Pyryliumverbindungen. 41. 7aH-Cyclopenta[b]pyran-7-one durch Ringtransformation von 2,4,6-Triaryl-pyryliumsalzen mit acyclischen 1,2-Diketonen

Pyrylium Compounds. 41. 7aH-Cyclopenta[b]pyran-7-ones by Ring Transformation of 2,4,6-Triarylpyrylium Salts with Acyclic 1,2-Diketones Reaction of 2,4,6-triarylpyrylium salts 1 with acyclic 1,2-diketones 2 (CH₃COCOR, R = Me, Ph) in the presence of an appropriate condensing agent (e.g. piperidine acetate, triethyl-amine/acetic acid, sodium acetate) yields the hitherto unknown 2,4,5,7a-substituted 7aH-cyclopenta[b]pyran-7-ones 3 as a result of a new type of ring transformation (2,5-[C₄O+C]/2,3-[C₂+C₃]). A characteristic feature of compounds 3 is their ability to undergo electrophilic substitutions. Thus, stepwise bromination of the 7a-methyl derivative 3a in acetic acid affords 6-bromo-7a-methyl-2,4,5-triphenyl-cyclopenta[b]pyran-7-one ( 4 ) and 3,6-dibromo-7a-methyl-2,4,5-triphenyl-cyclopenta[b]pyran-7-one ( 5 ); analogously, nitration of 3a leads to 7a-methyl-6-nitro-2,4,5-triphenyl-cyclopenta[b]pyran-7-one ( 6 ), indicating that position 6 is the favoured position for electrophilic substitutions. Contrary to the 3,5-unsubstituted pyrylium salts 1 , the 3,5-dimethyl-2,4,6-triphenyl-pyrylium perchlorate ( 13 ) reacts with 1,2-diketones 2 through a 2,6-[C₅+C] transformation to give arylsubstituted 1,2-diones 14 . The structure of the new compounds was determined by spectroscopic methods and by a single crystal X-ray analysis of the dibromo derivative 5 .