Diels-Alder reactions of quinones

The high pressure Diels-Alder reactions of quinone-mono-ketals [80]... 

Diels-Alder reactions of quinones generated in situ by electrochemical oxidation in lithium perchlorate-nitromethane [105]... 

Chiral boron catalysts had already been widely used in a variety of reactions before they were applied in Diels-Alder reactions220. Boron catalysts were first employed in the Diels-Alder reactions of quinones with electron-rich dienes. Kelly and coworkers221 found that stoichiometric amounts of a catalyst prepared from BH3, acetic acid and 3,3 -diphenyl-l,l/-bi-2-naphthol (344) catalyzed the reaction of 1-acetoxy-l,3-butadiene (341) with juglone (342) to afford cycloadduct 343 with 98% ee (equation 96). The reaction was supposed to proceed via a spirocyclic borate complex in which one face of the double bond of juglone was effectively shielded from attack by the diene. 

A variety of other heterogeneously catalyzed Diels-Alder reactions has been reported. Nafion-H, a perfluorinated resinsulfonic acid, catalyzed several Diels-Alder reactions and the isolated yield of the adducts was 80-95% [46]. We have found that a recently described Nafion-silica composite catalyst containing 13 % (w/w) Nafion [47] was approximately 30 times more active than the pure resin in the Diels-Alder reaction of 2,3-dimethylbutadiene with 1,4-naphthoquinone [48]. We also showed that another strong heterogeneous Brpnsted acid, tung-stophosphoric acid supported on silica gel, is a very active catalyst of Diels-Alder reactions of quinones [49] and other enones [50]. 

The ortho-quinone methides are difficult to isolate due to their high reactivity, which leads to rapid Diels-Alder dimerization or trimerization (Fig. 7.26). At 150°C, a partial retro-Diels-Alder reaction of the trimer can occur to form ortho-quinone methide and bis(2-hydroxy-3,5-dimethylphenyl) ethane (dimer).51... 

Enantiomers (M)- and (P)-helicenebisquinones [32] 93 have been synthesized by high pressure Diels-Alder reaction of homochiral (+)-(2-p-tolylsulfo-nyl)-l,4-benzoquinone (94) in excess with dienes 95 and 96 prepared from the common precursor 97 (Scheme 5.9). The approach is based on the tandem [4 + 2] cycloaddition/pyrolitic sulfoxide elimination as a general one-pot strategy to enantiomerically enriched polycyclic dihydroquinones. Whereas the formation of (M)-helicene is explained by the endo approach of the arylethene toward the less encumbered face of the quinone, the formation of its enantiomeric (P)-form can be the result of an unfavourable interaction between the OMe group of approaching arylethene and the sulfinyl oxygen of 94. 

Azulene quinones [49b] are compounds related to the family of tropones and are considered to possess great biological and physiological potential. Several polycyclic compounds have been prepared by high pressure (3kbar, PhCl, 130°C, 15h) Diels-Alder reaction of 3-bromo-l,5-azulene quinone (137) and 3-bromo-l,7-azulene quinone (138) with several dienophiles. The cycloadditions were regioselective and afforded cycloadducts in reasonable to good yields (Scheme 5.20). 

Mataka and coworkers reported the studies of the Diels-Alder reactions of [3.3] orthoanthracenophanes 96 and 97, of which anthraceno unit, the potential diene, has two nonequivalent faces, inside and outside. The reactions of 96 with dien-ophiles gave the mixtures of inside and outside adducts with the ratios between 1 1 and 1 1.5. However, the ratio changes drastically, in favor of the inside adducts, when 97 reacts with dienophiles such as maleic anhydride, maleimide and naphto-quinone [55] (Scheme 46). Mataka suggested that the Jt-facial selectivity is controlled by an orbital interaction between the electron-poor dienophiles and the Jt-orbital of the facing aromatics, which would lead to a stabilization of the transition state, while Nishio suggested that the selectivity is due to the attractive k/k or CH/jt interaction [53]. 

Wang, H. Wang, Y. Han, K.-L. Peng, X.-J. A DFT study of Diels-Alder reactions of o-quinone methides and various substituted ethenes selectivity and reaction mechanism. J. Org. Chem. 2005, 70, 4910-4917. 

Inubushi s synthesis of racemic serratinine commences with a Diels-Alder reaction of butadiene with substituted quinone derivative 26 (available in four steps from 25,... 

Brimble and coworkers172 reported the asymmetric Diels-Alder reactions between quinones 265 bearing a menthol chiral auxiliary and cyclopentadiene (equation 73). When zinc dichloride or zinc dibromide was employed as the Lewis acid catalyst, the reaction proceeded with complete endo selectivity, but with only moderate diastereofacial selectivity affording 3 1 and 2 1 mixtures of 266 and 267 (dominant diastereomer unknown), respectively. The use of stronger Lewis acids, such as titanium tetrachloride, led to the formation of fragmentation products. Due to the inseparability of the two diastereomeric adducts, it proved impossible to determine which one had been formed in excess. 

