Abstract:

2-Alkyl-2-(prop-2-ynyl)cyclopentane-1,3-diones 2, conveniently prepared from 2-alkylcyclopentane-1,3-diones 1 and prop-2-ynyl bromide, afford the triketones 3 by Hg²⁺-catalyzed hydration of the acetylenic triple bond. Treatment of these triketones with aqueous sodium hydroxide gives rise to the 2,3,5-trisubstituted cyclopent-2-enones 5, which are accompanied by the isomeric 2,3,4-trisubstituted cyclopent-2-enones 7 as byproducts. The formation of these isomers can be avoided, when the 2,2-disubstituted cyclopentane-1,3-diones 2 are first converted by ring cleavage into the 5-alkyl-4-oxooct-7-ynoic acids 4 and then by subsequent hydration into the 5-alkyl-4,7-dioxoalkanoic acids 6. An intramolecular aldolization of the latter forms exclusively the cyclopentenones 5. A mechanism explaining the simultaneous formation of 5 and 7 from 3 is based on the formation of the 2,3-diacylcyclopropanolates 11 and 16 by intramolecular aldolization and subsequent ring opening to the 2-acetylcyclohexane-1,4-diones 13 and 18. A further ring opening to the 4,7-dioxoalkanoates 15 and 20 followed by intramolecular aldol condensation then gives rise to the isomeric trisubstituted cyclopent-2-enones 5 and 7.