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1 Supporting Information Siloxy(trialkoxy)ethene Undergoes Regioselective [2+2] Cycloaddition to Ynones and Ynoates en route to Functionalized Cyclobutenediones Shin Iwata, Toshiyuki Hamura, and Keisuke Suzuki * Department of Chemistry, Tokyo Institute of Technology, and SORST JST, O-okayama, Meguro-ku, Tokyo , Japan Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, and PRESTO, Japan Science and Technology Agency (JST) 2- Gakuen, Sanda, Hyogo , Japan ksuzuki@chem.titech.ac.jp General Experimental Procedures All experiments dealing with air- and moisture-sensitive compounds were conducted under an atmosphere of dry argon. For thin-layer chromatography (TLC) analysis, Merck pre-coated plates (silica gel 60 F 254, Art 575, 0.25 mm) were used. For flash column chromatography, silica gel 60 (Merck Art 7734, mesh) was used. Silica gel preparative TLC (PTLC) was performed on Merck silica gel 60 PF 254 (Art 7747). Melting point (mp) determinations were performed by using a Yanako MP-S3 instrument and are uncorrected. H NMR and 3 C NMR were measured on a JEOL JNM lambda-400, a JEOL JNM ECA-400, and a Bruker AV400N spectrometer. Infrared (IR) spectra were recorded on a Jasco IR-Report-00, and a Horiba FT-70 spectrometer. Attenuated Total Reflectance Fourier Transformation Infrared (ATR-FTIR) spectra were recorded on a Perkin Elmer 600 FTIR. Elementary analyses were performed by using Perkin Elmer series II S

2 Synthesis of cyclobutene 3: A mixture of 3-butyne-2-one () (3 mg,.92 mmol) and KSA 2 (627 mg, 2.52 mmol) was heated at 60 C for 8 h. The crude product was purified by silica-gel flash column chromatography (hexane/etoac = 9/) to give 3 (564 mg, 92.6%) as a colorless oil. Me O cyclobutene (s, 3H), 0.4 (s, 3H), 0.90 (s, 9H), 2.29 (s, 3H), 3.33 (s, 3H), 3.36 (s, 3H), 3.39 (s, 3H), 7.33 (s, H); 3 3.4, 3.2, 9., 26.3, 27.5, 5.2, 5.3, 53., 07.0, 08.3, 45.8, 52.3, 94.9; 2937, 2856, 689, 606, 463, 36, 276, 25, 95, 095, 04, 998, 898, 838, 78 cm ; Anal. Calcd for C 5 H 28 O 5 Si: C, 56.93; H, Found: C, 56.77; H, 9.0. Me CO 2 Me,3-diene (s, 6H), 0.9 (s, 9H),.86 (s, 3H), 3.49 (s, 3H), 3.60 (s, 3H), 3.63 (s, 3H), 4.30 (s, H); 3 3.7, 8.6, 20.0, 26.0, 5., 55.6, 56., 76.3,.8, 48.8, 59.7, 69.3; 2950, 2897, 2858, 734, 667, 69, 462, 434, 38, 324, 254, 23, 69, 04, 076, 002, 939, 863, 838, 83, 780 cm. # The stereochemistry of 4 was determined on the basis of the observed NOE shown below. (H a ) 3 C CO 2 Me H b OC(H c ) 3 NOE H a :.86 ppm, H b : 4.30 ppm, H c : 3.63 ppm S2

3 Scheme. Determination of the regiochemistry Me O HO HO NaBH 4, CeCl 3 7H 2 O Me Me + MeOH, 0 C 99% 3 ia ib Synthesis of alcohol i: To a solution of cyclobutene 3 (33.5 mg, 0.06 mmol) in MeOH (0.3 ml) was added CeCl 3 7H 2 O (43.5 mg, 0.7 mmol) and NaBH 4 (4.5 mg, 0.9 mmol) at 0 C. After h, the reaction was stopped by adding water. The products were extracted with Et 2 O (X3), and the combined organic extracts were washed with brine, dried (Na 2 SO 4 ), and concentrated in vacuo. The residue was purified by PTLC (hexane/ch 2 Cl 2 /Et 2 O = 50/25/25) to give alcohol i (33.3 mg, 98.8%) as a mixturer of diasteromers. Recrystallization from hexane gave ia as colorless prisms. Mp C. Me HO alcohol ia 0.49 (s, 3H), 0.54 (s, 3H), 0.90 (s, 9H),.32 (d, 3H, J = 6.8 Hz), 3.27 (s, 3H), 3.30 (s, 3H), 3.4 (s, 3H), 3.89 (d, H, J = 5.3 Hz), (m, H), 6.47 (d, H, J =.4 Hz); 3 2.9, 2.7, 9.0, 22.8, 26.5, 50.87, 50.9, 53.2, 64.2, 07.7, 08.3, 3.4, 6.6; IR (ATR) 350, 2938, 2893, 2857, 467, 436, 387, 359, 333, 277, 243, 93, 65, 080, 058, 009, 982, 905, 829, 780, 727 cm ; Anal. Calcd for C 5 H 30 O 5 Si: C, 56.57; H, Found: C, 56.65; H, #2 The stereochemistry of alcohol ia was determined by X-ray analysis shown below. [] [] CCDC (ia) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via S3