The asymmetric Diels-Alder cycloadditions of enantiopure (5)-5-(/ -tolylsulfinyl)-1,4-benzoquinones with Dane s diene under thermal and Lewis acid conditions produce tetracyclic quinones after spontaneous elimination of the sulfinyl group.The Diels-Alder reaction of barrelene with o-benzoquinone produces tetracyclo[6.2.2.2 .0 ]tetradeca-9,ll,13-triene-4,5-dione. Under kinetic control, the Diels-Alder cycloaddition of 2,3-dicyano-p-benzoquinone (98) with cyclopentadiene in MeOH produces the single cycloadduct (99) (Scheme 38). ... 

Lebold and Kerr reported the total synthesis of the eustifolines-A (172), -B (173), -C (93),-D (227), and glycomaurrol (92) starting from the readily available quinine imine 1574 and diene 1575 (899). This methodology uses the Diels-Alder reaction of a quinone monoimine 1574 and subsequent Plieninger indolization of the adduct 1576 for the synthesis of the key tetrahydrocarbazole framework. 

Diels-Alder reactions with p-quinones (6. 65 66). The orientation of Diels-Alder reactions of 6-meihoxy-l-vinyl-3,4-dihydronaphthalene (1) with p-quinones is subject to reversal by addition of BF, etherate (1.3 equivalent). Thus the thermal reaction with 2,6-dimethyl-/>-bcnzoquinone (2) results in exclusive formation of 3, whereas the catalyzed reaction leads predominately to the isomer 4. The adduct 3 is stable to base, but the syn, m-isomer 4 on treatment with NaX O, is converted to the more stable anti, frau.s-isomer 5. 

Ab initio and density functional theoretical studies of the 4 + 2-cycloaddition of 2-azabutadiene with formaldehyde predict a concerted reaction that agrees well with experimental evidence.184 The azadiene A-plienyl-l-aza-2-cyanobuta-l,3-diene reacts with electron-rich, electron-poor, and neutral dipolarophiles under mild thermal conditions.185 5,6-Diliydro-4//-1,2-oxazines have been shown to be usefiil as synthon equivalents of 2-cyano-l-azabuta-1,3-dienes.186 The intramolecular Diels-Alder reaction of 1-aza-l,3-butadienes (106) can be activated by a 2-cyano substituent (Scheme 37).187 Stereoselectivity in the hetero-Diels-Alder reactions of heterobutadienes, nitrosoalkenes, and heterodienophiles has been extensively reviewed.188 The azadiene l-(f-butyldimethylsilyloxy)-l-azabuta-1,3 -diene (107) reacts with halobenzo-quinones, naphthoquinones, and A-phcnylmalcimidc to yield low to good yields of various pyridine heterocycles (108) (Scheme 38).189 The 4 + 2-cycloaddition of homophthalic anhydride with A-(cinnamylidcnc)tritylaminc produces the 3,4-adduct whereas with A -(cinnamylidcnc)bcnzylidinc the 1,2-adduct is produced.190... 

DFT theory at the B3LYP/6-31G(d,p) level was used to investigate the Diels-Alder reactions of o-quinone methides with various ethenes. Calculations show that solvent decreases the activation energy and increase the asynchronicity.116 The Diels-Alder reaction of ortho-quinone methides derived from 3//-1,2-benzoxathiole 2,3-dioxides with maleimides produces chroman 2,3-dicarboxylic acid derivatives.117... 

Molecular electrostatic potentials have been used to explain the regioselectivity exhibited in the Diels-Alder cycloaddition reactions between 1-trimethylsilyloxy-butadiene and the quinones 5-formyl-8-methyl-1,4-naphthoquinone, 5-methoxy-7-methyl-1,4-phenanthrenequinone, and 5,6,7-trimethyl-1,4-phenanthrenequinone.128 The intramolecular Diels-Alder reaction of masked o-benzoquinones (123) with a variety of dienes provides adducts (124) which rearrange to functionalized ris-decal ins (125) with complete stereocontrol of up to five stereocentres. This methodology ... 

The ready evolution of the adducts into aromatic quinones by spontaneous sulfinyl elimination and further aromatization prompted the use of sulfinyl naphthoquinones as a synthetic equivalent of the unknown compound naph-thynoquinone [103]. For this purpose, sulfinyl quinones represent a convenient synthetic alternative to haloquinones. The highly regioselective course of the Diels-Alder reactions of 2-phenylsulfinyl-1,4-naphthoquinones (as well as their corresponding thioethers and sulfones) unsymmetrically substituted by... 

Diels-Alder reactions. The laboratories of Breslow and of Grieco have reported that water can enhance the rate of Diels-Alder reactions of dienes that possess carboxylic acid or similar hydrophilic groups (12, 314). Liotta et al. have examined solvent effects on cycloaddition reactions of benzoquinones with dienes substituted with a relatively hydrophobic group, and report significant rate enhancement in ethylene glycol relative to benzene (26 1) or even to reactions in the absence of a solvent. They attribute the solvent effect to aggregation of the diene and the quinone. 

A Lewis-acid-catalysed Diels-Alder reaction of the diene (70) and the quinone (71) is the key step in the synthesis of compound (72) with the appropriate stereochemistry and functionality for conversion into quassin.53 The androstane derivative (73) has been transformed into (74) in model experiments towards the synthesis of quassin.54... 

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