Diels-alder cycloadditions of N-substituted-1,2-dihydropyridines with 1,2,4-triazoline-3,5-diones and maleimides

The reaction of N-substituted-1,2-dihydropyridines 1 with 1,2,4-triazoline-3.5-diones 2 and maleimides 9 proceeds stereospecifically to afford endo cycloaddition products. N-Acetyl-1,2-dihydropyridines react with 2 to afford a stereo isomeric mixture of 3 and 4 whereas those possessing a N-ethoxycarbonyl, methoxycarbonyl, methanesulfonyl or benzenesulfonyl substituent yield 3 exclusively: similar results are also obtained in reactions employing maleimides. Stereochemistry was assigned on the basis of nmr data and use was made of the anisotropic effects of the 7,8 unsaturation on the R₁ and R₂ substituents.     

Diels-alder reactions with ethyl 1-benzofuran-3-carboxylates

Substituted salicylaldehydes reacted with ethyl diazoacetate to give ethyl 1-benzofuran-3-carboxylates containing various substituents in the aromatic ring. The Diels-Alder reaction of these compounds with Danishefsky’s diene was regioselective, and it provided an effective method for the construction of the heterocyclic skeleton of hexahydrodibenzo[b,d]furan-7-one or tetrahydrodibenzo[b,d]furan-7-one. The adducts were found to undergo rearrangement to substituted 4′-hydroxybiphenyl-2-yl methyl carbonates during column chromatography on silica gel.     

Diels-alder cycloaddition reactions of 1,3-diazabutadienes with ketenes

Diels-Alder cycloaddition reactions of 1-aryl-4-dimethylamino-2-phenyl-1,3-diazabutadienes with monophenyl and diphenylketenes, resulting in high yields of pyrimidin-6-one derivatives are reported.     

In situ TEM studies of metal-carbon reactions.

The reactions which occur between amorphous carbon and a number of first transition metals (Ti, Cr, Fe, Co, Ni, and Cu) have been studied by transmission electron microscopy (TEM). The materials are in thin-film form with the metal layer sandwiched between thicker carbon layers. In four cases, the predominant reaction is the graphitization of the amorphous carbon, at temperatures well below 800 degrees C. This is brought about by the elements themselves in the case of Co and Ni, and by metastable carbides in the case of Fe (Fe3C) and Cr (Cr3C2-x). The Ti-C and Cu-C systems do not exhibit graphitization. For the former, only TiC is produced up to 1000 degrees C, while the carbon does not react at all with copper. In situ TEM studies show the mechanism to be of the dissolution-precipitation type, which is equivalent to the metal-mediated crystallization process for amorphous silicon and germanium. The heat of graphitization is found to be 18-19 kcal/mol-C by differential scanning calorimetry.     

1-AZA-1,3-dienes. Diels-alder reactions with α, β-unsaturated hydrazones.

The α,β unsaturated hydrazone $\text{3}$ reacts regioselectivity with a wide range of dienophiles to give the corresponding [4+2] adducts. Reductive cleavage of the NN bond in the adducts gives tetrahydropyridines.     

Ru and Sm catalyzed polyaddition of dialdehydes to give polyesters. Application of Tishchenko type reactions to polymer synthesis

RuH₂(PPh₃)₄ catalyzed Tishchenko type polyaddition of terephthal-aldehyde gives aromatic polyester ( 1 ), which contains three structural units, [OCH₂ C₆H₄ CH₂O] ( 1a ), [OCH₂ C₆H₄ CO] ( 1b ), and [CO C₆H₄ CO] ( 1c ). ¹H-NMR spectrum shows the presence of the three units in a 1 : 2 : 1 ratio. Isophthalaldehyde also undergoes similar polyaddition to give another aromatic polyester ( 2 ), while 1,12-dodecanedial gives an aliphatic polyester ( 3 ) containing the following structural units: [OCH₂ (CH₂)₁₀ CH₂O] ( 3a ), [OCH₂ (CH₂)₁₀ CO] ( 3b ), and [CO (CH₂)₁₀ CO] ( 3c ). The above polymers have M_n of 2.7 × 10³−5.4 × 10³ and M_w of 4.3 × 10³ − 9.7 × 10³, respectively. Mixtures of terephthalaldehyde and 1,12-dodecanedial produce copolymers, which contain the units 1a–1c and 3a–3c in a random sequence. In the copolymerization, terephthalaldehyde shows a strong tendency to give 1c units, whereas 1,12-dodecanedial predominantly affords 3a units. SmI₂ also catalyzes polyaddition of terephthalaldehyde to give the corresponding polyester with M_n of 1.7 × 10³ and M_w of 3.7 × 10³, respectively. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 1265–1273, 1997     

Diels-alder reactions with inverse electron demand. II. The reaction of benzamidine with π-deficient heteroaromatic compounds.

Benzamidine (I) reacts with several s-tetrazines to give as-triazines in moderate to good yield, together with some s-triazines; separate experiments have shown that these are due to the fact that (1) also undergoes Diels-Alder- reactions with as- and even with s-triazines.     

In situ version of radical-chain substitution reactions

Peroxy acetals derived from an ether and a hydroperoxide can be prepared in situ and used without isolation to effect sulfochlorination of n-alkanes or chlorination of toluene to benzyl chloride.     