4 Figure. X-ray structure of ia Me HO alcohol ib 0.4 (s, 3H), 0.4 (s, 3H), 0.89 (s, 9H),.29 (d, 3H, J = 6.6 Hz), 3.26 (s, 3H), 3.30 (s, 3H), 3.40 (s, 3H), 3.95 (d, H, J = 5. Hz), (m, H), 6.44 (d, H, J =.4 Hz); 3 3.2, 2.9, 9., 23.0, 26.3, 5.0, 5.2, 52.3, 63.9, 06.9, 09.0, 30.7, 6.7; 3454, 2952, 2935, 2856, 2832, 473, 463, 438, 373, 36, 276, 253, 93, 64, 089, 058, 08, 989, 92, 894, 836, 779, 730 cm Anal. Calcd for C 5 H 30 O 5 Si: C, 56.57; H, Found: C, 56.80; H, Synthesis of cyclobutene 6a: According to the general procedure for synthesis of cyclobutene 3, ynone 5a [2] (24 mg, mmol) and KSA 2 (38 mg,.28 mmol) gave, after purification by silica-gel flash column chromatography (hexane/etoac = 9/), 6a (337 mg, 93.4%) as a yellow oil. Ph O cyclobutene 6a 0.08 (s, 3H), 0.6 (s, 3H), 0.89 (s, 9H), 3.39 (s, 3H), 3.42 (s, 3H), 3.47 (s, 3H), 7.2 (s, H), (m, 2H), (m, H), (m, 2H); 3 [2] Y. Maeda, N. Kakiuchi, S. Matsumura, T. Nishimura, T. Kawamura, S. Uemura, J. Org. Chem. 2002, 67, 678. S4

5 3.2, 3., 9.0, 26.3, 5.3, 5.4, 53.4, 08.2, 08.9, 29.6, 29.8, 34.2, 37.9, 45.3, 5.3, 90.4; 3066, 2937, 2904, 2856, 2835, 660, 598, 580, 472, 463, 448, 360, 38, 278, 256, 93, 50, 07, 090, 065, 03, 000, 94, 892, 874, 839, 80, 78, 72 cm ; Anal. Calcd for C 20 H 30 O 5 Si: C, 63.46; H, Found: C, 63.34; H, Synthesis of cyclobutene 6b: According to the general procedure for synthesis of cyclobutene 3, ynone 5b [2] (78.3 mg, 0.50 mmol) and KSA 2 (78 mg, 0.77 mmol) gave, after purification by silica-gel flash column chromatography (hexane/etoac = 9/), 6b (80 mg, 88.8%). Recrystallization from hexane gave 6b as yellow prisms. Mp C. Ph O cyclobutene 6b 0.3 (s, 3H), 0.8 (s, 3H), 0.93 (s, 9H), 3.38 (s, 3H), 3.4 (s, 3H), 3.48 (s, 3H), 7.4 (d, H, J = 5.8 Hz), (m, 3H), 7.54 (s, H), 7.7 (d, H, J = 5.8 Hz), (m, 2H); 3 3.2, 3., 9., 26.4, 5.3, 5.4, 53.3, 07.2, 08.6, 24.0, 29.5, 29.8, 3.6, 35.6, 44.3, 45.3, 53.2, 86.2; IR (ATR) 3062, 3026, 2937, 2903, 2856, 2834, 666, 636, 597, 576, 495, 47, 462, 450, 408, 388, 339, 277, 80, 093, 066, 00, 999, 92, 887, 838, 796, 78, 766, 727 cm ; Anal. Calcd for C 22 H 32 O 5 Si: C, 65.3; H, Found: C, 65.5; H, 8.4. Synthesis of cyclobutene 6c: According to the general procedure for synthesis of cyclobutene 3, ynone 5c [3] (53.5 mg, mmol) and KSA 2 (4 mg, mmol) gave, after purification by PTLC (hexane/etoac = 7/3), 6c (5 mg, 75.4%) as a colorless oil. O cyclobutene 6c 0.05 (s, 3H), 0.3 (s, 3H), 0.89 (s, 9H), (m, 4H), (m, 4H), 3.34 (s, 3H), 3.36 (s, 3H), 3.43 (s, 3H), 6.93 (s, H), (m, H); 3 3.4, 3., 8.9, 22.3, 22.5, 23.5, 26.3, 26.7, 5.6, 5.23, 53.4, 08.5, 08.7, 39.9, 4.8, 45.3, 52.0, 9.4; [3] E. Schroeder, M. Lehmann, I. Boettcher, Eur. J. Med. Chem. 979, 4, 309. S5

