- Quassinoid Synthesis via o-Quinone Diels-Alder Reactions
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The reaction of 3,5-disubstituted o-quinones (2a, 2c) and 4-chloro-3,5-disubstituted o-quinones (2b, 2d) with simple dienes was investigated as a potential route to the quassinoid skeleton.Quinones 2a and 2c reacted in high yield at the 3,4-position with only a small excess of diene.Attempted equilibration of cis-fused cycloadducts to the trans-fused system failed due to the intervention of a stable enol form, as in 20.Compound 2c with ethyl 3,5-hexadienoate gave 15a, which upon reduction and lactonization provided BCD-ring tricyclic quassinoid analogues 18a and 19a.Again isomerization to the BC trans-fused system was not possible.The chloroquinones showed some preference for Diels-Alder reaction at the 5,6-position, but the additions were characterized generelly by low yields, side reactions, and lessened stereoselectivity.
- Weller, Dwight D.,Stirchak, Eugene P.
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- Metal ion catalysis of the decomposition of transient 2-carboxy-2,5-cyclohexadienones in aqueous solution
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Bromide ion induced debromination of the anion of 4-bromo-4-methyl-2,5-cyclohexadienone-2-carboxylic acid (1) is catalyzed by cupric ions and ferric ions.Similarly, the enolization of the anion of the benzocyclohexadienone 3, which is formed during the bromination of 1-naphthol-2-carboxylic acid, is catalyzed by some metal ions.The origin of the catalysis in these reactions is strong metal ion binding to the incipient dianion products that are of the salicylate type.Evidence for this is that the efficiency of the metal (and hydrogen) ion catalysis parallels the stability of the analogous complexes with the salicylate dianion.
- Tee, Oswald S.,Iyengar, N. Rani
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p. 1194 - 1198
(2007/10/02)
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