Experiments on the total synthesis of Lysolipin I. Part III . Preparation and transformations of substituted 1,2,3,4‐Tetrahydrodibenzofuran‐1‐ones

1,2,3,4‐Tetrahydrodibenzofuran‐1‐ones were obtained by Michael addition of 1,3‐cyclohexadione ( 2 ) to o ‐benzoquinone ( 3 ) and to p ‐benzoquinones 8 and 11 ( Scheme 2 ). In addition to the expected 7,8‐disubstituted adduct 14 , the ZnCl 2 ‐catalyzed reaction of dione 2 with methoxy‐ p ‐benzoquinone ( 11 ) afforded a small amount of the 6,8‐disubstituted regio‐isomer 13 ( Scheme 2 ). The projected cleavage of these dibenzofuranones to 3‐methoxy‐2‐phenyl‐2‐cyclohexenone 22 could be effected by treatment with NaOH followed by methylation ( Scheme 3 ). Attempted acetalization of such dibenzofuranones resulted in a retro‐Claisen ‐type cleavage, giving the benzofuryl‐butyrate 16 . Other transformations include reduction of the ketone, of the C(4a)=C(9b) bond, and alkylation with Li‐ethoxyacetylide ( Scheme 3 ). Oxidation of 8‐hydroxy‐7‐mehoxydibenzofuran derivatives led to o ‐quinones instead of the desired ring cleavage to p ‐quinones ( Scheme 4 ).