6 3068, 2935, 2857, 2834, 647, 637, 472, 460, 449, 437, 422, 388, 378, 360, 343, 306, 278, 248, 92, 39, 099, 064, 03, 998, 97, 927, 889, 838, 799, 78, 749, 706 cm ; Anal. Calcd for C 20 H 34 O 5 Si: C, 62.79; H, Found: C, 63.02; H, Synthesis of cyclobutene 6d: According to the general procedure for synthesis of cyclobutene 3, ynone 5d [4] (48 mg, mmol) and KSA 2 (323 mg,.30 mmol) gave, after purification by silica-gel flash column chromatography (hexane/etoac = 9/), 6d (377 mg, 97.6%) as a colorless oil. Ph O cyclobutene 6d 0.3 (s, 3H), 0.7 (s, 3H), 0.9 (s, 9H), 3.38 (s, 3H), 3.4 (s, 3H), 3.45 (s, 3H), (m, 3H), 7.69 (s, H), (m, 2H); 3 3.4, 3.3, 9., 26.2, 5.3, 5.5, 53.4, 86.9, 92.8, 06.6, 08., 20.3, 29.8, 32.2, 34., 49.9, 52.8, 73.3; 3065, 2938, 2856, 2836, 2200, 644, 597, 490, 47, 463, 443, 38, 273, 90, 093, 009, 896, 838, 798, 78, 758, 688 cm ; Anal. Calcd for C 22 H 30 O 5 Si: C, 65.64; H, 7.5. Found: C, 65.42; H, Synthesis of cyclobutene 6e: According to the general procedure for synthesis of cyclobutene 3, ynoate 5e [5] (40.0 mg, mmol) and KSA 2 (89.5 mg, mmol) gave, after purification by PTLC (hexane/etoac = 9/), 6e (0 mg, 93.5%) as a colorless oil. PhO O cyclobutene 6e 0.7 (s, 3H), 0.9 (s, 3H), 0.93 (s, 9H), 3.38 (s, 3H), 3.4 (s, 3H), 3.5 (s, 3H), (m, 2H), (m, H), (m, 2H), 7.54 (s, H); 3 3.3, 3., 9.0, 26.2, 5.3, 5.4, 53.0, 06.9, 08.6, 22.4, 26.9, 30.4, 45.8, 49., 5.2, 60.5; [4] D. H. Wadsworth, S. M. Geer, M. R. Detty, J. Org. Chem. 987, 52, [5] P. V. Ramachandran, M. T. Rudd, M. V. R. Reddy, Tetrahedron Lett. 2005, 46, [6] L. G. Beholz, P. Benovsky, D. L. Ward, N. S. Barta, J. R. Stille, J. Org. Chem. 997, 62, 033. S6

7 3075, 3044, 2938, 2897, 2857, 2836, 744, 69, 59, 492, 472, 463, 409, 389, 360, 275, 239, 92, 62, 094, 07, 052, 02, 902, 868, 839, 798, 78, 756, 733 cm ; Anal. Calcd for C 20 H 30 O 6 Si: C, 60.89; H, Found: C, 60.65; H, Synthesis of cyclobutene 8a: To a solution of ynoate 7a [5] (80. mg, mmol) in toluene (.0 ml) was added Me 3 Al (.03 M in hexane, 0.48 ml, 0.49 mmol) and KSA 2 (4 mg, mmol) in toluene (.0 ml) at 78 C. After warmed up to 20 C and further stirred for further 69 h, the reaction was stopped by adding sat. aq. NaHCO 3. The products were extracted with EtOAc and combined organic extracts were washed with brine, dried (Na 2 SO 4 ), and concentrated in vacuo. The residue was purified by PTLC (hexane/ch 2 Cl 2 /Et 2 O = 70/5/5) to give cyclobutene 8a (89.4 mg, 44.4%) and adduct 9a (57.5 mg, 28.6%). MeO 2 C cyclobutene 8a (colorless oil) 0.0 (s, 3H), 0.4 (s, 3H), 0.9 (s, 9H), (m, 4H), (m, 4H), 3.40 (s, 3H), 3.4 (s, 3H), 3.42 (s, 3H), 3.73 (s, 3H), (m, H); 3 3., 3.0, 9.0, 22.2, 23.0, 26.3, 26.5, 26.9, 5.4, 52.4, 52.5, 52.6, 06.7, 09.2, 32.8, 33.5, 37.5, 56.8, 63.6; 2936, 2857, 720, 627, 46, 434, 268, 245, 28, 99, 087, 025, 998, 952, 890, 836, 802, 779, 728 cm ; Anal. Calcd for C 2 H 36 O 6 Si: C, 6.3; H, Found: C, 6.; H, t-bume 2 SiO CO 2 Me MeO adduct 9a (colorless oil) (s, 3H), 0.2 (s, 3H), 0.90 (s, 9H), (m, 4H), (m, 4H), 3.38 (s, 3H), (s, 3H), 3.40 (s, 3H), 3.65 (s, 3H), (m, H); 3 3.4, 2.6, 8.8, 22.0, 22.8, 26.08, 26.3, 29.3, 5.85, 5.92, 52.0, 52.02, 84.2, 90., 97.3, 04.3, 20.6, 36.5, 67.5; 2935, 2857, 227, 75, 46, 434, 388, 36, 284, 249, 24, 26, 072, 08, 960, 937, 879, 840, 782, 77 cm ; Anal. Calcd for C 2 H 36 O 6 Si: C, 6.3; H, Found: C, 6.23; H, S7

