4-Azaspiro[2.3]hexane, an Overlooked Piperidine Isostere: Multigram Synthesis and Physicochemical and Structural Evaluation

An expedient approach to the synthesis of 4-azaspiro[2.3]hexane derivatives is described. The synthetic scheme consists of Tebbe olefination of N-Boc-protected 2-azetidinone (including the first use of the deuterated Petasis reagent Cp₂Ti(CD₃)₂ in the building block preparation) and cyclopropanation of the resulting intermediate. The developed protocols allowed for the preparation of target building blocks on a multigram scale (up to 52 g). To illustrate the potential of the obtained 4-azaspiro[2.3]hexane derivatives for isosteric replacements in drug discovery, their physicochemical and structural characterization was performed, i.e., basicity (pK_a) and lipophilicity (Log P) measurements, X-ray diffraction studies, and exit vector plot (EVP) analysis.