Alternatively, according to scheme two the 2-oxaspiro[4.5]dec-7-ene intermediate can be made by D.A. addition of 2-Methylenebutyrolactone to 1,3-Butadiene followed by reduction and intramolecular ring closure.
^Kamei J, Tanihara H, Kasuya Y. Antitussive effects of two specific kappa-opioid agonists, U-50,488H and U-62,066E, in rats. European Journal of Pharmacology. 1990 Oct 9;187(2):281-6. PMID 2272363
^Holtzman SG. Further characterization of the discriminative stimulus effects of spiradoline. Pharmacology, Biochemistry and Behaviour. 2000 Jul;66(3):517-22. PMID 10899364
^Wadenberg ML. A review of the properties of spiradoline: a potent and selective kappa-opioid receptor agonist. CNS Drug Reviews. 2003 Summer;9(2):187-98. PMID 12847558
^Halfpenny, Paul R. (1990). "Highly selective .kappa.-opioid analgesics. 3. Synthesis and structure-activity relationships of novel N-[2-(1-pyrrolidinyl)-4- or -5-substituted cyclohexyl]arylacetamide derivatives". Journal of Medicinal Chemistry33 (1): 286–291. doi:10.1021/jm00163a047.edit