8 Synthesis of cyclobutene 8b: According to the general procedure for synthesis of 8a, ynoate 7b [5] (90. mg, mmol), KSA 2 (47 mg, mmol), and Me 3 Al (.03 M in hexane, 0.50 ml, 0.5 mmol) gave, after purification by PTLC (hexane/etoac = 9/), cyclobutene 8b (05 mg, 48.7%) and adduct 9b (6.8 mg, 28.7%). EtO 2 C cyclobutene 8b (color less oil) 0.2 (s, 3H), 0.5 (s, 3H), 0.9 (s, 9H),.28 (t, 3H, J = 7.0 Hz), (m, 4H), (m, 2H), (m, 2H), (s, 3H), (s, 3H), 3.43 (s, 3H), 4.7 (qd, H, J = 7.0, J 2 =. Hz), 4.24 (qd, H, J = 7.0, J 2 =. Hz), (m, H); 3 3.0, 2.9, 4.5, 9.0, 22.2, 23.0, 26.3, 26.5, 27.0, 52.2, 52.6, 52.8, 60.9, 06.8, 09., 32.0, 34., 37., 56.4, 63.4; 2935, 2857, 2834, 76, 627, 46, 446, 388, 365, 307, 268, 245, 28, 9, 09, 029, 998, 952, 894, 836, 802, 779, 725 cm ; Anal. Calcd for C 22 H 38 O 6 Si: C, 6.94; H, Found: C, 6.7; H, 9.6. t-bume 2 SiO OEt CO 2 Me MeO adduct 9b (colorless oil) 0.8 (s, 3H), 0.20 (s, 3H), 0.90 (s, 9H),. (t, 3H, J = 7.0 Hz), (m, 4H), (m, 4H), 3.38 (s, 3H), 3.4 (s, 3H), 3.66 (s, 3H), 3.75 (q, H, J = 7.0 Hz), 3.75 (q, H, J = 7.0 Hz), (m, H); 3 3.4, 2.5, 5.6, 8.7, 22.0, 22.8, 26.07, 26., 29.3, 5.85, 5.88, 6., 84.8, 89.9, 96.9, 04.6, 20.8, 36.3, 67.7; 293, 2857, 227, 75, 46, 434, 388, 36, 284, 249, 2, 37, 06, 068, 08, 937, 890, 840, 779, 72 cm ; Anal. Calcd for C 22 H 38 O 6 Si: C, 6.94; H, Found: C, 6.70; H, 9.6. Synthesis of cyclobutene 8c: According to the general procedure for synthesis of cyclobutene 8a, ynoate 7c (47.0 mg, mmol), KSA 2 (72.0 mg, mmol), and Me 3 Al (.02 M in hexane, 0.24 ml, 0.24 mmol) gave, after purification by PTLC (hexane/etoac = 9/), cyclobutene 8c (63.9 mg, 59.3%) and adduct 9c (0.8 mg, 0.0%). S8

9 i-pro 2 C cyclobutene 8c (colorless oil) 0.4 (s, 3H), 0.6 (s, 3H), 0.9 (s, 9H),.27 (d, 3H, J = 6.4 Hz),.28 (d, 3H, J = 6.4 Hz), (m, 4H), (m, 2H), (m, 2H), (s, 3H), 3.40 (s, 3H), 3.43 (s, 3H), 5.08 (sept, H, J = 6.4 Hz), (m, H); 3 2.9, 2.8, 9.0, 2.9, 22.0, 22.2, 23.0, 26.4, 26.4, , 52.9, 68.7, 06.9, 09.0, 3.9, 34.5, 36.7, 55.9, 63.0; 2935, 2857, 2838, 72, 627, 465, 384, 357, 307, 268, 249, 222, 95, 09, 022, 998, 952, 92, 890, 836, 806, 779, 725 cm ; Anal. Calcd for C 23 H 40 O 6 Si: C, 62.69; H, 9.5. Found: C, 62.68; H, t-bume 2 SiO Oi-Pr CO 2 Me MeO adduct 9c (colorless oil) 0.23 (s, 3H), 0.24 (s, 3H), 0.90 (s, 9H),.6 (d, 6H, J = 6.4 Hz), (m, 4H), (m, 4H), (s, 3H), 3.40 (s, 3H), 3.65 (s, 3H), 4.28 (sept, H, J = 6.4 Hz), (m, H); 3 3., 2.4, 8.8, 22.0, 22.7, 23.5, 24.7, 26.0, 26.2, , 52.3, 68., 85.6, 88.7, 96.6, 04., 20.7, 36.2, 67.8; 2935, 2857, 227, 75, 627, 465, 434, 380, 36, 284, 249, 24, 4, 064, 04, 956, 97, 887, 836, 802, 779, 77 cm ; Anal. Calcd for C 23 H 40 O 6 Si: C, 62.69; H, 9.5. Found: C, 62.86; H, Synthesis of cyclobutene 8d: According to the general procedure for synthesis of cyclobutene 8a, alkyne 7d (8 mg, 0.52 mmol), KSA 2 (67 mg, mmol), and Me 3 Al (.03 M in hexane, 0.50 ml, 0.5 mmol) gave, after purification by PTLC (hexane/etoac = 8/2), 8d (82 mg, 73.5%) as a colorless oil. PhO 2 C 0.6 (s, 3H), 0.9 (s, 3H), 0.92 (s, 9H), (m, 4H), (m, 2H), (m, 2H), S9

