From central to helical chirality: synthesis of P and M enantiomers of [5]helicenequinones and bisquinones from (SS)-2-(p-tolylsulfinyl)- 1,4-benzoquinone

The reaction of 1,4-divinyl-1,3-cyclohexadiene, 5,8-dimethoxy- or tert-butyldimethylsilyloxy-3-vinyl-1,2-dihydrophenanthrene or 6-vinyl-7,8-dihydro-1,4-phenanthrenequinone with an excess of enantiopure (SS)-2-(p-tolylsulfinyl)-1,4-benzoquinone (2) led to the direct formation of enantioenriched dihydro[5]helicenequinones or bisquinones (50-->98 % ee). A domino Diels-Alder cycloaddition/sulfoxide elimination/partial aromatization process occurs, being the absolute configuration of the final helicene defined in the aromatization step. Both M and P helimers are accessible through a stepwise enantiodivergent process if the pentacyclic dihydroaromatic intermediate resulting in the two first steps is aromatized in the presence of (+/-)-2, DDQ, CAN or DBU.     

Influence of the Sulfinyl Group on the Chemoselectivity and pi-Facial Selectivity of Diels-Alder Reactions of (S)-2-(p-Tolylsulfinyl)-1,4-benzoquinone

Diels-Alder reactions of (S)-2-(p-tolylsulfinyl)-1,4-benzoquinone (1a) with cyclic (cyclopentadiene and cyclohexadiene) and acyclic dienes (1-[(trimethylsilyl)oxy]-1,3-butadiene and trans-piperylene) under different thermal and Lewis acid conditions are reported. Chemoselectivity (reactions on C(2)-C(3) versus C(5)-C(6) double bonds) is mainly related to the cyclic (on C(5)-C(6)) or acyclic (on C(2)-C(3)) structure of the diene. The high pi-facial selectivity observed could be controlled by choosing adequate experimental conditions.     

Influence of diene substitution on transition state stabilization in Diels-Alder reaction

A theoretical analysis of transition state stabilization in D-A reactions of substituted dienes according to the nature and position of the substituent has been carried on. Results revealed that substituents (de)stabilize TS through four effects (steric, mesomeric, inductive, and polarizability) acting principally by favoring the electronic transfer between the two partners. The correlations observed point out nevertheless that the reactivity of substituted dienes in [4 + 2] cycloadditions on ethylene may principally be predicted by the sole use of the F + R electronic parameters.     

Enantioselective Diels-Alder approach to C-3-oxygenated angucyclinones from (SS)-2-(p-tolysulfinyl)-1,4-naphthoquinone

Chiral racemic vinylcyclohexenes 2, bearing oxygenated substituents and/or a methyl group at the C-5 position of the cyclohexene ring, were submitted to Diels-Alder reactions with enantiomerically pure (SS)-(2-p-tolylsulfinyl)-1,4-naphthoquinone [(+)-1]. The domino cycloaddition/pyrolytic sulfoxide elimination process led to the formation of enantiomerically enriched angularly tetracyclic quinones anti-6 and syn-7, which were obtained from the kinetic resolution of the racemic diene. In all cases, (SS)-(2-p-tolylsulfinyl)-1,4-naphthoquinone reacted from the less hindered face of the more reactive s-cis conformation, to form products in good enantiomeric excesses. Steric effects and torsional interactions in the corresponding approaches account for the observed pi-facial diastereoselectivities at both partners. The usefulness of this methodology is illustrated with the four-step totally asymmetric synthesis of the C-3-oxygenated angucyclinone derivative (-)-8-deoxytetrangomycin 10 in 26% overall yield and with 50% enantiomeric purity.     

Substrate dependence in aqueous Diels-Alder reactions of cyclohexadiene derivatives with 1,4-benzoquinone

A reactivity difference based on the position of substituents on cyclohexa-1,3- diene was observed for the title reaction. The effect of water as solvent was more distinct for 1-methyl-4-isopropylcyclohexa-1,3-diene than for 2-methyl-5-isopropylcyclohexa- 1,3-diene or non-substituted cyclohexa-1,3-diene. The effect of NaCl (salting-out) and guanidium chloride (salting-in) was also large for 1-methyl-4-isopropylcyclohexa-1,3- diene.     

