▁anal ogs ▁the ▁nit ro gen ▁l one ▁pair ings ▁rig id ▁fix ity ▁makes ▁it ▁res ide ▁in ▁an ▁equ atorial ▁placing ▁for ▁the ▁pi per id ine ▁ring - part ▁of ▁the ▁trop ane ▁nucle us , ▁pointing ▁to ▁the ▁two - car bon ▁& ▁three ▁m eth yl ene ▁unit ▁bridge head ; ▁giving ▁the ▁att ested ▁front - brid ged ▁c oca ine ▁analog ues ▁preference ▁for ▁S ERT ▁over ▁DAT . ▁ ▁Tr icy clic ▁c oca ine ▁analog ues ▁ ▁T ether ing ▁the ▁nit ro gen ▁ 8 ▁trop ane ▁position ▁one ▁position ▁further ▁( be yond ▁ 2 β ▁and ▁crossed - over ▁it ▁/ ▁leaving ▁it ▁open ▁as ▁hydro gen ▁and ▁thus ▁possible ▁of ▁having ▁additional ▁un const rained ▁substitution s ▁there ) ▁and ▁linking ▁all ▁the ▁way ▁across ▁to ▁the ▁ 3 β ▁ar yl , ▁replacing ▁it ; ▁yields ▁an ▁exp ans ive ▁front - brid ged ▁structure ▁to ▁create ▁a ▁struct ur ally ▁tr icy clic ▁series ▁of ▁c oca ine ▁analog ues . ▁▁ 8 ▁to ▁ 3 ▁position ▁ ▁Az ab orn ane ▁trop ane ▁ring ▁contra ction ▁Alter ations ▁short ening ▁the ▁trop ane ▁ring ▁system ▁while ▁including ▁the ▁ben zo y lo xy ▁length ▁at ▁the ▁C 3 ▁have ▁been ▁made , ▁contrast ing ▁the ▁az ab orn ane ▁ph eny lt rop anes ; ▁likely ▁rem ed ying ▁the ▁shall ow ▁pen etr ation ▁( for ▁good ▁effic acy ) ▁of ▁the ▁latter . ▁ 5