10 3.455 (s, 3H), (s, 3H), 3.53 (s, 3H), (m, H), (m, 3H), (m, 2H); 3 3.0, 2.8, 9.0, 22., 23.0, 26.3, 26.6, 27.2, 52.3, 52.7, 53.0, 06.9, 09.2, 22.5, 26.8, 30.3, , 38.6, 5.3, 59.0, 6.6; IR (KBr) 3062, 2935, 2857, 73, 69, 592, 492, 249, 9, 64, 087, 08, 998, 952, 890, 836, 802, 779 cm ; Anal. Calcd for C 26 H 38 O 6 Si: C, 65.79; H, Found: C, 65.58; H, Synthesis of cyclobutene a: According to the general procedure for synthesis of cyclobutene 8a, ynoate 0a (03 mg, mmol), KSA 2 (53 mg, 0.66 mmol), and Me 3 Al (.03 M in hexane, 0.50 ml, 0.5 mmol) gave, after purification by PTLC (hexane/ch 2 Cl 2 /Et 2 O = 70/5/5), a (20 mg, 9.5%) as a colorless oil. PhO 2 C Bu cyclobutene a 0.7 (s, 3H), 0.9 (s, 3H), 0.9 (t, 3H, J = 7.3 Hz), 0.93 (s, 9H), (m, 2H), (m, 2H), 2.68 (t, 2H, J = 7.0 Hz), 3.45 (s, 3H), 3.47 (s, 3H), 3.50 (s, 3H), (m, 3H), (m, 2H); 3 3.2, 3.0, 3.9, 9.0, 23.4, 26.3, 28.3, 30.2, 5.5, 5.7, 52.9, 06.3, 08.6, 22.4, 26.7, 30.3, 37.3, 5., 6.2, 67.5; 3064, 2958, 2937, 2857, 737, 656, 592, 492, 463, 288, 247, 9, 074, 024, 937, 838, 78, 736 cm ; Anal. Calcd for C 24 H 38 O 6 Si: C, 63.97; H, Found: C, 63.73; H, PhO 2 C OCH 3 2 OCH 3 0 OCH 3 Key HMBC correlations position δh (ppm) carbon no. (ppm) C-2 (08.6) C- (06.3) C-2 (08.6) 3.47 C-2 (08.6) S0

11 Synthesis of cyclobutene b: According to the general procedure for synthesis of cyclobutene 8a, ynoate 0b (.09 g, 4.90 mmol), KSA 2 (.48 g, 5.96 mmol), and Me 3 Al (.05 M in hexane, 4.6 ml, 4.8 mmol) gave, after purification by silica-gel flash column chromatography (hexane/etoac = 9/), cyclobutene b (.88 g, 8.4%) as a colorless oil. PhO 2 C Ph cyclobutene b 0.9 (s, 3H), 0.24 (s, 3H), 0.96 (s, 9H), 3.43 (s, 3H), 3.44 (s, 3H), 3.6 (s, 3H), (m, 3H), (m, 5H), (m, 2H); 3 2.9, 2.7, 9.0, 26.3, 52.4, 52.6, 53.2, 07.0, 09.4, 22.5, 26.9, 29., 30.4, 30.6, 3.6, 32.0, 35.2, 5., 58.7, 6.4; 3068, 2938, 2856, 73, 63, 590, 490, 276, 93, 087, 022, 995, 894, 836, 78cm ; Anal. Calcd for C 26 H 34 O 6 Si: C, 66.35; H, Found: C, 66.2; H, Synthesis of cyclobutene c: According to the general procedure for synthesis of cyclobutene 8a, ynoate 0c (27 mg, mmol), KSA 2 (5 mg, mmol), and Me 3 Al (.03 M in hexane, 0.50 ml, 0.52 mmol) gave, after purification by PTLC (hexane/etoac = 7/3), cyclobutene c (209 mg, 85.3%) as a colorless oil. PhO 2 C Br cyclobutene c 0.6 (s, 3H), 0.9 (s, 3H), 0.92 (s, 9H), (m, 2H), (m, 2H), 3.37 (t, 2H, J = 7.0 Hz), 3.46 (s, 3H), 3.48 (s, 3H), 3.49 (s, 3H), (m, 2H), (m, H), (m, 2H); 3 3.2, 3.0, 6.9, 9.0, 26.3, 29.7, 32.0, 5.7, 5.9, 52.9, 06.3, 08.7, 22.5, 26.8, 30.3, 38.5, 5., 6., 65.2; 2937, 2903, 2856, 2835,738, 657, 592, 492, 472, 462, 388, 359, 283, 247, 92, 62, 09, 078, 04, 024, 943, , 780, 755, 739 cm ; Anal. Calcd for C 23 H 35 O 6 BrSi: C, 53.59; H, Found: C, 53.35; H, Synthesis of cyclobutene d: According to the general procedure for synthesis of cyclobutene 8a, ynoate 0d (00 mg, S

