3-Substituted cyclopenta[c]pyrans:: Synthesis and electrophilic substitutions

The literature procedure for the preparation of the parent cyclopenta[c]pyran (1) was shown to provide only a low yield on mechanistic grounds. Intended as an alternative starting material for the synthesis of 1, the cyclopenta[c]pyran-3-ones 10 were prepared. However, their conversion into 1 could be achieved at best in 1%, yield. Derivatives of I with an oxygen functionality in the 3-position were obtained by deprotonation, silylation and benzoylation of 10b, giving rise to the lithium enolate 15, the ketene acetals 16-18 and the ketene hemiacetal ester 19, respectively. The olefination of 10b with substituted methylenetriphenylphosplioranes furnished cyclopenta[c]pyran-3-ylacetonitrile (21) and methyl cyclopenta[c]pyran-3-ylacetate (22) in modest yields. These Compounds as well as the primary alcohol 23, which resulted from 22 on reduction with LiA1H(4), underwent an electrophilic aromatic substitution reaction with trifluoroacetic anhydride to form the 7-(trifluoroacetyl)cyclopenta[c]pyrans 24-27. In the case of the acetonitrile 27, a second and a third trifluoroacetyl group were readily introduced, giving rise to the highly resonance-stabilised anions 28 and 29, the latter exhibiting a strong green fluorescence. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006).