the catalysis of a tertiary phosphine, N-(thio)phosphorylimines readily undergo the [3+2] cycloaddition reaction with ethyl 2,3-butadienoate or ethyl 2,3-pentadienoate, affording the corresponding N-(thio)phosphoryl 3-pyrrolines in moderate to high yields with good diastereoselectivity. Removal of the (thio)phosphoryl group from the adducts has been successfully achieved via the acidic methanolysis of the P-N bond, giving the free amine 3-pyrrolines in fair to good yields without severe aromatization. Thus, a facile synthesis of N-unsubstituted 3-pyrrolines is established from the phosphine-catalyzed [3+2] cycloaddition reaction of allenes with imines.     

Generation and trapping reaction of an efficient 1,2-dehydrocarborane precursor, phenyl[o-(trimethylsilyl)carboranyl]iodonium acetate

An efficient 1,2-dehydrocarborane precursor, phenyl[o-(trimethylsilyl)carboranyl]iodonium acetate, was readily prepared by reaction of [o-(trimethylsilyl)carboranyl]lithium and IPh(OAc)2. The facile 2+4 cycloaddition of with dienes such as anthracene, naphthalene, norborna-2,5-diene and 2,5-dimethylfuran gave high yields of the 1,2-dehydrocarborane adducts in the presence of a desilylating agent. The reaction of with a cyclic alkene and strained cycloalkynes afforded the adducts formed by the ene reaction and the 2+2 cycloaddition reaction. The reaction of with a bicyclopalladacycle yielded the cyclization product. The structures of compounds and were determined by single-crystal X-ray crystallography.     

Cu(I)-catalyzed enantioselective [3 + 2] cycloaddition reaction of 1-alkylallenylsilane with alpha-imino ester: asymmetric synthesis of dehydroproline derivatives

[reaction: see text] The catalytic, enantioselective [3 + 2] cycloaddition reaction of 1-alkyl-substituted allenylsilanes with alpha-imino ester has been achieved by means of [Cu(MeCN)(4)]BF(4)/(R)-DM-SEGPHOS catalyst to afford silyl-substituted dehydroproline derivatives in high yields and enantioselectivities.     

Cobalt-catalyzed intramolecular [2 + 2 + 2] cocyclotrimerization of nitrilediynes: an efficient route to tetra- and pentacyclic pyridine derivatives

[reaction: see text] In this paper, we wish to report the intramolecular [2 + 2 + 2] cocyclotrimerization of nitrilediynes catalyzed by the CoI2(dppe)/Zn system at 80 degrees C in CH3CN. Under these reaction conditions, various highly substituted nitrilediynes having steric conjunction at the alpha and beta positions to a nitrile group and a bulkier substitution at the terminal carbon of alkyne undergo [2 + 2 + 2] cocylotrimerization to afford tetra- and pentacyclic pyridine derivatives in good to excellent yields.