An efficient synthesis of chiral isoquinuclidines by Diels-Alder reaction using Lewis acid catalyst

The Diels-Alder reaction of 1,2-dihydropyridine derivatives (1-phenoxycarbonyl-1,2-dihydropyridine 1 or 1-methoxycarbonyl-1,2-dihydropyridine 4) with N-acryloyl (1S)-2,10-camphorsultam (1S)-2 {or N-acryloyl (1R)-2,10-camphorsultam (1R)-2} in the presence of Lewis acid, such as titanium tetrachloride, zirconium tetrachloride, and hafnium tetrachloride afforded the endo-cycloaddition product, 2-azabicyclo[2.2.2]octane derivatives in good yields with excellent diastereoselectivity. The absolute stereochemistry assignment of the endo-cycloaddition product (1S)-5a starting from N-acryloyl (1S)-2,10-camphorsultam (1S)-2 has been established to be (1S,4R,7S) and the reaction mechanism was proposed.     

Synthesis and ring opening reactions of 2-glyco-1,4-dimethyl-3-nitro-7-oxabicyclo[2.2.1]hept-5-enes

The high-pressure asymmetric Diels-Alder reactions of d-galacto- (1a) and d-manno-3,4,5,6,7-penta-O-acetyl-1,2-dideoxy-1-nitrohept-1-enitol (1b) with 2,5-dimethylfuran (2) afforded mixtures of cycloadducts, from which the (2S,3R)-3-exo-nitro (3a and 3b), (2R,3S)-3-exo-nitro (4a and 4b), and (2R,3S)-1′,2′,3′,4′,5′-penta-O-acetyl-1′-C-(1,4-dimethyl-3-endo-nitro-7-oxabicyclo[2.2.1]hept-5-en-2-exo-yl)-d-galacto-pentitol (5b) were isolated pure. Deacetylation of these compounds led to new chiral mono-, bi-, and tricyclic ethers, being their asymmetric centers arising from the chiral inductor used in the cycloaddition reaction. A ring opening mechanism through a 1-nitro-1,3-cyclohexadiene intermediate has been proposed.     

Fluoropropenoates as dienophiles in the Diels-Alder reaction

Several fluorinated alkenes were prepared from known ethyl (R)-2-fluoro-4,5-dihydroxyisopropylidine-2-pentenoate (1). The fluoroalkenoates were tested as dienophiles with several dienes and showed cycloaddition only with the very reactive diene, 1,3-diphenylisobenzofuran (5). Fluorobutenolide 2 reacted with 5 to produce the endo-syn adduct 9 in a highly stereoselective manner.     

Experimental and theoretical studies of Diels-Alder dimerization of 1,2,3,4,5-pentachlorocyclopentadiene and of Diels-Alder cycloaddition of polychlorinated cyclopentadienes to norbornadiene

Diels-Alder cyclodimerization of 1,2,3,4,5-pentachlorocyclopentadiene (1) affords 2a as the exclusive reaction product. Diels-Alder cycloaddition of 1 to norbornadiene also proceeds stereoselectively to produce a single [4+2] cycloadduct, 4c. The structures of 2a and 4c were established unequivocally via application of single crystal X-ray crystallographic techniques. The origins of the observed diastereofacial selectivity in each of these cycloaddition processes have been investigated by application of semiempirical (AM1 Hamiltonian) and ab initio (Hartree-Fock 3-21G*) calculations. The computational results thereby obtained, which are based upon consideration of the kinetically favored transition state for each of the two cycloaddition reactions studied, are consistent with experiment. These semiempirical and ab initio methods also have been used to investigate the mechanisms of the Diels-Alder reactions that have been used to prepare aldrin and isodrin (7 and 8, respectively). The results thereby obtained suggest that isodrin formation via Diels-Alder cycloaddition of cyclopentadiene to 1,2,3,4,7,7-hexachloronorbornadiene proceeds with kinetic control of product stereochemistry.     

Synthesis of (±)-kuwanon V and (±)-dorsterone methyl ethers via Diels-Alder reaction

The mulberry Diels-Alder adducts, the pentamethyl ethers of kuwanon V 1a and dorsterone 2a were synthesised via a biomimetic intermolecular [4+2] cycloaddition reaction between a highly electron-rich dienophile and a Lewis acid sensitive diene derived from chalcone. Cycloaddition under thermal conditions afforded 1a and 2a (in 3:2 ratio). Cycloaddition catalysed by AgOTf/Bu₄NBH₄ gave higher yields of the adducts.     

Intramolecular Diels-Alder reaction leading to tricyclic derivatives as intermediates of natural products synthesis

Tricyclic compounds 4a and 4b possessing a bicyclo[4.3.0] moiety, were successfully synthesized by using the intramolecular Diels-Alder reaction. The siloxy- rather than acyloxy-substituents increased the ratio of the endo-cylcoadducts 10 and 12. The oxygen substitution of 9 influenced conformation of the transition state, which was stereochemically restricted by the butenolide moiety. In addition, 9b carrying a hydroxyl group also produced 10b in a similar ratio to 9a. Compound 11 was the only exo-adduct produced in all of the entries.     

