Acid-catalysed transformations of substituted 4-hydroxy-2-(prop-2-enyl)cyclopent-2-enones

Abstract

Treatment of the substituted 4-hydroxy- or 4-methoxy-3-methyl-2-(prop-2-enyl)cyclopent-2-enones (1a–c; R²= H or Me) with pyridine hydrochloride (PHC) or 6N-HCl gives the corresponding substituted 4-methyl-5-n-propylcyclopent-4-ene-1,3-diones (2a–c). Under the same conditions 4-methyl-5-(prop-2-enyl)cyclopent-4-ene-1,3-dione (3; R = H) produces only 4-methyl-5-(prop-1-enyl)cyclopent-4-ene-1,3-dione (4). Reduction of (3; R = H) with ZnHg–HCl or treatment of the hydroxycyclopentenone (1a; R²= H) with NaOMe–MeOH led to the crystalline enol 3-hydroxy-4-methyl-5-(prop-2-enyl)cyclopent-2-enone (6), which produced the cyclo-pentenedione (2a) with PHC or 6N-HCl. 4-Hydroxy-3-methyl-2-n-propylcyclopent-2-enone (7), showing no side-chain unsaturation, produced only 3-hydroxy-4-methyl-5-n-propylcyclopent-2-enone (8) with PHC. These data suggested that the isomerisations (1)→(2) probably proceed via intermediate enols [viz.(6)]. Conversion (1)→(6) is envisaged as an acid-catalysed enone–dienol type rearrangement and conversion (6)→(2) as a simple double bond migration proceeding by a series of prototropic shifts. An alternative scheme, involving initial side-chain double bond migration, followed by enol formation is also proposed under certain conditions.

The analogue 4-hydroxy-3-methyl-2-[(Z)-penta-2,4-dienyl]cyclopent-2-enone (14) containing additional unsaturation in the prop-2-enyl side-chain produced largely 4-methyl-5-[(E)-pent-1-enyl]cyclopent-4-ene-1,3-dione (17) with PHC, accompanied by small amounts of positional isomers (16) and (18), In contrast to earlier reports, (14) produced only the enol 3-hydroxy-4-methyl-5-[(Z)-penta-2,4-dienyl]cyclopent-2-enone (19) in NaOMe–MeOH.