12 mmol), KSA 2 (0 mg, mmol), and Me 3 Al (.03 M in hexane, 0.35 ml, 0.36 mmol) gave, after purification by PTLC (hexane/etoac = 8/2), cyclobutene d (56 mg, 86.6%) as a colorless oil. PhO 2 C PMBO cyclobutene d 0.5 (s, 3H), 0.8 (s, 3H), 0.92 (s, 9H), 2.95 (td, H, J = 7.3, J 2 = 4.6 Hz), 2.98 (td, H, J = 7.3, J 2 = 4.6 Hz), 3.44 (s, 3H), 3.46 (s, 3H), 3.48 (s, 3H), 3.76 (s, 3H), 3.76 (t, 2H, J = 7.3 Hz), 4.45 (s, 2H), 6.87 (d, 2H, J = 8.7 Hz), (m, 2H), (m, 3H), (m, 2H); 3 3., 3.0, 9.0, 26.3, 29.4, 5.6, 5.7, 53.0, 55.4, 67.6, 72.8, 06.4, 08.4, 4.3, 22.5, 26.7, 29.9, 30.2, , 5., 60.0, 6., 63.7; 3066, 2935, 2900, 2857, 739, 658, 62, 592, 5, 492, 46, 36, 284, 249, 9, 083, 037, 94, 836, 782, 752 cm ; Anal. Calcd for C 30 H 42 O 8 Si: C, 64.49; H, Found: C, 64.30; H, Synthesis of cyclobutene e: According to the general procedure for synthesis of cyclobutene 8a, alkyne 0e (5 mg, mmol), KSA 2 (47 mg, mmol), and Me 3 Al (.03 M in hexane, 0.50 ml, 0.52 mmol) gave, after purification by PTLC (hexane/etoac = 8/2), cyclobutene e (245 mg, 9.9%) as a colorless oil. PhO 2 C PMBO cyclobutene e 0.6 (s, 3H), 0.8 (s, 3H), 0.92 (s, 9H), (m, 2H), (m, 2H), 3.44 (s, 3H), 3.46 (s, 3H), 3.48 (s, 3H), 3.5 (t, 2H, J = 6.4 Hz), 3.76 (s, 3H), 4.40 (s, 2H), 6.84 (d, 2H, J = 8.7 Hz), (m, 2H), 7.22 (d, 2H, J = 8.7 Hz), (m, H), (m, 2H); 3 3.2, 3.0, 9.0, 25.6, 26.3, 28.5, 5.6, 5.8, 52.9, 55.4, 70.0, 72.9, 06.4, 08.6, 4.3, 22.5, 26.7, 29.8, 30.3, , 5.2, 60.0, 6.2, 67.0; 2936, 2855, 736, 655, 62, 59, 53, 492, 463, 388, 360, 287, 247, 90, 078, 024, 937, 838, 780, 755 cm ; Anal. Calcd for C 3 H 44 O 8 Si: C, 65.0; H, Found: C, 64.79; H, S2

13 Scheme 2. Preparation of ynoate 0d. CO 2 Ph n-buli ClCO 2 Ph PMBO ii THF, 78 C 88% PMBO 0d Synthesis of alkyne 0d: To a mixture of alkyne ii (2.45 g, 2.9 mmol) in THF (60 ml) was added n-buli (.60 M in hexane, 8.5 ml, 4 mmol) at 78 C. After h, ClCO 2 Ph (2.00 g, 2.8 mmol) in THF (5 ml) was added to the mixture, and the reaction was stirred for further h. The reaction was stopped by adding sat. NH 4 Cl. The products were extracted with EtOAc and combined organic extracts were washed with brine, dried (Na 2 SO 4 ), and concentrated in vacuo. The residue was purified by silica-gel flash column chromatography (hexane/etoac = 9/) to give ynoate 0d (3.50 g, 87.6%) as a colorless oil. CO 2 Ph PMBO alkyne 0d H NMR (CDCl 3, δ) 2.69 (t, 2H, J = 7.3 Hz), 3.65 (t, 2H, J = 7.3 Hz), 3.80 (s, 3H), 4.5 (s, 2H), (m, 2H), (m, 2H), (m, 3H), (m, 2H); 3 C NMR (CDCl 3, δ) 20.4, 55.3, 66.5, 72.8, 73.5, 88.8, 3.9, 2.4, 26.3, 29.4, 29.5, 29.7, 50., 5.8, 59.4; 3064, 304, 3002, 2955, 2935, 293, 2838, 2865, 2236, 728, 62, 589, 53, 49, 457, 42, 362, 328, 302, 234, 90, 6, 097, 043, 004, 949, 93, 822, 745 cm ; Anal. Calcd for C 9 H 8 O 4 : C, 73.53; H, Found: C, 73.29; H, 6.5. Scheme 3. Preparation of ynoate 0c and 0e. CO 2 Ph CO 2 Ph CO 2 Ph CO 2 Ph PMBO iii as above 88% PMBO 0e DDQ, H 2 O MsCl, Et 3 N CH 2 Cl 2 CH 2 Cl 2, 0 C 78% 79% HO iv MsO v LiBr acetone reflux 95% Br 0c Synthesis of alkyne 0e: According to the general procedure for synthesis of alkyne 0d, alkyne iii (.3 g, 6.4 mmol), S3