Diels-Alder reactions of masked o-benzoquinones with acrylonitrile

Regioselective Diels-Alder reactions of masked o-benzoquinones (MOBs) 2a-i derived from the corresponding 2-methoxyphenols 1a-i with acrylonitrile leading to highly functionalized bicyclo[2.2.2]octenone derivatives in high yields are described.     

Conformational chirality and chiral crystallization of N-sulfonylpyrimidine derivatives

New N-sulfonylpyrimidine derivatives 1-(p-toluenesulfonyl)uracil (1), 1-(p-toluenesulfonyl)thymine (2), 5-bromo-1-(p-toluenesulfonyl)uracil (3), 1-(methanesulfonyl)uracil (4), 1-(1-naphthylsulfonyl)uracil (5), and 1-(1-naphthylsulfonyl)thymine (6) were prepared by the condensation reaction of silylated pyrimidine derivatives with selected sulfonyl chlorides in acetonitrile. Some members of the series showed unexpected crystal properties as a consequence of their conformational chirality in the solid state. Compounds 1 and 5 exhibited chiral crystallization, which was, in the case of 1, accompanied by the formation of racemically twinned crystals regardless of the solvent used, while 5 gave a conglomerate of enantiomorphous crystals. For 2, 3, and 6, substituents at the C-5 position of the pyrimidine ring prevented chiral crystallization by influencing the crystal packing. Analysis of the crystal structures of 1, 4, and 5, reveals the influence of the arylsulfonyl group on the occurrence or absence of chiral crystallization.     

Studies concerning the double reduction of Diels-Alder derived bicylic sulfonamides

A series of bicyclic sulfonamides, 14-17, was prepared by thermal, intramolecular Diels-Alder cycloaddition. The ratio of exo:endo-diastereoisomers formed following this process was found to be dependent on the substitution pattern of the dienophile. Styryl substituted sulfonamides preferentially formed the corresponding exo-adducts, whereas vinyl substituted sulfonamides preferentially gave the endo-adducts. These adducts were treated under dissolved metal reduction conditions and it was found that compounds possessing an N-benzyl substituent underwent preferential debenzylation and resisted further reduction. The complementary N-isobutyl functionalised materials only underwent the double reduction when a phenyl substituent was present.     

Sequential homo-coupling Diels-Alder/retro Diels-Alder reaction of 5,5′-bi-1,2,4-triazine-containing thiamacrocycles as a new route to thiacrown ethers incorporating a 2,2′-bipyridine subunit

A new route to thiacrown ethers 5a-d and 6a-d incorporating a 2,2′-bipyridine subunit is elaborated using, (1) homo-coupling of 1,2,4-triazine sulfides 3a-d tethered to poly(ethylene glycol) chains with potassium cyanide and (2) Diels-Alder/retro Diels-Alder reaction with norbornadiene or 1-pyrrolidino-1-cyclopentene as the key steps.     

Cycloaddition of methyl 2-(2,6-dichorophenyl)-2H-azirine-3-carboxylate to electron-rich 2-azadienes

tert-Butyldimethylsililoxy-2-aza-1,3-butadienes react with 2H-azirine 3 leading to Diels-Alder cycloadducts in moderate yields. The reactions are endo- and regioselective with the azirine being added by its less hindered face. There is only one product in the case of 1b, 4b. There are two isomers (4 and 5) from 1a, 1c and 1d. A different result was obtained with the diene 1e. Diene 1e formed products 4e and 8. Some of compounds 4 and 5 have been hydrolysed leading to functionalised aziridines 7. Compound 8 gave aziridine 9.     

Stereoselective intramolecular hetero Diels-Alder reactions of 1-oxa-1,3-butadienes: a novel approach for the synthesis of complex annulated uracils

The intramolecular hetero Diels-Alder reactions of 1-oxa-1,3-butadienes 4, obtained from salicylaldehyde 1 via O-allylation followed by Knoevenagel condensation with barbituric acids 3 in the presence of hydrochloric acid as catalyst, affords the tetracyclic uracil derivatives 5 and 6 in a stereoselective manner and high overall yields.     

A mechanistic study on the intramolecular ionic Diels-Alder reaction of 2-methyl-3,9,11-tridecatriene-2-ol and 2,11-dimethyl-1,3,9,11-dodecatetraene

Intramolecular ionic Diels-Alder reaction of 2-methyl-3,9,11-tridecatriene-2-ol (1) was studied under acidic conditions. Treatment of 2-methyl-3,9,11-tridecatriene-2-ol (1) with trifluoromethanesulfonic acid yielded 7-methyl-8-isopropenyl-1,2,3,4,4aR^∗,7R^∗,8R^∗,8aS^∗-octahydronaphthalene (4) and (1Z)-1-((E)-but-2-enylidene)-2-(2-methylpropenyl)cyclohexane (5) through regioselective intramolecular ionic Diels-Alder reaction. The reaction appeared to proceed partly through a stepwise mechanism involving a carbocation intermediate. However, a concerted pathway rather than a stepwise one is suggested to be involved in the acid-catalyzed intramolecular Diels-Alder reaction of 2,11-dimethyl-1,3,9,11-dodecatetraene (13).     

