Cycloaddition reactions of phthalimide substituted cyclic polyenes with heteroatom dienophiles

The cycloaddition of 6‐phthalimidobicyclo[5.1.0]octa‐2,4‐diene and 7‐phthalimido‐1,3,5‐cycloheptatriene with nitrosobenzene and with 4‐phenyl‐1,2,4‐triazoline‐3,5‐dione each gave a single heterocyclic product. X‐ray crystallographic analysis of 8a indicated the regio‐ and stereoselectivity of this cycloaddition.     

Some new dienophiles in the diels-alder reaction with active cyclic dienes

A comparative study of the Diels-Alder reaction have been carried out for three types of cyclic dienes: cyclopentadiene (CPD), hexachlorocyclopentadiene (HCP), and tetrachloro-1,2-benzoquinone (TCBQ) with dienophiles of five types: monosubstituted acetylenes (propargyl alcohol and its esters), conjugated amines (allyl acetylene, allyl ethynyldimethylcarbonol and its esters), conjugated dienes (2-methylhexadiene-3,5-ol-2 and its esters, 1-ethoxy- and 1-ethoxycarbonylbutadiene-1,3), and conjugated trienes (hexatriene-1,3,5 and 5-methylhexatriene-1,3,5).It was found that conjugated trienes react with dienes of all types under mild conditions and give only the monoadducts through the terminal vinyl or ethynyl group.     

Diels-Alder reaction of furan with some dienophiles

The reaction of furan and maleic acid was carried out in several solvents. The endo-adduct was isolated and the structure established by its spectral properties and conversion into known compounds. The adducts of furan with fumaric acid, diethyl fumarate and diethyl maleate are reported.     

A new access to quinazolines from simple anilines

A new synthetic pathway to quinazolines is described. This new method uses hexamethylenetetramine in TFA and potassium ferricyanide in aqueous ethanolic KOH, starting from simple N-protected anilines. The method affords substituted quinazolines with high selectivities and good yields, reducing reaction-time and work-up operations.     

Convenient synthetic approach to 2,4-disubstituted quinazolines

[reaction: see text] 2,4-Dialkyl or aryl quinazolines have been prepared in three steps starting from easily available anilides. A photochemically induced Fries rearrangement of the anilides gave several ortho-aminoacylbenzene derivatives that were acylated at the NH2. These acylamides underwent rapid cyclization to 2,4-disubstituted quinazolines (and benzoquinazolines) in the presence of ammonium formate under microwave activation. This procedure is compatible with different functional groups and allowed also the preparation of new quinazolines derived from naturally occurring amino acids.     

Electron transfer initiated Diels-Alder reaction with allenes as dienophiles

The first examples of using aryl allenes as dienophiles in the electron transfer induced Diels-Alder reaction are described. According to mechanistic tests the reaction proceeds via a cycloaddition of the diene^+· to the allene.     

[4+2] Cycloaddition reactions of 4-sulfur-substituted 2-pyridones with electron-deficient dienophiles

[4+2] Cycloaddition reactions of 4-(phenylthio)-1-tosyl-2-pyridone (6a) and 4-(phenylsulfonyl)-1-tosyl-2-pyridone (6b) with electron-deficient dienophiles 7 (N-methylmaleimide, N-phenylmaleimide, and methyl acrylate) gave new isoquinuclidine products 8-10. The N-tosyl group of 6a and 6b was also efficiently converted to N-alkyl derivatives 6c-f, which showed different stereoselectivity toward reactions with dienophiles 7. Several other dienophiles 15 (dimethyl acetylenedicarboxylate, methyl vinyl ketone, ethyl vinyl ether, and methyl methacrylate) were found not to react with 6a or 6b, but led to the formation of tosyl migration products 4-(phenylthio)-O-tosyl-pyridinol (16a) and 4-(phenylsulfonyl)-O-tosyl-2-pyridinol (16b), respectively. The reactivity, regioselectivity, and stereoselectivity of the cycloaddition reactions were also compared with semi-empirical calculations.     

A new approach to anodic substitution reaction using acoustic emulsification

We have developed a novel electrolytic system for anodic substitution reactions using acoustic emulsification. This new system involves the generation of a carbocation by anodic oxidation of a substrate, and then its reaction with a nucleophile droplet formed by ultrasonication. In this system, even if the oxidation potential of the nucleophile is lower than that of the substrate, the substrate was predominantly oxidized to give the corresponding cation intermediate because the nucleophile phase, which was insoluble in the electrolytic medium, was electro-inactive. In addition, the overoxidation of the desired products was considerably suppressed by the extraction of products from the electrolyte solution into the nucleophile phase. As a result, the anodic substitution reaction of several carbamates with allyltrimethylsilane was carried out to provide the corresponding products in good to moderate yields.     

A novel Ireland-Claisen rearrangement/Diels-Alder tandem reaction of propargylic acrylates with acyclic dienophiles

A novel DABCO-catalyzed Ireland-Claisen rearrangement/Diels-Alder tandem reaction of propargylic acrylates 3 with dienophiles 4 was developed in the presence of an excess TMSCl and DBU with 1% hydroquinone as polymerization inhibitor in acetonitrile at refluxing temperature under air.This protocol gave cyclicα,β-unsaturated carboxylic acids 5 with complete regioselectivity.     

A new facile approach to N-alkylpyrroles from direct redox reaction of 4-hydroxy-l-proline with aldehydes

An unprecedented acetic acid-catalyzed efficient access to N-alkylpyrroles from reaction of 4-hydroxy-L-proline with a variety of aldehydes has been achieved in good to excellent yields under mild reaction conditions.     

Cycloaddition reactions of carbohydrate derivatives. Part V. A hetero Diels-Alder approach to swainsonine analogs

Diastereoselective hetero Diels-Alder reactions of sugar derived azomethines 4a–4g have been used for the construction of chiral piperidones 6 and 7. Configuration of the major products was 6,1′-threo in every case. The enone system of 6 and 7 was reduced diastereoselectively and subsequently swainsonine-analog hydroxyindolizidines 28–32 were prepared by an intramolecular reductive amination.     

Stereochemical studies on the intramolecular diels alder reaction of furans with doubly activated dienophiles.

The stereochemical outcome of intramolecular Diels-Alder reactions of furans with Z- and E-2-ene-1,4-dione units attached by a bridging chain to C-2 of the furan results from modification of the steric demands of the bridging chain by the preference of the external activating group to adopt an endo- stereochemistry.