In situ synthesis of polysulfides covalently bonded to silica

Silanol groups, triple bond SiOH, on the surface of silica were esterified with unsaturated alcohols and long-chain alcohols bearing thiol groups. The modified silicas obtained were used as substrates for a vulcanization-analogous reaction with sulfur catalyzed by zinc dimethyldithiocarbamate. Surface-esterified thiols could be smoothly converted to bridged polysulfides bonded to the silica surface, whereas the use of surface-esterified unsaturated alcohols led to removal of the surface-esterified alcohol from the silica surface. The materials were characterized by solid-state NMR and thermal and microanalytical analysis. The linking of surface-esterified alkenols and thiols by sulfide bridges was investigated by a numerical model for a flat surface. This showed that for a typical density of 3-4 micromol/m(2) surface groups, a statistical maximum of 70-75% of groups could be linked by S(n) bridges (n=2-4).     

Diels-alder reactions of 1,3-dienes with 4-nitrobenzenediazonium salt as a dienophile

Further examples of the title reaction, discovered by Carlson, Sheppard, and Webster, reveal scope and regiochemistry of the formation of 1,6-dihydropyridazines; all the evidence points to a concerted primary addition.     

Studies directed toward the total synthesis of the rubradirin antibiotics. 3. Diels-alder reactions of unsymmetrically substituted quinones-a confirmation of structure.

The regiochemical course of the Diels-Alder reaction of 2-acetamido-3-iodo-p-benzo-quinone with 1-methoxy-2-methyl-3-trimethylsilyloxy-1,3-butadiene has been unambiguously established through a correlation study.     

Diels-alder reactions of 2-acetyl-2-cyclohexenone with enol ethers and emamines

2-Acetyl-2-cyclohexenone ($\text{2}$) undergoes inverse electron demand Diels-Alder reactions with enol ethers at 25°C. The adduct in which the oxygen adds endo is favored. Trans substituted enol ethers are ≈ 35 times as reactive as the cis isomer. Enamines react immediately with $\text{2}$ at 25°C.     

Diels-alder reactions of 2-pyrones with acyclic dienophiles and hydrolyses of the mono-adducts

Diels-Alder reactions of 2-pyrone (1) with fumaronitrile and maleonitrile afforded regio- and stereo-specific mono-adducts 3a and 3b , respectively. The reaction of 1 with acrylonitrile gave a bis-adduct. The reaction of 4,6-dimethyl-2-pyrone (7) with dimethyl fumarate and dimethyl maleate at higher temperature gave benzene derivatives. The low reactivity of 7 and the instability of the mono-adducts with 7 were considered from the existence of back reaction and easy elimination of methyl formate, respectively. Hydrolyses of the mono-adducts of 2-pyrones gave polyfunctionalized cyclohexenes and elimination products generated from the preferencial trans-elimination of the leaving groups.     

Mechanism of the Fischer indole synthesis. Quantum-chemical interpretation of the rearrangement of substituted cyclohexanone arylhydrazones to tetrahydrocarbazoles

Calculations of a number of model structures within the scheme of the Fishcer indole synthesis were made on the basis of a bonding variant of perturbation theory in the self-consistent-field (SCF) MO LCAO method. A quantum-chemical interpretation of the effect of substituents on the course of the thermal process is given. The kinetics of the thermal and acid-catalyzed indolization of substituted cyclohexanone arylhydrazones to tetrahydrocarbazoles were studied by spectrophotometry. It was shown that the experimental data are in satisfactory agreement with the calculated values. It was concluded that a concerted mechanism (a [3,3]-sigmatropic shift) for the step involving the formation of a carbon-carbon bond in the Fischer reaction is preferred.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 898–907, July, 1986.     

Diels-alder reactions of pyrroles as an entry to substituted 3-oxatropanes and tetrasubstituted pyrrolidines

The Diels-Alder adduct of N-(p-toluenesulfonamido)pyrrole and dimethyl acetylenedi-carboxylate, 2,3-dimethyl N-(p-toluenesulfonamido)-7-azabieyclo[2.2.1]-2,5-heptadiene-2,3-di-carboxylate ( 1c ), was obtained in good yield and converted to remarkably stable 3-oxatropanes ( 6 and 8 ) in 25% yield.     

Brønsted acid catalyzed Diels-Alder reactions of 2-vinylindoles and 3-nitrocoumarins: an expedient synthesis of coumarin-fused tetrahydrocarbazoles

A Brønsted acid catalyzed Diels-Alder reaction of 2-vinylindoles and 3-nitrocoumarins has been described. The methodology allows a rapid and expedient synthesis of a variety of coumarin-fused polycyclic indoles in good yields (up to 82%) with high diastereoselectivities (up to >19:1).     

Quinine-derived thiourea promoted enantioselective Michael addition reactions of 3-substituted phthalides to maleimides

A highly diastereoselective and enantioselective Michael addition/desymmetrization reaction of maleimides with prochiral 3-substituted phthalides catalyzed by quinine-derived bifunctional thiourea was realized. A broad range of the 3,3′-disubstituted phthalides bearing vicinal quaternary-tertiary stereogenic centers were synthesized in moderate to good yields(up to 96%) with high diastereoselectivities(up to 19:1 dr) and enantioselectivities(up to 96:4 er).     

Chiral primary amine thiourea promoted highly enantioselective Michael reactions of isobutylaldehyde with maleimides

Chiral primary amine thiourea catalysts were first successfully applied to promote Michael addition of isobutyraldehyde to maleimides. A variety of N-aryl and N-aliphatic maleimides provided Michael adducts in excellent yields (up to 98%) and enantioselectivities (up to 99% ee) with 5 mol % catalyst.