A series of threo-1-aza-3 or 4-substituted-5-phenyl[4.4.0]decanes (quinolizidines), which were envisioned as restricted rotational analogues (RRAs) of methylphenidate (MP), was synthesized and tested for inhibitory potency against [(3)H]WIN35,428, [3H]citalopram, and [3H]nisoxetine binding to the dopamine, serotonin, and norepinephrine transporters, respectively. Two different synthetic schemes were used; a Wittig reaction or acylation (followed by an intramolecular condensation) was a key feature of each scheme. The unsubstituted RRA, threo(trans)-1-aza-5-phenyl[4.4.0]decane (12a), was equipotent to unconstrained threo-MP against [(3)H]WIN35,428 binding. The extra ring in these RRAs (which reduces the conformational freedom) and the orientation and polarity of substituents at the 4-position on this extra ring are of critical importance to the biological activity. Generally, the RRAs paralleled the corresponding unconstrained MP derivatives in binding affinity to the three transporters. The results suggest that the conformation of MP in which the carbonyl group of the methyl ester is H-bonded to the piperidinyl N-H may be the bioactive form of the molecule.

A series of 8-substituted-3-azabicyclo[3.2.1]octanes (isotropanes) were synthesized and tested for inhibitor potency using [(3)H]WIN 35,428 binding at the dopamine (DA) transporter, [(3)H]citalopram binding at the serotonin (5-HT) transporter, and [(3)H]DA uptake assays. The synthesis started with a Mannich condensation of cyclopentanone, benzylamine, and fomaldehyde to afford N-benzyl-3-azabicyclo[3.2.1]octan-8-one (6). The 8-phenyl group was introduced by Grignard addition to ketone 6 or nucleophilic displacement via a triflate of the corresponding alcohol 7a. The 8beta-phenyl-8alpha-alcohols from Grignard addition generally have low affinity for the two transporters and do not effectively inhibit the uptake of [(3)H]DA. The 8beta-phenyl compound (14) without the hydroxyl group at C-8 was much more potent (22-fold) for [(3)H]WIN 35,428 binding inhibition than the corresponding 8beta-phenyl-8alpha-hydroxy compound (7a). The 8alpha-phenyl compound 8a was almost as potent as cocaine in binding to the DA transporter (IC(50) = 234 nM vs 159 nM for cocaine), whereas the C-8 epimer, compound 14, was somewhat less potent (IC(50) = 785 nM). The lower potency of 14 (beta-orientation of 8-phenyl group) as compared to 8a (alpha-orientation) was unexpected, based on modeling studies comparing the new compounds to WIN 35,065-2, an analogue of cocaine. The benzhydryl ethers at C-8 (17), analogous to the benztropines, had better selectivity than the corresponding phenyl compounds, 8a and 14, for the DA transporter as compared to the 5-HT transporter. The isotropane and benzisotropine analogues seem to bind in a manner that is more similar to that of the benztropine compounds 5 rather than those of cocaine and WIN 35,065-2.