Conformational analysis of 7-alkyl-3-oxabicyclo-[3.3.1]nonanes and complexes with lanthanide shift reagents

The conformation of 3-oxabicyclo3,3.1]nonane and of some of its 7α- and 7β-alkyl substituted derivatives has been studied with the use of ¹³C and ¹H NMR spectroscopy. A comparison is made with the carbocyclic analogues; it turns out that the replacement of the 3-methylene group by oxygen has no substantial influence on the conformational preferences. With the aid of ³J_HH coupling constants it is shown that the geometry of the cyclohexane rings is about the same as in the corresponding carbocyclic compounds. The results of calculations on the lanthanide induced shifts indicate that the tetrahydropyran ring is not flattened but probably somewhat puckered. The calculated location of Eu(III) in complexes of Eu(dpm)₃ with the 3-oxabicyclo 3.3.1]nonanes is compared with that in the complexes of the related compounds 2-oxaadamantane and 4-methyltetrahydropyran. The data indicate that the lanthanide ion coordinates “axially” to the latter compound.