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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.

A regiospecific approach to each N‐methyl‐5‐(1H‐indol‐2‐yl)‐6,7‐dihydro‐2H‐indazole isomer is reported. The 1‐methyl isomer 1 was prepared from 5‐bromo‐1‐methyl‐6,7‐dihydro‐1H‐indazole 3 and indole‐2‐boronate 5 by palladium catalyzed Suzuki coupling. The 2‐methyl regioisomer 2 was synthesized via addition of lithium (1‐carboxylato‐1H‐indole‐2‐yl)lithium 6 with 2‐methyl‐2,4,6,7‐tetrahydro‐indazol‐5‐one 8 followed by acid catalyzed dehydration.

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-methyl-5-(1H-indol-2-yl)-6,7-dihydro-2H-indazole isomer is reported. The 1-methyl isomer 1 was prepared from 5-bromo-1-methyl-6,7-dihydro-1H-indazole 3 and indole-2-boronate 5 by palladium catalyzed Suzuki coupling. The 2-methyl regioisomer 2 was synthesized via addition of lithium (1-carboxylato-1H-indole-2-yl)lithium 6 with 2-methyl-2,4,6,7-tetrahydro-indazol-5-one 8 followed by acid catalyzed dehydration.