Synthesis and dienophilic behavior of (S)-2-cyano-3-(p-tolylsulfinyl)-1,4- benzoquinone

Hydrocyanation of 2-(p-tolylsulfinyl)-1,4-benzoquinone (2) followed by oxidation with PhI(OAc)(2) gives 2-cyano-3-(p-tolylsulfinyl)-1,4-benzoquinone (1). The generation of 1 in the presence of cyclic and acyclic dienes affords the Diels-Alder adducts with a complete chemo- (only reaction with the sulfinyl-substituted double bond takes place), regio- (controlled by the cyano group), and endo selectivity (with respect to the quinone moiety), whereas the pi-facial selectivity is dependent on the structure of the diene.     

Studies of Diastereoselectivity in Diels-Alder Reactions of Enantiopure (SS)-2-(p-Tolylsulfinyl)-1,4-naphthoquinone and Chiral Racemic Acyclic Dienes

Enantiopure sulfinylnaphthoquinone (+)-5 reacted with racemic acyclic dienes 1a-f bearing a stereogenic allylic center, through a tandem cycloaddition/pyrolytic sulfoxide elimination, to afford optically enriched compounds 8a-f and 9a-f with good like/unlike selectivities (ca. 75:25) and good enantiomeric excesses (68-82%), arising from the partial kinetic resolution of the racemic dienes. The opposite diastereoselection (8g-i: 9g-i, up to 5:95) was observed in reactions with dienes 1g-i, having an additional methyl group at C-3, the enantiomeric purities being moderate (14-25%). Steric effects and torsional interactions in the corresponding approaches account for the observed diastereoselectivities.     

An Improved and Efficient Preparation of the Chiral NAD(P)H Model (SS)-1-Benzyl-3-(p-tolylsulfinyl)-1,4-dihydropyridine

An improved procedure for the preparation of chiral NAD(P)H model, (SS)-1-benzyl-3-(p-tolylsulfinyl)-1,4-dihydropyridine, with satisfactary chemical yield and excellent enantiopurity is reported.     

Synthesis of dihydronaphthalenes via aryne Diels-Alder reactions: scope and diastereoselectivity

Novel aryne Diels-Alder reactions with functionalized acyclic dienes are reported. These give useful cis-substituted dihydronaphthalene building blocks in good yield which are not easily accessible via other means, as demonstrated in the synthesis of sertraline. The first asymmetric aryne cycloaddition with an acyclic diene is also reported, giving excellent diastereoselectivities with Oppolzer's sultam as a chiral auxiliary.     

Studies of Diastereoselectivity in Diels-Alder Reactions of (S)S-4a,5,8,8a-Tetrahydro-5,8-methane-2-(p-tolylsulfinyl)-1,4-naphtho- quinones with Cyclopentadiene

The title compounds 6 and 7 have proved to be adequate rigid models to evaluate the ability of the sulfinyl group to control the diastereoselectivity of the [4 + 2] cycloadditions of cyclopentadiene on the ene-dione moiety. The results of thermal and Lewis acid-catalyzed reactions allowed us to establish that both reactivity and endo/exo selectivity were modulated by the presence of the sulfinyl group, the endo-anti-endo or the exo-anti-endo bisadducts being obtained as major products depending on experimental conditions. The role of the association between the SOTol group and several Lewis acids (BF(3).OEt(2), Eu(fod)(3), ZnBr(2)), which shifted the conformational equilibrium around the C-S bond, was used to explain the stereochemical course of the cycloadditions mainly controlled by steric factors. The synthesis of the exo-anti-endo bisadduct 5 was described for the first time.     

Hydrohalogenation of N-acetyl(aroyl)-1,4-benzoquinone monoimines

New N-acetyl-1,4-benzoquinone monoimines alkyl-substituted in the quinoid ring were synthesized. The hydrohalogenation of N-acetyl(aroyl)-1,4-benzoquinone monoimines proceeds exclusively in keeping with the 1,4-addition. The hydrochlorination occurs along the ionic mechanism, in the hydrobromination grows the role of the radical mechanism.     

