The enantiomers of 7aH-cyclopenta[b]pyran-7-ones and their thermal racemization

Abstract

3952H-Pyrans are known to undergo ring opening reactions to their valence isomers, the pentadienones, which are capable to recyclize forming the starting pyrans [1,2]. These electrocyclic interconversions can be induced photochemically or by heat. In some cases an equilibrium between both isomers is observed. The favoured type of compound at given conditions strongly depends on the substitution pattern. Electron donating groups at position 5 of the pentadienone usually stabilize it by conjugation with the carbonyl group at C-1 whereas substituents at C-3 on the 2H-pyrans lead to a decrease of the tendency of electrocyclic ring opening [1,2].A possibility for the determination of kinetic data such as rate constants k or free enthalpies of activation ∆G ≠ for the electrocyclic ring opening/ring closure consists in the determination of the concentration of 2H-pyrans or pentadienones as a function of time. However, this method fails if one of the valence isomers is too stable that no formation of the other one by electrocyclic conversion can be detected. Investigations on spiro[indoline-chromenes], spiro[indoline-benzoxazines] [3] and spiro[cyclohexadiene-indolines][4] have shown that kinetic data for ring opening reactions can also be determined by liquid chromatographic separation of the enantiomers on non-racemic stationary phases followed by thermal enantiomerization, i.e. measuring the time-dependent loss of optical activity with a polarimeter.Racemic 7aH-cyclopenta[b]pyran-7-ones 1, the products of ring transformation of 2,4,6-triarylpyrylium salts with acyclic 1,2-diketones (butan-2,3-dione, 1-phenyl-propan-1,2-dione) [5] represent a novel type of 2H-pyran derivatives in which the pyran system is 2,3-anellated with a carbocyclic five-membered ring. At room temperature the isomeric cyclopentadienones 2 cannot be observed. Nevertheless, kinetic data for this interconversion should be obtained as in the case of spiro In this paper, we report such investigations and compare the data obtained with those of structurally related systems.According to the substitution pattern of the chiral center at C-7a, the 7aH-cyclopenta[b]pyran-7-ones studied can be divided into the 7a-methyl derivatives 1a-g and the compounds 1h -o bearing a phenyl group at C-7a. The separation of the enantiomers (R)-1 and (S)-1 of both types of pyrans by liquid chromatography on triacetylcellulose using methanol as eluent failed with the exception of the derivatives 1a, 1l, 1n and 1o (cf. Table 1). When the sorbent was changed to tribenzoylcellulose, an improved separation of the 7a-methyl substituted pyrans 1a -g could be observed whereas this system was inactive for the 7a-phenyl compounds 1h -o. The latter ones were successfully separated on (+)-poly(trityl methacrBarriers to ring opening were determined for two 7a-methyl and two 7a-phenyl substituted 2,4,5-triaryl derivatives by thermal racemization of the enantiomers. Substituted cyclopentadienones 2 are discussed as intermediates in the thermal electrocyclic ring opening/ring closure reacti...