^US Patent 3792080 - Process for Substituted Cyclohexenes & it's Products
^J Clin Pharmacol. 2002 Nov ;42 (11):1257-61 - Sequential first-pass metabolism of nortilidine: the active metabolite of the synthetic opioid drug tilidine
^Opiates: George R. Lenz page 439, Table 9-30 (78)