A mixture of tert-butyl 3-acetyl-4-oxospiro[chroman-2,4′-piperidine]-1′-carboxylate (250 mg, 0.70 mmol), tert-butyl N-amino-N-methyl-carbamate (120 μL, 0.84 mmol) and p-toluene sulfonic acid monohydrate (27 mg, 0.14 mmol) in ethanol (8 mL) was heated at 80° C. overnight. Additional tert-butyl N-amino-N-methyl-carbamate (120 μL, 0.84 mmol) was added and the mixture was heated at 80° C. for 6.5 h, then another portion of ten-butyl N-amino-N-methyl-carbamate (120 μL, 0.84 mmol) was added and the mixture was heated at 80° C. for 2 h. HCl (0.87 mL of 4.0 M in dioxane, 3.5 mmol) was added and the mixture was stirred at 80° C. for 1 h. After cooling, the precipitate was collected via filtration and washed with cold EtOH to provide 1,3-dimethyl-1H-spiro[chromeno[4,3-c]pyrazole-4,4′-piperidine]dihydrochloride (295 mg, >100%, may contain some methyl hydrazine HCl salt) as a solid. ESI-MS m/z calc. 269.2. found 270.5 (M+1)+; Retention time: 1.11 minutes (3 min run). 1H NMR (400 MHz, DMSO) δ 7.71 (dd, J=7.8, 1.3 Hz, 1H), 7.33-7.27 (m, 1H), 7.17-7.14 (m, 1H), 7.11-7.05 (m, 1H), 4.02 (s, 3H), 3.28-3.15 (m, 4H), 2.40-2.30 (m, 2H), 2.23 (s, 3H), 2.07-1.97 (m 2H).
