Heterodiene synthesis—XI : A study of the isomerization of tetrahydropyran[2′,3′:2,3] 2,3-dihydropyran[6,5-c] pyrazoles

The stereoisomers (1–4) of the tetrahydropyran2′,3′:2,3] 2,3-dihydropyran6,5-c] pyrazole system equilibrate in deuterated trifluoroacetic acid at 35°.The mechanism which causes incorporation of deuterium in the 4a-position is discussed.The isomers distribution is rationalized in terms of steric and electronic interactions, and the energy values for the interaction between the phenyl group in 5 and the substituent in position 6 on the pyrazole ring are given.