An extended compound library of spiro[cycloalkane-pyridazinones] with high Fsp3 character is targeted. There are two possibilities to improve the physicochemical parameters of a drug candidate molecules or building blocks, either to replace the aromatic systems with bioisoster heteroaromatic moieties, e.g. with one or two nitrogen containing ring systems (pyridines, pyridazines, pyrimidines, etc.), or to increase the Fsp3 character of the compounds. Using a new synthetic ap-proach, the Grignard reaction of 2-oxaspiro[4,5]decane-1,3-dione and 2-oxaspiro[4,4]nonane-1,3-dione with p-halophenyl- or p-alkylphenyl-magnesium bromide resulted in the formation of the corresponding 2-oxoethyl-cycloalkanecarboxylic ac-ids, which served as starting materials for the formation of pyridazinone with hydrazine or phenylhydrazine. The pyridazi-nones obtained were alkylated with methyliodide or benzylbromide. 16 Novel 4-tolyl- or 4-halophenyl-2,3-diazaspiro[5.5]undec-3-en-1-one and 4-tolyl- or 4-halopyhenyl-7,8-diazaspiro[4.5]dec-8-en-6-one, and their N-methyl, N-benzyl, and N-phenyl derivatives were synthetized.The physicochemical parameters and the Fsp3 character of the novel compounds obtained were studied. A few of them showed excellent logP and clogP values, but introduction of further phe-nyl group seemed to be disadvantageous