Further BF₃·Et₂O-catalyzed cycloadditions of sesquiterpenic p-benzoquinones

Cycloaddition reactions of O-methylperezone [4], O-methyl-6-methoxyperezone [13], the mixture of O-angeloyl-6-methoxyperezone [14] and O-methyl-6-angeloxyperezone [15], 6-methoxyperezone [16], and O-methyl-6-hydroxyperezone [17] are described. The results obtained from 16 and 17 allow confirmation of previous suggestions indicating that BF₃ Et₂O-catalyzed reactions of these quinones afford pipitzols or pipitzol derivatives when position 6 is either free or has a protected alcohol, while they afford perezinone: BF₂ [10] and/or perezinone [21] when a free hydroxyl group is present at C-6. The structures of 7-methoxy-α-pipitzol [18] and 7-methoxy-β-pipitzol [19], obtained from 13, were independently confirmed by single crystal X-ray diffraction analyses.