14 ClCO 2 Ph (.2 g, 7.5 mmol), and n-buli (.60 M in hexane, 4. ml, 6.6 mmol) gave, after purification by silica-gel flash column chromatography (hexane/etoac = 9/), alkyne 0e (.83 g, 88.0%) as a colorless oil. CO 2 Ph PMBO alkyne 0e H NMR (CDCl 3, δ).89 (tt, 2H, J = 6., J 2 = 7.0 Hz), 2.52 (t, 2H, J = 7.0 Hz), 3.55 (t, 2H, J = 6. Hz), 3.79 (s, 3H), 4.45 (s, 2H), (m, 2H), (m, 2H), (m, 3H), (m, 2H); 3 C NMR (CDCl 3, δ) 5.8, 27.7, 55.3, 67.9, 72.7, 72.9, 9.6, 3.8, 2.4, 26.3, 29.3, 29.5, 30.3, 50., 52.0, 59.2; 3063, 304, 300, 2954, 2934, 2858, 2230, 728, 62, 590, 53, 490, 457, 442, 363, 30, 232, 90, 62, 02, 063, 038, 00, 95, 820, 74 cm ; Anal. Calcd for C 20 H 20 O 4 : C, 74.06; H, 6.2. Found: C, 74.27; H, Synthesis of alkyne iv: To a solution of ester 0e (.2 g, 3.45 mmol) in CH 2 Cl 2 (2 ml) and H 2 O ( ml) was added DDQ (82 mg, 3.57 mmol) at 0 C. After warmed to room temperature, and stirred for further 2 h, the reaction was stopped by adding sat. aq. NaHCO 3. The products were extracted with EtOAc and combined organic extracts were washed with brine, dried (Na 2 SO 4 ), and concentrated in vacuo. The residue was purified by silica-gel flash column chromatography (hexane/etoac = 7/3) to give alcohol iv (547 mg, 77.6%) as a colorless oil. CO 2 Ph HO alcohol iv H NMR (CDCl 3, δ).88 (quint, 2H, J = 7.0 Hz), 2.55 (t, 2H, J = 7.0 Hz), 3.55 (t, H, J = 6.0 Hz), 3.78 (dt, 2H, J = 6.0, J 2 = 7.0 Hz), (m, 2H), (m, H), (m, 2H); 3 C NMR (CDCl 3, δ) 5.2, 30.0, 60.8, 72.8, 9.6, 2.3, 26.3, 29.5, 49.9, 52.0; 3495, 2954, 288, 2232, 728, 590, 490, 457, 425, 326, 235, 90, 6, 047, 00, 95, 838, 742 cm ; Anal. Calcd for C 2 H 2 O 3 : C, 70.57; H, Found: C, 70.42; H, S4

15 Synthesis of mesylate v To a mixture of alcohol iv (43 mg, mmol) and Et 3 N (2 mg,.20 mmol) in CH 2 Cl 2 (3 ml) was added MsCl (9 mg, mmol) in CH 2 Cl 2 ( ml) at 0 C. After 5 min, the reaction was stopped by adding water. The products were extracted with EtOAc and combined organic extracts were washed with brine, dried (Na 2 SO 4 ), and concentrated in vacuo. The residue was purified by silica-gel flash column chromatography (hexane/etoac = 5/5) to give mesylate v (57 mg, 79.4%) as a colorless oil. CO 2 Ph MsO mesylate v H NMR (CDCl 3, δ).89 (quint, 2H, J = 6.3 Hz), 2.60 (t, 2H, J = 6.3 Hz), 3.06 (s, 3H), 4.36 (t, 2H, J = 6.3 Hz), (m, 2H), (m, H), (m, 2H); 3 C NMR (CDCl 3, δ) 5.3, 27.0, 37.4, 67.7, 73.8, 89.3, 2.4, 26.4, 29.6, 50.0, 5.7; 3029, 2940, 2235, 727, 590, 489, 355, 236, 90, 75, 972, 929, 832, 748 cm ; Anal. Calcd for C 3 H 4 O 5 S: C, 55.3; H, Found: C, 55.5; H, 5.2. Synthesis of alkyne 0c To a solution of mesylate v (99.2 mg, 0.35 mmol) in acetone (20 ml) was added LiBr (52 mg,.75 mmol) at 0 C. The mixture was heated under reflux conditions for h. The reaction was stopped by adding water, and the products were extracted with EtOAc and combined organic extracts were washed with brine, dried (Na 2 SO 4 ), and concentrated in vacuo. The residue was purified by silica-gel flash column chromatography (hexane/etoac = 7/3) to give bromide 0c (89. mg, 94.9%) as a colorless oil. CO 2 Ph Br ynoate 0c H NMR (CDCl 3, δ) 2.5 (quint, 2H, J = 6.5 Hz), 2.62 (t, 2H, J = 6.5 Hz), 3.52 (t, 2H, J = 6.5 Hz), (m, 2H), (m, H), (m, 2H); 3 C NMR (CDCl 3, δ) 7.5, 30.2, 3.6, 73.5, 89.8, 2.4, 26.4, 29.6, 50., 5.8; 3064, 2965, 2232, 728, 590, 489, 456, 435, 233, 90, 6, 07, 038, 002, 740 cm ; S5

