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8 A representative experimental procedure for the NBS-promoted ring expansion reaction: (5
S
*
,7
S
*,10
S
*)-10-Bromo-7-ethoxy-6-[(4-methylphenyl)sulfonyl]-6-azaspiro[4.5]decan-1-one (
3): To a stirred solution of cyclobutanol 1 (100 mg, 0.285 mmol, 1 equiv) in propylene oxide (2 mL) and i-PrOH (2 mL) at -78 °C was added N-bromosuccinimide (60.8 mg, 0.341 mmol). The mixture was stirred at -78 °C for 2 h. The reaction was warmed to r.t. over 1 h and solvents were removed in vacuo. Column chromatography on silica gel (10% EtOAc-hexanes) gave 117 mg (96%) of a white solid; mp 100-103 °C(dec). IR (KBr): 2938, 1740, 1328, 1137 cm-1. 1H NMR (400 MHz, CDCl3): δ = 7.79 (d, J = 8.5 Hz, 2 H), 7.29 (d, J = 8.2 Hz, 2 H), 4.87 (dd, J = 5.2, 1.5 Hz, 1 H), 4.61 (dd, J = 11.6, 4.3 Hz, 1 H), 3.89 (dq, J = 9.8, 7.0 Hz, 1 H), 3.52 (dq, J = 9.8, 7.0 Hz, 1 H), 2.78 (dt, J = 14.0, 9.5 Hz, 1 H), 2.70 (ddd, J = 19.2, 10.4, 8.9 Hz, 1 H), 2.55-2.29 (m, 3 H), 2.42 (s, 3 H), 2.26-2.01 (m, 3 H), 1.86-1.68 (m, 2 H), 1.19 (t, J = 7.0 Hz, 3 H). 13C NMR (75 MHz, CDCl3): δ = 214.2, 143.6, 138.1, 129.4, 127.5, 83.4, 68.2, 63.7, 50.8, 40.3, 37.4, 28.6, 25.3, 21.5, 18.4, 15.2; MS (ESI): m/z 424 [M + 2 - EtOH + K]+, 422 [M - EtOH + K]+, 408 [M + 2 - EtOH + Na]+, 406 [M - EtOH + Na]+, 386(47), 384(45).
