1-Azabicyclo undecanes conformations

However, in more complicated amines, this straight correlation is violated. The bicyclic tertiary amine l-azabicyclo[4.4.4]tetradecane (22) and the acyclic tertiary amine n-Bu3N have nearly the same first IP (7.84 and 7.90 eV, respectively), but the proton affinity of the bicyclic amine is 20 kcal mol 1 lower than that of the acyclic52. On the other hand, for other bridge-head tertiary amines like l-azabicyclo[2.2.2]octane (quinuclidine, 20) and l-azabicyclo[3.3.3]undecane (manxine, 21) the expected relation between proton affinities and IP values is observed. The extraordinary properties of l-azabicyclo[4.4.4]tetradecane (22) are caused by its unusual conformation the nitrogen lone-pair is directed inward into the bicycle where protonation is not possible. In the protonated form, the strained out-conformation is adopted. This makes it the least basic known tertiary amine with purely saturated alkyl substituents. Its pKa, measured in ethanol/water, is only +0.693. Strain effects on amine basicities have been reviewed by Alder88. 

Spectral investigations of daphnezomines F (42) and G (43), whose molecular formulas are C27H35NO8 and C27H35NO7, respectively, revealed that they are structurally related and possess a l-azabicydo[5.2.2]undecane moiety. The conformation of the l-azabicyclo[5.2.2]undecane ring in 43 was elucidated by a low-temperature NMR study and computational analysis [46],... 

The structure of daphmanidin B (67), C25H36NO6, was elucidated by 2D NM R data to possess a l-azabicydo[5.2.2]undecane moiety, like daphnezomines F and G [46]. The relative stereochemistry was deduced from NOFSY correlations. The conformation of the unit (C-2-C-5, C-18 to C-2, C-19, and N) in the l-azabicyclo[5.2.2]undecane moiety, with a twist-chair form as shown in Figure 18.16, was consistent with the results of a conformational search using MMFF force field [60] implemented in the Macromodel program [59]. 

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