2-Benzopyran-3-ones, Diels-Alder reaction

Both inter- and intra- molecular Diels-Alder reactions of 2-benzopyran-3-ones occur with high endo-selectivity and have been used to synthesise (-)-podophyllotoxin (14) and 4a-substituted cis-BC fused hexahydrophenenthrenes (15), respectively <96JCS(P1)151, 96JCS(P1)705>. 

The use of isopropylidine acetals (112) as tethers in the intramolecular Diels-Alder reactions of dienes with alkenes facilitates the formation of civ-fused cycloadducts (113) from an endo transition state (Scheme 41).218 The intramolecular Diels-Alder reaction of 4-[tris-(2-mcthylcthyl)silyl]oxy-2//-thiopyran derivatives with potential dienophiles tethered at C(2), C(3), C(5), and C(6) positions yielded cycloadducts when the dienophiles were activated with a carbomethoxy group.219 By the substitution of a phenylsulfonyl group on the dienophile of 2-benzopyran-3-ones, it is possible to enhance exo addition during intramolecular Diels-Alder cyclizations to yield a predominance of trans-fused hexaphenanthrenes related to natural products.220 The intramolecular Diels-Alder reaction of 2-furfuryl fumarates has been investigated by molecular mechanics (SIBFA)/continuum reaction field computations.221 The intramolecular 4 + 2-photo-cycloaddition of A-benzylcinnamamides (114) in the presence of C(,H6 gives 3-azatricyclo[5.2.2.01,5]undeca-8,10-dien-4-ones (115) with high stereoselectivity (Scheme 42).222... 

The reactivity of 47/-benzopyran-4-ones in Diels-Alder reactions is well documented <1987T3075>, and recently high asymmetric induction has been achieved in the reaction of 3-alkoxycarbonyl-substituted chromones with chiral auxiliaries and Danishefsky s diene <1991JOC2058>. It should be noted that 3-formylchromones can react as heterodienes in the stereoselective inverse electron Diels-Alder reaction with enol ethers <1994T11755> to provide a route to pyrano[4,3-A][l]benzopyrans a heterocyclic nucleus which occurs naturally in the fungal metabolite fulvic acid <1984CC1565>. The thermal Diels-Alder reaction of 477-pyran-4-one 405 in the presence of an excess of Danishefsky s diene 404 provided cycloadduct 406 (Equation 32) <1996H(43)745>. 

Benzopyran-3-one 399 is also a very reactive diene in the intermolecu-lar Diels-Alder reaction [70JCS(C)536 76JCS(P1)1647]. (See Schemes 74 and 75.) 2-Benzopyran-3-one 399a-b derived from 400a-b undergoes... 

The stereochemistry of the products of intramolecular Diels-Alder reactions of 4-substituted 2-benzopyran-3-ones is influenced by the nature of the terminal substituent X. Thus, (B-unsubstituted compounds yield the endo-chain adduct (42), whereas when X is -S02Ph exo-chain addition is predominant. Subsequent reductive elimination gives reduced phenanthracenes (Scheme 10) <97JCS(P1)1929>. 

Van Broeck PI, Van Doren PE, et al (1992) Diels-Alder reactions of pyrano[3,4-b]indol-3-ones and a 2-benzopyran-3-one with hetero substituted olefins generation of carbazole and naphthalene derivatives by elimination instead of dehydrogenation. J Chem Soc, Perkin Trans 1 415 19... 

Diels-Alder reactions of the o-quinodimethanes (167) and (168) with cyclopentene, cJ5-but-2-ene, norbornadiene, and benzonorbornadiene gave mainly endo-adducts similar behaviour was noted for benzo[c]furan except that the exo-adduct predominated for addition to norbornadiene. " Since it is thought that steric effects for endo- or exo-addition to (167) and (168) are similar, an attractive interaction between endo-alkyl groups and the diene systems is proposed to explain the predominant endo-addition pathway. In contrast to the preference for endo-addition to (167)—(169) by various dienophiles, reaction of l,4-diphenyl-2-benzopyran-3-one (170) leads mainly to the production of exo-adducts. Since 1,3-diphenylinden-2-one also reacted by endo-addition, the exo-selectivity in additions to (170) is explained as resulting from inhibition of attractive endo secondary interactions by non-coplanarity of the phenyl substituents. Related to these results is the finding that isodicyclopentadiene (171) reacts with methyl acrylate under kinetic control to give the syn-adduct (172) a similar result was observed in the addition of methyl... 

The enantioselective intramolecular formal 2+4-cycloaddition of acrylates and a, -unsaturated imines (99) catalysed by chiral phosphines (100), derived from amino acids, produced A-heterocycles (101) (Scheme 31). Chiral dirhodium(II) carboxamidates (102) catalysed the hetero-Diels-Alder reactions between 2-aza-3-silyloxy-l,3-butadienes and aldehydes to yield all cw-substituted l,3-oxazinan-4-ones in high yields and high enantioselectivity (98% ee)P The nickel-catalysed 4 + 2-cycloaddition of a, -unsaturated oximes with alkynes yielded 2,3,4,6-tetrasubstituted pyridine derivatives. The reaction of isoquinoline, an activated alkyne, and 4-oxo-4//-l-benzopyran-3-carboxaldehyde (103), in ionic solvents, produced 9a//,15//-benzo[a][l]benzopyrano[2,3-/t]quinolizine derivatives (105) via the zwitterion (104) selectively and in good yields (Scheme 32).The Diels-Alder cycloaddition of ethyl 3-(tetrazol-5-yl)-l,2-diaza-l,3-butadiene-l-carboxylates with -rich heterocycles, nucleophilic olefins, and cumulenes formed 3-tetrazolyl-l,4,5,6-tetrahydropyridazines regioselectively. The silver-catalysed formal inverse-electron-demand Diels-Alder... 

In contrast, the adduct (41) obtained from singlet oxygen and l,4-diphenyl-3/7-2-benzopyran-3-one (40) is a stable, crystalline solid (Scheme 2) <78JA2564>. Nonetheless, (41) decomposes in boiling benzene giving o-dibenzoylbenzene (43) and phenyl o-benzoylbenzoate (45) in 85% and 5% yield respectively. No luminescence is detected. The major reaction course is decarboxylation to the intermediate o-xylylene peroxide (42), which can be trapped as its Diels-Alder adduct with maleic anhydride. The ester (45) could have arisen directly from (41) by decarbonylation and concomitant... 

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