[C:1]1(C)C=CC(S(O)(=O)=O)=CC=1.[CH:12]([O-:16])([O-])[O:13][CH3:14].[NH:17]1[CH2:22]C(=O)[CH2:20][CH2:19][C:18]1=[O:24].C(N(CC)CC)C>CO>[CH3:1][O:16][C:12]1([O:13][CH3:14])[CH2:22][NH:17][C:18](=[O:24])[CH2:19][CH2:20]1About 5 mg of p-toluenesulfonic acid was added to a solution of methyl orthoformate (5 ml), and the resulting mixture was then added to a solution of 2,5-piperidinedione (1.8 g, 16 mmol) in methanol (20 ml). The mixture was heated under reflux for 12 hours. After cooling, triethylamine (1 ml) was added to the mixture, and then the mixture was concentrated by using a rotary evaporator. The residue was purified by silica gel column chromatography (CHCl3 :MeOH=15:1), and then recrystallized from ethyl acetate/ether to obtain 5,5-dimethoxy-2-piperidone as the desired compound (1.85 g, 81%). m.p. 89-90° C.; 1H-NMR (CDCl3, 300 MHz) δ: 2.01 (t, 2H, J=7.0 Hz, NCOCH2 --), 2.43 (t, 2H, J=7.0 Hz, NCOCH2CH2 --), 3.25 (s, 6H, OMe), 3.36 (d, 2H, J=2.5 Hz, NCH2C(OMe)2), 5.89 (bs, 1H, NH). P-SIMS (Glycerin matrix) 182 (M+ +1.8%), 160 (100%), 128 (34%), 101 (5%).
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