Investigation of the Diels-Alder cycloadditions of 2(H)-1,4-oxazin-2-ones

[Reaction: see text]. A variety of 5-chloro-2(H)-1,4-oxazin-2-ones bearing a range of substituents at their 3- and 6-positions undergo Diels-Alder cycloaddition as a 2-azadiene component with electron-rich, electron-deficient, and electron-neutral dienophiles. These reactions proceed with moderate regio- and stereoselectivity to afford relatively stable and readily isolable bridged bicyclic lactone cycloadducts. Chemical manipulation of these cycloadducts affords highly substituted and functionally rich piperidines. The regio- and stereochemical preferences of the cycloadditions of 5-chloro-2(H)-1,4-oxazin-2-ones are investigated computationally using density functional theory (B3LYP/6-31G).     

Synthesis,structure, and properties of 2-(triphenylstannyloxy)-3,6-di-tert butyl-1,4-benzoquinone

2-(Triphenylstannyloxy)-3,6-di-tert-butyl-1,4-benzoquinone (1) was obtained from the potassium salt of 3,6-di-tert-butyl-2-hydroxy-1,4-benzoquinone and triphenyltin bromide in methanol and characterized by x-ray diffraction analysis, electronic, IR, and NMR spectroscopy. According to x-ray diffraction data, the single crystal of1 has the structure of a chain metal polymer with bridging hydroxy-para-quinone ligands; the coordination number of tin is six. Complex1 is reduced by cobaltocene to the corresponding radical anion. In solutions,1 reacts with metal-centered radicals , and with formation of paramagnetic binuclearortho-semiquinone complexes.Institute of Organometallic Chemistry, Russian Academy of Sciences, 603600 Nizhnii Novgorod. A. N. Nesmeyanov Institute of Heteroorganic Compounds, Russian Academy of Sciences, 117813 Moscow. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 12, pp. 2798–2804, December, 1992.     

Synthesis and polymerization of 7,7-bis(alkoxycarbonyl)-1,4-benzoquinone methides

7,7-Bis(methoxycarbonyl)-, 7,7-bis(ethoxycarbonyl)-, and 7,7-bis(isopropoxycarbonyl)-1,4-benzoquinone methides ( 4a, 4b , and 4c ) were successfully prepared as pure, isolable yellow-orange needles. The values of the first reduction potential for 4a, 4b , and 4c were measured in dichloromethane containing tetrabutylammonium perchlorate by cyclic voltammetry to be −0.54, −0.55, and −0.55 V, respectively, indicating that the alkyl groups do not significantly affect their electron-accepting properties. An anionic initiator butyllithium induced the homopolymerizations of 4a–c at 0°C, but a cationic initiator boron trifluoride etherate did not of 4a–c at 0°C. Compounds 4a and 4b homopolymerized with a radical initiator 2,2′-azobis(isobutyronitrile) (AIBN), but 4c did not, probably due to a larger steric hindrance effect of the isopropyl group compared with methyl and ethyl groups. Homopolymerizable compound 4a copolymerized with styrene in benzene in the presence of AIBN in a random fashion to give the monomer reactivity ratios r₁ ( 4a ) = 2.40 ± 0.40 and r₂ (styrene) = 0.01 ± 0.02 at 60°C and the Q and e values of 4a were 21.2 and +1.13, respectively, indicating that 4a is a highly conjugative and electron-accepting monomer, while the nonhomopolymerizable compound 4c copolymerized with styrene in a perfectly alternating fashion in benzene in the presence of AIBN at 60°C. No copolymerizations of 4a or 4c with 7,7,8,8-tetracyanoquinodimethane took place in dichloromethane in the presence of AIBN at 60°C. © 1996 John Wiley & Sons, Inc.     

Synthesis and Diels-Alder reactions of 9-(4-benzyloxazolin-2-yl) anthracene

The synthesis of 9-(4-benzyloxazolin-2-yl)anthracene is described employing a new approach for the cyclisation of β-hydroxy amides to oxazolines. Thermal Diels-Alder reactions with N-methyl maleimide were found to be considerably slower than those previously observed. Essentially no diastereoselectivity was observed in these reactions as the benzyl stereodirecting group is remote from the reactive site. Minor rate enhancements were noticeable in the presence of some added Lewis acids, but with no diastereoselection.