Bromination of 9 dimedonyl 3,3 dimethyl 2,3,4,9 tetrahydro 1H xanthen 1 one was car ried out to provide a route to a new series of substituted hydrochromeno[2,3,4 kl]xanthenones, which may find application for the synthesis of poorly accessible compounds including the heteroanalogs. A dependence of transformations of dimedonyltetrahydroxanthenone on the nature of the solvent was elucidated: bromination at the α methylene groups of the dimedonyl substituent is accompanied by heterocyclization involving 1 and 5 oxo groups. Quantum chemical HF/6 311(d,p) ab initio calculations for the proposed mechanism of hemiacetalization and halogenation were carried out.

Keywords: 9-dimedonyltetrahydroxanthene, oxo 1,5-diketone, hydrochromenoxanthylium 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,10,11,12,13-octahydrochromeno[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.

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