New condensed systems-substituted octahydro-chromeno[2,3,4-k,l]xanthylium salts
Semicyclic 1,5-diketones of the 2-(3-oxopropyl)cyclohexane-1,3-dione series are capable of cyclization only in severe conditions on boiling in acetic anhydride to give substituted tetrahydrochromenones [1]. Salt formation is also difficult and takes place only with a donor substituent on C(1 ) of the aliphatic chain of the oxo compounds [2]. Reactions of analogous bicyclic oxo 1,5-diketones have not appeared in the periodic literature. We have shown for the first time that it is possible to aromatize the tetrahydroxanthene ring of 3,3-dimethyl-9(4,4-dimethyl-2,6-dioxocyclohexyl)-1,2,3,4-tetrahydro-9H-xanthen-1-one (1) under the influence of perchloric acid of boron trifluoride etherate with simultaneous polyacetalization of the 1,5-dicarbonyl unit into a hydroxytetrahydrochromen unit and formation of 8a-hydroxy-7,7,11,11-tetramethyl-13-oxo-6,7,8,8a,10,11,12,13octahydrochromeno[2,3,4-k,l]xanthen-5-ylium perchlorate (2) or tetrafluoroborate (3) respectively. The yields of the salts were 42.2 and 57.5% respectively. 5a-Hydroxy-3,3,7,7-tetramethyl-1H-2,3,4,5a,6,7,8,13b-octahydrobenzopyrano[2,3,4-k,l]xanthen-1-one (4) was isolated along with salts 2 and 3 (12.2 and 21.0% yield respectively). The formation of 4 permits the suggestion that polyacetalization precedes aromatization of the heterocyclic fragment of the substrate.