Structure determination of an endogenous sleep-inducing lipid, cis-9-octadecenamide (oleamide): A synthetic approach to the chemical analysis of trace quantities of a natural product

The pursuit of endogenous sleep-inducing substances has been the focus of an extensive, complicated body of research. Several compounds, including Delta-sleep-inducing peptide and prostaglandin D-2, have been suggested to play a role in sleep induction, and yet, the molecular mechanisms of this physiological process remain largely unknown. In recent efforts, the cerebrospinal fluid of deep-deprived cats was analyzed in search of compounds that accumulated during sleep deprivation. An agent with the chemical formula C18H35NO was found to cycle with sleep-wake patterns, increasing in concentration with sleep deprivation and decreasing in amount upon recovery sleep. Since the material was generated in minute quantities and only under the special conditions of sleep deprivation, efforts to isolate sufficient material for adequate characterization, structure identification, and subsequent detailed evaluation of its properties proved unrealistic. With the trace amounts of the impure endogenous compound available, extensive MS studies on the agent were completed, revealing key structural features of the molecule including two degrees of unsaturation, a long alkyl chain, and a nitrogen substituent capable of fragmenting as ammonia. Additionally, HPLC traces suggested a weak UV absorbance for the unknown material. With this data in hand and encouraged by the relatively small size of the molecule, MW = 281, a synthetic approach toward the structural identification of the natural compound was initiated. Herein, we report the full details of the synthesis and comparative characterization of candidate structures for this endogenous agent that led to the unambiguous structural correlation with synthetic cis-9-octadecenamide.