Initial naphthoquinone

Turning from the intramolecular process to the intermolecular ones, we now extend our comparison of the thermal and cation-radical cyclizations. It is also interesting to take sonication into account as a route to initiate cyclizations. The reaction between 2-butenal A,A-dimethylhydrazone (a diene) and 5-hydroxy-l,4-naphthoquinone (a dienophile) gives such an opportunity. In toluene, at 20°C, the reaction follows as depicted in Scheme 7.28 (Nebois et al. 1996). 

A -Sulfonyl-2,2 -biindoles undergo a formal [4-I-2] cycloaddition with various dienophiles to produce indolocarba-zoles <2004TL4009>. Thus, reaction of 171 with A -benzylmaleimide in chlorobenzene in a sealed tube produces the cycloadduct in moderate yield (Equation 105) as the only observable product (Table 11). It is proposed that the reaction proceeds by an initial Michael addition, followed by a rapid cyclization and loss of the phenylsulfonyl group during the aromatization process. Similar cyclization reactions are observed with A -methylmaleimide, dimethylace-tylene dicarboxylate, and 1,4-naphthoquinone. 

Addition of water to quinones. The yields of the known Michael addition of water or an alcohol to a 1,4-naphthoquinone such as 5,8-dimethoxy-l,4-naphtho-qninonc arc improved by addition of an oxidant such as Fe2(S04)3 to convert the initial 2-hydroxynuphlhohydroquinone to the corresponding quinone. The presence til ll 1 rcc pert-hydroxy group interferes with the reaction. 

Naphthalene oxidation is very similar to benzene oxidation except for the much greater importance of naphthoquinone, compared with benzo-quinone, as a reaction intermediate. Roughly equal amounts of phthalic anhydride and naphthoquinone are initially formed from naphthalene. A suitable simplified reaction scheme is... 

Diels-Alder reactions of a,fi-unsaturatedN,N-dimethylhydrazones.1 These readily available hydrazones can function as 1-amino-l-aza-l,3-dienes in Diels-Alder reactions. Thus, 1 undergoes regioselective cycloaddition with various electrophilic dienophiles to give tetrahydropyridines such as 2 and 3. Unfortunately, removal of the dimethylamino group with zinc and acetic acid (or other reagents) also effects reduction of the double bond. The initial adduct from cycloaddition of 1 with naphthoquinone is unstable and undergoes spontaneous elimination of the elements of dimethylamine to give the aromatic adduct 4. 

Asymmetric epoxidation catalyzed by chiral phase-transfer catalysts is another reaction which has been extensively studied following an initial report by Wynberg [2,44]. Shioiri et al. further improved the enantioselective epoxidation of naphthoquinones under cinchona alkaloid-derived chiral phase-transfer catalysis [45],... 

Kinetics of the addition of PI13P to p-naphthoquinone in 1,2-dichloromethane, using the initial rate method, revealed the order of reaction with respect to the reactants the rate constant was obtained from pseudo-first-order kinetic studies. A variable time method using UV-visible spectrophotometry (at 400 nm) was employed to monitor this addition, for which the following Arrhenius equation was obtained log k = 9.14- (13.63/2.303RT). The resulting activation parameters a, AH, AG, and Aat 300 K were 13.63, 14.42 and 18.75 kcalmol-1 and —14.54 calmol 1K 1,... 

As mentioned above, the natural photosynthetic reaction center uses chlorophyll derivatives rather than porphyrins in the initial electron transfer events. Synthetic triads have also been prepared from chlorophylls [62]. For example, triad 11 features both a naphthoquinone-type acceptor and a carotenoid donor linked to a pyropheophorbide (Phe) which was prepared from chlorophyll-a. The fluorescence of the pyropheophorbide moiety was strongly quenched in dichloromethane, and this suggested rapid electron transfer to the attached quinone to yield C-Phe+-Q r. Transient absorption studies at 207 K detected the carotenoid radical cation (kmax = 990 nm) and thus confirmed formation of a C+-Phe-QT charge separated state analogous to those formed in the porphyrin-based triads. This state had a lifetime of 120 ns, and was formed with a quantum yield of about 0.04. The lifetime was 50 ns at ambient temperatures, and this precluded accurate determination of the quantum yield at this temperature with the apparatus employed. 

