Diels-Alder reactions 1.4- dihydronaphthalenes

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. 

Vinylbenzo[6]thiophene 90 reacted with benzoquinone to give a fully aromatized 1 1 cycloadduct 151 as result of a Diels-Alder reaction followed by dehydrogenation by excess quinone. Isolation of a dihydro derivative 152 was possible using a weaker solution of 3-vinylbenzo[b] thiophene, whereas a further excess of the vinyl compound and a very short reaction time afforded the terahydro derivative 153 (79AJC145). On the other hand, primary Diels-Alder adducts are isolated from 1-(3-benzo[6]thiophene)cyclohex-1 -ene and 1 -(3-benzo[fc]thiophene)-3,4-dihydronaphthalene on reaction with MA (50JA571). 

Naphthalene oxides. The Diels-Alder reaction of 1,4-diacetoxybutadiene (1) with benzyne (generated from o-benzenediazonium carboxylate, 1, 46) gives the diacetate of l,4-dihydronaphthalene-cis-l,4-diol (2) in 53% yield. The product is converted into 3 by reaction with hydrogen peroxide and catalytic amounts of osmium tetroxide. The monotosylate of 3 is converted into 4 by treatment with base. This epoxy diol has the stereochemistry found in the carcinogenic epoxy diols formed as metabolites of aromatic hydrocarbons. The dimesylate of 3 is converted into the syft-diepoxide 5. 

Three successive [2+4] cycloadditions were used in the synthesis of the pentacyclic methyl ether of G-2N by Kraus and Zhao [92] and later, by a slightly modified procedure, also of the natural product G-2N (118) [93] (Scheme 31). Thermal reaction of the cyclobutanol 112 with acrylic ester gave the dihydronaphthalene 113 which was demethylated by treatment with boron tribromide and converted into the exocyclic ketene acetal 114. This unstable diene was reacted in a second cycloaddition with 2,6-dichlorobenzoquinone (115) to afford the tetracyclic chloroquinone 116. In a last Diels-Alder reaction, ring E was anella-ted by treatment of 116 with l-methoxy-l,3-bis[(trimethylsilyl)oxy]-l,3-buta-diene (117) to yield the pentacyclic natural product G-2N (118) [93]. 

Diels-Alder Reactions. Reagent 1 is useful as an efficient chiral dienophile in asymmetric Diels-Alder reactions. Reaction of 1 with cyclopentadiene in the presence of a Lewis acid occurs with high stereoselectivity. Reaction with 6-methoxy-l-vinyl-3,4-dihydronaphthalene in the presence of EtAlCh proceeds with... 

A novel C2-chiral bis(amidinium) salt can be synthesized from 5-(tert-butyl)isophthalic acid. The salt [tetrakis(3,5-bistrifluoromethylphenyl)borate (TFPB)] contributes to not only rate acceleration but also asymmetric induction in the Diels-Alder reaction of 1 -vinyl-3,4-dihydronaphthalene and cylopentendione, owing to hydrogen bond mediated association of chiral auxihary with dienophile [91] (Scheme 3.NaN). 

Diels-Alder reaction with cyclopentadiene gave the corresponding adduct (Eq. 31) [28]. (Dihydronaphthalene)chromium complex gave addition product by reaction with diazomethane with high diastereoselectivity [29]. The acrylate having chiral arene chromium complex underwent Diels-Alder reaction with high selectivity in the presence of Lewis acid (Eq. 32) [30]. 

Diels-Alder Reactions. Bp3-OEt2 is used to catalyze and reverse the regiospecificity of some Diels-Alder reactions, e.g. with pen-hydroxylated naphthoquinones, sulfur-containing conpounds, the reaction of 1-substituted trans-1,3-dienes with 2,6-dimethylbenzoquinones, and the reaction of 6-inethoxy-l-vinyl-3,4-dihydronaphthalene with p-quinones. BFs-OEta has a drastic effect on the regioselectivity of the Diels-Alder reaction of quinoline- and isoquinoline-5,8-dione with piperylene, which produces substituted azaanthraquinones. This Lewis acid is the most effective catalyst for the Diels-Alder reaction of furan with methyl acrylate, giving high endo selectivity in the 7-oxabicyclo[2.2.1]heptene product (eq 35). ... 

The benzyne created from 2-(trimethylsilyl)phenyl triflate was used in a highly diastereoselective aryne Diels-Alder reaction with a diene bearing Oppolzer s sultam. This approach to cis-functionalized 1,4-dihydronaphthalenes was reportedly the first aryne Diels-Alder reaction to provide enantioenriched cyclo-adducts. An unusual route to /3-aminoketones involves the treatment of 2-(trimethylsilyl)phenyl triflate with TBAF and an asymmetric vinyldihydropyridone. The resultant aryne Diels-Alder cycloadduct undergoes aromatization/eUmination to create the )V-acyl-/3-aminoketone (eq 13). This method was featured in a multistep synthesis of an unnatural a-amino acid. 

Fused Systems.—Naphthalene and Derivatives. A new method of aromatization of partially hydrogenated aromatic hydrocarbons overcomes the difficulty of the competing Diels-Alder reaction of reactive arenes. The method is based on deprotonation-hydride elimination in which potassium fencholate (277) serves as the base and fenchone (278) as the hydride acceptor, catalytic amounts only of fencholate being required since it is regenerated in the aromatisation step. In this way, the conversion of 1,2-dihydronaphthalene into naphthalene and of 9,10-dihydroanthracene into anthracene proceeds almost quantitatively. 

Dumitrascu and co-workers (52) transformed 4-halosydnones into 5-halopyr-azoles by cycloaddition with DMAD and methyl propiolate followed by retro-Diels-Alder loss of CO2. Turnbull and co-workers (194) reported that the cycloadditions of 3-phenylsydnone with DMAD and diethyl acetylenedicarboxylate to form pyrazoles can be achieved in supercritical carbon dioxide. Nan ya et al. (195) studied this sydnone in its reaction with 2-methylbenzoquinone to afford the expected isomeric indazole-4,7-diones. Interestingly, Sasaki et al. (196) found that 3-phenylsydnone effects the conversion of l,4-dihydronaphthalene-l,4-imines to isoindoles, presumably by consecutive loss of carbon dioxide and A-phenylpyrazole from the primary cycloadduct. Ranganathan et al. (197-199) studied dipolar cycloadditions with the sydnone 298 derived from A-nitrosoproline (Scheme 10.43). Both acetylenic and olefinic dipolarophiles react with 298. In... 

Use of the chiral pool typically requires a series of subsequent transformations to achieve the substitution pattern desired and sometimes may be limited by the availability of only one enantiomer. Microbial oxidations of benzene derivatives have provided an excellent route to cyclohexadienediols in enantiomerically pure form. Although this provides only one enantiomer, synthetic methods have been devised to circumvent this problem [36]. Far fewer methods exist for the enantioselective synthesis of cycloheptenes for which there exists no reaction analagous to the Diels-Alder process [37,38,39,40,41,42]. The enantioselective hydroalumination route to dihydronapthalenols may prove to be particularly important. Only one other method has been reported for the enantioselective synthesis of these compounds microbial oxidation of dihydronaphthalene by P. putida generates the dihydronaphthalenol in >95% ee and 60% yield... 

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