The Synthesis of a Dopamine D2 Partial Agonist for the Treatment of Schizophrenia

The synthesis of the phosphoric acid salt of dopamine D2 partial agonist 2-{4-[4-(7-fluoro-naphthalen-1-yl)-piperazin-1-yl]-butoxy}-5,6,7,9-tetrahydro-1,7,9-triaza-benzocyclohepten-8-one (1) is reported. The most prominent feature of the molecule is a seven-membered ring urea functionality that has been prepared via an efficient one-pot, three-step transformation. The original synthesis from the Medicinal Chemistry group provided precursor 13 in 10 steps and 2% overall yield, required four chromatographies and employed unsafe reagents such as 2-iodoxybenzoic acid (IBX) and HClO4. The optimized synthetic route for the preparation of phosphate salt 1 consists of 12 linear steps with a 10% overall yield. Safer and more robust reaction conditions have been developed with only one required chromatography. Another key step in the synthesis is the coupling of iodide 25 with naphthalenopiperazine 12 to provide 13. Due to the difficulty to purify this intermediate, a protocol had to be developed to obtain crude material with the required purity, suitable for use in the subsequent salt formation step. Finally, considerable work was carried out to determine the most stable polymorph of the API. As a result, a robust set of conditions has been developed for the formation of phosphoric acid salt 1, providing the desired polymorph in excellent yield and purity.