Benzo[3]thiete is a source of o-thioquinone methides, heterodienes which react with 1,4-naphthoquinones or 1,4-epoxynaphthalenes to give the benzo[ ]thioxanthene system. With the former reagents, the initial products undergo an autooxidation and 6,ll-dihydro-12//-benzo[7]thioxanthen-6,ll-diones result. The 6,11-epoxy adducts which are formed from the epoxynaphthalenes can be dehydrated to 12//-benzo[7]thioxanthenes or converted into the dihydrobenzothioxanthene-6-ol (Scheme 131) <1995JPR379>. 

A photoinduced substitution reaction initiated by electron transfer has also been proposed to account for the formation of 2-chloro-3-(2-thienyl)-1,4-naphthoquinones (217) from 2,3-dichloro-l, 4-naphthoquinone (215) and thiophenes (216)623 (equation 160). 

Another substitution reaction that is initiated by photochemical hydrogen abstraction is the replacement of the bromine atom in 2-bromo-8-methoxy-l, 4-naphthoquinone by an acyl group757. Irradiation of a solution in benzene of the quinone, butyraldehyde or capraldehyde and pyridine yields mixtures of acylated quinone and acylated hydro-quinone. In the first step, the excited quinone abstracts the aldehyde hydrogen atom and this is followed by bond formation between the acyl radical and C-2 of the quinone. The radical that is formed after departure of a bromine atom may either lose a hydrogen atom and yield acylated quinone or take up a hydrogen atom and become acylated hydro-quinone. 

Addition of thermally-generated ethoxycarbonylnitrene to 1,4-naphthoquinone in the presence of 02 gives a small amount of a 1,3,4-dioxazolidine derivative, thought to arise from an initially-formed 1,2,3-dioxazolidine by a process analogous to the molozonide-ozonide rearrangement (77CJC2363). 

Naphtho[2, 3 3,4]-[l,2,5]triazocino[8,l+]quinazolinone 137 was obtained by refluxing of 2-substituted 3-amino-quizalin-4-one with 2,3-dichloro-l,4-naphthoquinone in DMF (Scheme 35 <2001MOL267>). Initial formation of 5,6-dihydrotriazocine derivative was followed by elimination of arylsulfinic acid and formation of ring system 137. 

Murphy and Neville found that dihydrofuran undergoes Me2AlCl-promoted [2 + 2] cycloaddition with benzo- and naphthoquinone [55]. It was proposed that this cycloaddition occurs by initial Lewis acid coordination to the quinone, then Michael addition to dihydrofuran, followed by rapid ring closure of the intermediate dipolar ion, as shown in Sch. 31. 

Dissolve 1 g of the aminonaphthoquinonimine hydrochloride (2) reserved from Experiment 2 in 25 mL of water, add 2 ml of concentrated ammonia solution (den 0.90), and boil the mixture for 5 min. The free quinonimine initially precipitated is hydrolyzed to a mixture of the aminoquinone 7 and the isomer 6. Cool, collect the precipitate and suspend it in about 50 mL of water, and add 25 mL of 10% sodium hydroxide solution. Stir well, remove the small amount of residual 2-amino-1,4-naphthoquinone (6) by filtration, and acidify the filtrate with acetic acid. The orange precipitate of 7 is collected, washed, and crystallized while still wet from 500-600 mL of hot water (the separation is slow). The yield of orange needles, which decomposes about 270°C, is about 0.4 g. 

A novel O-deprotection protocol involving anodic cleavage of aromatic ether (LXXXV) was effectively used in the total synthesis of the natural products alkannin and shikonin [Eq. (42)]. The electrolysis was conducted at a carbon anode in MeCN/ H2O with LiC104 as electrolyte, giving an 80% yield at 50% conversion [97]. Although both naphthoquinone tautomers were initially formed after trapping of the radical cation by water and loss of CH2O, the tautomer with alkyl substitution at the quinone double bond was more thermodynamically stable. 

After some initial structural uncertainties, vitamin Kj, 2-methyl-3-farnesylgeranylgeranyl-1,4-naphthoquinone, was obtained following a route similar to that for the phytyl analogue (ref.88). 

Oxidative coupling of furanes with quinones. Furanes do not react with mtphlhoquinone under usual conditions. However, Bridson and co-workers1 have effected this addition in the presence of a high-potential quinone to oxidize the Initial adduct to a 2-fury 1-1,4-naphthoquinone and displace the equilibrium. The reliction with 2-methoxyfurane is particularly interesting because the adduct can be Converted to a naphthacenequinone (equation I). 

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