Structure determination of N-methyl-tetrahydro-5H-indazol-5-ones

This paper communicates the (regio) synthesis and a convenient NMR structural assignment method for N‐methyl‐tetrahydro‐5H‐indazol‐5‐one isomers. The cyclization reaction of 7‐(hydroxymethylene)‐1,4‐dioxaspiro[4,5]decan‐8‐one (3) with methylhydrazine yields, after de‐protection predominately the N‐2 methyl isomer 2. Analysis of the product ratio and structural assignments are based on NMR data including NOE difference experiments and subsequently confirmed with X‐ray crystallography. These findings are in sharp contrast with the literature. The experimental conditions used to optimize the synthesis of the individual isomers are discussed.