Quinone-Diels-Alder adducts

Nitrodienes undergo intermolecular Diels-Alder reactions with appropriate dienophiles. The resulting nitro compounds can then be cyclized via a nitrile oxide intermediate.49 Thus, the 2-chloroacrylonitrile Diels-Alder adduct of 8-nitro-l,3-octadiene was prepared and cyclized to give (105) as a 3 1 mixture of diastereomers (Scheme 30). The Diels-Alder adduct of dimethyl acetylenedicarboxylate and 8-nitro-l,3-octadiene cyclized exclusively at the conjugated double bond, activated by the ester groups. Similarly, the quinone Diels-Alder adduct (106) cyclized at the conjugated double bond reduction of the conjugated double bond permitted cyclization on the cycloalkenyl double bond. 

Suggest a reasonable pathway for the formation of each of the photoproducts formed on irradiation of the Diels-Alder adduct of 2,3-dimethylbutadiene and quinone ... 

The reduction of ,/S-unsaturated y-diketones can conveniently be done with zinc in acetic acid. The following procedure is applicable to the reduction of the Diels-Alder adduct of quinone and butadiene (Chapter 8, Section II). 

Engler and colleagues256 demonstrated that the way in which catalyst 406 is prepared has a strong effect on the regioselectivity and enantioselectivity of quinone Diels-Alder reactions. The most effective catalyst was prepared from a 1 1 1 mixture of titanium tetrachloride, titanium tetraisopropoxide and chiral diol 416. The cycloadditions of 2-methoxy-l,4-benzoquinones such as 414 with simple dienes to give adducts like 415 proceeded with high yields and enantioselectivities of up to 80% ee using this catalytic system (equation 123). 

A number of side products can arise with this quinone. They include Diels-Alder adducts (DDQ is a powerful dienophile) and Michael adducts derived from the hydroquinone. 

We characterized and further studied this basic mechanism of covalent affinity labeling using spectroelectrochemical techniques. The kinetics and stability of quinone oxidation products at high dilution and low pH were consistent with the proposed mechanism, as was the concentration dependence of rapid labeling reactions of the more reactive catechol with the receptor.1215 Spectroelectrochemical and direct cyclic voltametric determination of the half-potentials of the hydroxylated quinones were further consistent with their intermediacy in the labeling reactions of TMC.15 The quinone oxidation products of 4- and 5-HTMC were characterized in part as cyclopentadiene Diels-Alder adducts.15 The instantaneous reactions of these hydroxy TMCs with receptor were consistent with their intermediacy in the TMC reactions. From the concentration dependence of the half-of-sites labeling reactions we could deduce Kd for each isomer fC,(4-HTMC) = 224 98 pM, K/5-HTMC) = 39 17 juM. 

The Diels-Alder adduct (Formula 190) from cyclopentadiene and benzo-quinone gives a photoisomer formulated as Structure 191 on the basis of the absence of double bonds and the infrared carbonyl absorption of the photoprbduct (74). Similar products are obtained from the hexachlorocyclopentadiene-benzoquirione adduct and the cyclo-pentadiene-chloranil adduct (74). Irradiation of the cyclooctatetraene-benzoquinone adduct (Formula 192) gives a photoproduct formulated as Structure 193 (74). 

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). 

Eq. (29)], and phenanthrene quinone [Eq. (28b)], leads to the [4 + 2] Diels-Alder adducts the two benzoquinone adducts add a second molecule of the quinone 1 resp. 2, forming the trioxyphosphoranes. The mechanism of this [4 + 2] cycloaddition, either symmetry allowed or stepwise, is not yet clear (73). 

Cyclitols. A novel synthesis of conduritol (6) from benzoquinone utilizes 1 for protection of one C=C bond and for differentiation of the carbonyl groups. Reaction of the quinone with 1 gives the Diels-Alder adduct 2, which is converted selectively into 4,... 

It is possible to form cyclobutene adducts from a quinone which has a lowest (n,n ) excited state by a sequence involving protection of the quinone as a Diels-Alder adduct with anthracene (equation 95) . The enedione which undergoes photocycloaddition with alkyne reacts at the C=C bond rather than at the C=0 bond (see also equation 48). 

The cage compound (301) is formed efficiently on irradiation of the adduct (302).Irradiation of the ene dione (303) affords the cage compound (304) by a (6 + 2)-cycloaddition. The Diels-Alder adducts (305) are photochemically reactive. Irradiation of (305) affords either the cage compounds (306) by a (2+2)-cycloaddition reaction, a process favoured by the adducts (305 a,b), or the oxetanes (307). The structures (306, 307) were verified by 2L ray diffraction studies The quinone adduct (308) also undergoes photochemical cyclization to afford the cage diketone (309). This material was subjected to flash vacuum pyrolysis to yield the diketone (310) important in the synthesis of perhydroindacenes. ... 

All the precursors are easily prepared by epoxidation and alkylation of the Diels-Alder adduct of p-benzoquinone and dimethylfulvene, and subsequent pyrolyses of the precursors in a sealed tube under rather mild conditions to yield these epoxy compounds [20-22]. Although there are other methods to prepare such quinone epoxides and a-epoxycyclohexenones, the advantages of the retro Diels-Alder method are as follows 1. epoxidation of quinone adducts proceeds regioselectively to give an epoxide, in which the more substituted double bond in the starting quinone is epoxidized, 2. the adducts are usually stable, and are able to afford appreciably modified quinone moieties, 3. stereoselectivities are expected in the modification of the ewiAj-adducts. 

Dehydrogenation.1 Treatment of the Diels-Alder adducts of 1,3-dienes and citracononitrile and mesacononitrile (1) with potassium f-butoxide and anthraquinone (3 eq.) in benzene at room temperature gives the substituted benzonitrile (2). This is apparently the first recorded instance of quinone dehydrogenation of carbanions. 

The usefulness of the approach for the preparation of highly reactive cycloproparenes is further illustrated by its application to benzocyclopropene-2,5-quinone In this case, flash-vacuum pyrolysis of the precursor (500 °C) afforded only polymeric material pyrolysis in a static system at 255 C in the presence of 3 equivalents of anthracene for 10 minutes gave the quinone as the Diels —Alder adduct with anthracene (12%), while unreacted starting material (41 %) and dimethyl phthalate (16%) were also recovered. 

The use of cyclobuteno-heterocycles is of course dependent on a convenient synthesis (for an example, see 14.13.2.5), but when available, they are excellent precursors, only rather moderate heating being required for ring opening, as shown by the example below, in which the initial Diels-Alder adduct is aro-matised by reaction with excess quinone. ... 

The study of the Diels-Alder adducts of levopimaric acid has continued with an examination of the structure and stereochemistry of the adducts with cyclopentenone and cyclopent-l-ene-3,5-dione. The major product with cyclo-pentenone is the endo,cis adduct (83). The enedione gives a mixture of enolic endo,cis adducts whose stereochemistry was determined by photocyclisation to give compounds such as (84). A correlation was also achieved with the benzo-quinone adduct through a Favorskii-type ring-contraction of the epoxide (85) to (86). The 13(14) double bond of these adducts is hindered to oxidation, excepting the adduct with acetylenedicarboxylic acid. Condensation of 12-hydroxymethylabiet-7(8)-en-18-oic acid with formaldehyde gives the cyclic ether (87). 

Oxidations. 4-Hydoxy-2-cyclobutenones are oxidized to y-acetoxybutenolides with PhI(OAc)2. 2-Methoxyphenols give o-quinones that can be trapped as Diels-Alder adducts. 

In the methylalizarin series a Diels-Alder adduct from naphtho-1,4-quinone and... 

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