ISSN:
0021-8383
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Pyrylium Compounds. 41. 7aH-Cyclopenta[b]pyran-7-ones by Ring Transformation of 2,4,6-Triarylpyrylium Salts with Acyclic 1,2-DiketonesReaction 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.
Additional Material:
2 Ill.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/prac.19893310216
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