Research on 1-azabicyclic systems

Cis-trans-conversion of the bicycle in pyrrolizidines: The effect of the preferred conformations on the properties of the bases and the thermodynamics of conformational transformation of the ring fusion type. (Review)

“…In both solvents the latter is almost 1.5 times weaker as a base than its isomer 18 [34]. Like the unstrained cis-fused pyrrolizidines, the base 11 in the 11A conformation no longer has such an accessible unshared electron pair of the nitrogen, which can moreover undergo partial delocalization according to the familiar mechanism [47][48][49][50]. Taken all together this makes the similar values of the basicity of compound 11 (pK a 10.28 ± 0.10), indolizidine (pK a ± 0.05) [46], and quinolizidine (pK a 10.19 ± 0.07) [51] understandable.…”

“…On the other hand, in the trans-fused conformation A it experiences further stabilization in connection with the delocalization of the unshared electron pair of the nitrogen atom according to the above-mentioned mechanism [47][48][49][50]. From this it had to be expected that ∆H i 0 of the trans-cis conversion of pyrrolizidine 1 is smaller in absolute value than the difference between the enthalpies of compounds 3 and 2 [4].…”