Highly efficient synthesis of extended triptycenes via Diels-Alder cycloaddition in water under microwave radiation

Microwave-irradiated Diels-Alder reactions of anthracene and endoxides (6a-g) in water afforded the cycloadducts (8a-g) with high efficiencies. The extended triptycenes (2a-g) were readily obtained by dehydration of 8a-g in a mixture of AcOH and Ac₂O with good overall yields.     

Intramolecular Diels-Alder reactions of oxazole-olefins: synthesis of the Rauwolfia alkaloids suaveoline and norsuaveoline

A full account of the highly stereoselective total synthesis of two indole alkaloids, suaveoline (4) and norsuaveoline (5), is presented. Central features of the synthetic strategy include the conversion of l-tryptophan methyl ester (12) into the oxazole derivative 11 and the intramolecular Diels-Alder reaction of the oxazole-olefin 19 leading to the pentacyclic pyridine derivative 21.     

Synthesis and highly selective Diels-Alder cycloadditions of the new dienes N-substituted 2,3,5,6-tetrahydrobenzoxazol-2-ones

The synthesis of the new dienes N-substituted 2,3,4,5-tetrahydrobenzoxazol-2-ones 8a-8c is described, through a one-step convergent process from 1,2-cyclohexanedione (7a) and the corresponding isocyanates 2a-2c. The presence of electron-donor substituents in the aryl ring of the isocyanate gave rise to the exclusive formation of the captodative olefins 10. Diene 8a proved to be reactive and stereoselective in Diels-Alder additions with a cyclic olefin. The reaction with acetylenic dienophiles yielded the 2,3-dihydrobenzoxazol-2-ones 21 and 24, as the products of sequential [4+2] cycloaddition and retro-Diels-Alder reactions. Methyl vinyl ketone (22) underwent regio- and stereoselective tandem Diels-Alder and Michael additions to give propellane 29a. Evidence of an endo π-pyramidalization of the central double bond of adduct 19 would rationalize the exo stereoselection in the formation of 29a. The regioselectivity in these reactions has been rationalized in terms of FMO theory by ab initio calculations.     

[60]Fullerene adducts with 9-substituted anthracenes: mechanochemical preparation and retro Diels-Alder reaction

Three 9-substituted anthracene derivatives, that is, 9-hydroxymethylanthracene (2), 9-methoxymethylanthracene (3) and bis(9-anthrylmethyl) adipate (6), were chosen as the model compounds to evaluate the reactivity in their Diels-Alder reactions with [60]fullerene and in retro Diels-Alder reactions of the formed cycloadducts. Corresponding adducts 4, 5 and 7 were prepared in high yields under solvent-free conditions using high-speed vibration milling technique. In order to determine thermal stabilities of adducts 4, 5 and 7, their dissociations in the temperature range of 40-65 °C were investigated. Fitting the dissociation rates and temperatures to the Arrhenius equation gives the activation energies of 25.8, 21.8 and 24.9 kcal/mol for compounds 4, 5 and 7, respectively.     

Synthesis of 2,2′-bipyridyl derivatives using aza Diels-Alder methodology

Amidrazone 1 and the tricarbonyl derivatives 2a-c gave the triazines 3a-c, respectively, which reacted with 2,5-norbornadiene 4 in boiling ethanol yielding the corresponding novel 2,2′-bipyridines 5a-c in good yield. Triazine 6 gave the 2,2′-bipyridyl derivative 7 (65%) with compound 4 in 1,2-dichlorobenzene at 140°C.     

Total synthesis of suaveoline and norsuaveoline via intramolecular oxazole-olefin Diels-Alder reaction

Total synthesis of two Rauwolfia alkaloids, suaveoline (1) and norsuaveoline (2), has been achieved through a highly stereoselective route starting from l-tryptophan methyl ester (9) and exploiting the intramolecular Diels-Alder reaction of the oxazole-olefin 16.     

Cinchona-derived ammonium salts-catalyzed aza Diels-Alder reaction of Danishefsky’s diene with imines

Described is the efficiency of cinchona-derived quaternary ammonium salts as Lewis acid organocatalysts in aza Diels-Alder reaction of Danishefsky’s diene 1 with imines 2 and 16, providing 1,2-dialkyl-2,3-dihydro-4-pyridinones 3 and cyclic dihydropyridones 17, respectively. Among the nine of cinchonidine-derived quaternary ammonium catalysts prepared, N-2′,3′,4′-trifluorobenzyl-O-benzylcinchonidinum bromide (6) exhibited the highest chemical yield (up to 99%). Systematic study on structure-catalytic efficiency relationship revealed that 2′,3′,4′-trifluorobenzyl, quinuclidine, and quinoline moieties are essential.