CONVERSION OF 2-ALKYL-2-(2-OXOPROPYL)CYCLOPENTANE-1,3-DIONES INTO 2,3,5-TRISUBSTITUTED AND 2,3,4-TRISUBSTITUTED CYCLOPENT-2-ENONES BY INTRAMOLECULAR ALDOLIZATIONS TO 2,3-DIACYLCYCLOPROPANOLATES FOLLOWED BY REMARKABLE SKELETAL REARRANGEMENTS

2-Alkyl-2-(prop-2-ynyl)cyclopentane-1,3-diones 2, conveniently prepare d from 2-alkylcyclopentane-1,3-diones 1 and prop-2-ynyl bromide, affor d the triketones 3 by Hg2+-catalyzed hydration of the acetylenic tripl e bond. Treatment of these triketones with aqueous sodium hydroxide gi ves rise to the 2,3,5-trisubstituted cyclopent-2-enones 5, which are a ccompanied by the isomeric 2,3,4-trisubstituted cyclopent-2-enones 7 a s byproducts. The formation of these isomers can be avoided, when the 2,2-disubstituted cyclopentane-1,3-diones 2 are first converted by rin g cleavage into the 5-alkyl-4-oxooct-7-ynaic acids 4 and then by subse quent hydration into the 5-alkyl-4,7-dioxoalkanoic acids 6. An intramo lecular aldolization of the latter forms exclusively the cyclopentenon es 5. A mechanism explaining the simultaneous formation of 5 and 7 fro m 3 is based on the formation of the 2,3-diacylcyclopropanolates 11 an d 16 by intramolecular aldolization and subsequent ring opening to the 2-acetylcyclohexane-1,4-diones 13 and 18. A further ring opening to t he 4,7-dioxoalkanoates 15 and 20 followed by intramolecular aldol cond ensation then gives rise to the isomeric trisubstituted cyclopent-2-en ones 5 and 7.