5431exchangeable), 1.8-2.1 (5 H, complex m), 2.1-2.6 (4 H, complex m), 3.15-3.3(4H,m),3.4(4H,t,J=1.5Hz),4.0(1H,t),4.58(1H,d, J = 3 Hz); MS, exact mass calcd for C,,H2,0S4 (M' + 1) 309.048, found 309.047.Keto Thiolactol 27. To a solution of oxa-tricyclic ketone 13 (50 mg, 0.36 mmol) and 1,2-ethanedithiol (30 kL, 0.36 mmol) in THF (1 mL) was added at -78 O C 1 M ZnCl, in THF (2% equiv). The mixture was slowly warmed to -10 O C , the reaction was quenched with 25% aqueous ammonium acetate, and the solution was extracted with Et20. The organic layer was dried and evaporated. The residue was flash chromatographed on silica gel, eluting with 20% EtOAc/hexane to afford 27 (53 mg, 64%) as an oil: 'H NMR (250 MHz, CDCI,) 6 1.65-1.8 (3 H, complex m), 2.1-2.25 (1 H, m), 2.3-2.42 (1 H, m), 2.6 (1 H, dd, J = 14, 2 Hz), 2.7-3.1 (8 H, complex m), 4.25 (1 H, s(b)), 5.4 (1 H, s(b)).Ketone 28. To a solution of 27 (23 mg, 0.10 mmol) in CH,CI, (1 mL) was added, at room temperature, boron trifluoride etherate (2% equiv) and the solution was stirred for 10 min. The mixture was quenched with 25% aqueous ammonium acetate, extracted with CH,C12, dried, and evaporated to afford 28 (20 mg, 87%): 'H NMR (250 MHz, CDCI,) 6 1.6 (1 H, s(b), exchangeable), 1.65-1.9 (4 H, complex m), 2.0-2.15 (1 H, m), 3.5 ( 1 H, dd, J = 12, 9 Hz), 3.65 (1 H, dd, J = 12, 3 Hz), 3.7-3.85 (2 H, m), 3.15 (4 H, s(b)), 4.2 (1 H, t), 4.52 (1 H, d, J = 4 Hz); MS, exact mass calcd for C,0H1702S2 (M' + 1) 233.067, found 233.067. Ketone 29a. A solution of oxa-tricyclic ketone 14 (358 mg, 2.18 mmol) and 1,2-ethanedithiol (0.21 mL, 2.18 mmol) in THF (6 mL) was cooled to -78 O C and TiCI4 (5 fiL, 0.044 mmol) added. The mixture was warmed to room temperature for 30 min and finally heated to 50 O C for 48 h. The reaction mixture was quenched with 25% aqueous ammonium acetate, extracted with ether, dried, and evaporated. The residue was flash chromatographed on silica gel, eluting with 40% EtOAc/hexane to afford 29a (460 mg, 82%) (substantial amount of hemiacetal 29b): 'H NMR (250 MHz, CDCI3) 6 1.35 (3 H, s), 1.4-2.3 (11 H, complex m), 3.0 (1 H, s, exchangeable), 3.15-3.25 (4 H, m), 4.52 (1 H, d, J = 4 Hz); MS, exact mass calcd for CI2Hl90SZ (M' + 1 -H,O) 243.088, found 243.088. Registry No. HS(CH2),SH, 540-63-6; methyl 3,4-dihydro-2H-pyran-2-acetate, 109390-70-7; sodium 3,4-dihydro-2H-pyran-2-acetate, 109390-71-8; ethyl frans-3-(3,4-dihydro-2H-pyran-2-yl)acrylate, 76919-60-3; ethyl cis-3-(3,4-dihydro-2H-pyran-2-yl)acrylate, 109390-73-0; 3-(3,4-dihydro-2H-pyran-2-yl)propanal, 109390-91-2; ethyl trans-5-(3,4-dihydro-2H-pyran-2-yl)-2-pentenoate, 109390-92-3; ethyl trans-5-(3,4-dihydro-2-methyl-2H-pyran-2-yl)-2-pentenoate, 109391-01-7; ethyl trans-3-(3,4-dihydro-2-methyl-2H-pyran-2-yl)acrylate, 109390-96-7; ethyl cis-3-(3,4-dihydro-2-methyl-2H-pyran-2-yl)acrylate, 109390-97-8; methyl 4-(3,4-dihydro-2-methyl-2H-pyran-2-yl)butanoate, 10939 1-00-6.Supplementary Material Available: X-ray crystal structure analysis of 25 and tables of functional coordinates and temperature factors, bon...