16 Anal. Calcd for C 2 H BrO 2 : C, 53.96; H, 4.5. Found: C, 54.06; H, Synthesis of cyclobutenone 2: To a solution of cyclobutene b (456 mg, mmol) in CH 2 Cl 2 (7.0 ml) and H 2 O (0.5 ml) was added BF 3 OEt 2 (45 mg, 3.9 mmol) in CH 2 Cl 2 (3.0 ml) at 78 C. After the mixture was warmed to 0 C and stirred for further 0.5 h, the reaction was quenched by adding sat. aq. NaHCO 3. The products were extracted with EtOAc ( 3), and the combined organic extracts were washed with brine, dried (Na 2 SO 4 ), and concentrated in vacuo. The residue was purified by silica-gel flash column chromatography (hexane/etoac = 20/) to give cyclobutenone 2 (266 mg, 84.7%). Recrystallization from Et 2 O gave 2 as colorless prisms. PhO O Ph O cyclobutenone (s, 6H), (m, 3H), (m, 2H), (m, 2H), (m, H), (m, 2H); , 8.0, 22.5, 27., 29.2, 29.9, 30.3, 32.5, 35.7, 38., 5.2, 58.9, 83., 88.8; IR (ATR) 3066, 3000, 2942, 2838, 774, 735, 585, 488, 450, 346, 303, 253, 95, 4, 03, 056, 995, 92, 898, 840, 798, 77, 725 cm ; Anal. Calcd for C 9 H 6 O 5 : C, 70.36; H, Found: C, 70.6; H, Figure 2. X-ray structure of 2 [7] [7] CCDC (2) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via S6

17 Synthesis of cyclobutene 3: To a solution of cyclobutene b (50 mg, 0.39 mmol) in THF (5.0 ml) was added LiAlH 4 (20 mg, 0.52 mmol) at 0 C. After 5 min, the reaction was quenched by adding sat. aq. NaHCO 3. The products were extracted with EtOAc ( 3), and the combined organic extracts were washed with brine, dried (Na 2 SO 4 ), and concentrated in vacuo. The residue was purified by PTLC (hexane/etoac = 7/3) to give cyclobutene 3 (06 mg, 87.4%) as a colorless oil. HO Ph cyclobutene (s, 6H), 0.92 (s, 9H), 3.35 (s, 3H), 3.47 (s, 6H), (m, H), (m, 2H), (m, 3H), 7, (m, 2H); , 2.89, 9.0, 26.3, 52.2, 52.7, 55.7, 07.4, 09.8, 28.9, 29.0, 29.4, 33.9, 45.8, 48.4; 3448, 3056, 2937, 2856, 2834, 656, 573, 494, 463, 274, 253, 2, 78, 39, 085, 002, 993, 92, 892, 869, 836, 779 cm ; Anal. Calcd for C 20 H 32 O 5 Si: C, 63.2; H, Found: C, 62.90; H, Synthesis of cyclobutenone 4: To a solution of cyclobutene 3 (88.0 mg, 0.23 mmol) in CH 3 CN (3.0 ml) was added sat. aq. KF (0. ml) and n-bu 4 NCl (4.8 mg, mmol) at room temperature. After 24 h, the reaction was stopped by adding water. The products were extracted with EtOAc ( 3), and the combined organic extracts were washed with brine, dried (Na 2 SO 4 ), and concentrated in vacuo. The residue was purified by PTLC (hexane/etoac = 5/5) to give cyclobutenone 4 (47.2 mg, 87.2%) as a pale yellow oil. HO O Ph cyclobutenone (s, 6H), (m, 3H), (m, 3H), (m, 2H); , 8., 29.6, 30.4, 30.5, 32.6, 52.8, 74.8, 93.9; 3472, 3064, 2998, 2944, 2837, 754, 63, 573, 494, 448, 346, 337, 37, 302, 254, 205, 7, 096, 060, 026, 000, 928, 888, 77, 74 cm ; Anal. Calcd for C 3 H 4 O 4 : C, 66.66; H, Found: C, 66.42; H, Synthesis of cyclobutenedione 5: To a solution of cyclobutene 4 (37.3 mg, 0.59 mmol) in CH 2 Cl 2 (.5 ml) and H 2 O (0.05 ml) was added BF 3 OEt 2 (0.05 ml, 0.40 mmol) at 0 C. After 0.5 h, the reaction was quenched by adding S7

18 sat. aq. NaHCO 3. The products were extracted with EtOAc ( 3), and the combined organic extracts were washed with brine, dried (Na 2 SO 4 ), and concentrated in vacuo. The residue was purified by silica-gel flash column chromatography (hexane/etoac = 5/5) to give cyclobutenedione 5 (23.7 mg, 79.%). Recrystallization from hexane/et 2 O gave 5 as yellow needles. Mp C. HO O Ph O cyclobutenedione (br s, H), 5.0 (s, 2H), (m, 3H), (m, 2H); , 29., 30.0, 30.8, 34.2, 90.5, 96.6, 96.7, 97.8; IR (ATR) 3472, 3065, 2894, 780, 76, 597, 58, 568, 49, 448, 388, 334, 32, 294, 24, 84, 54, 05, 085, 056, 999, 94, 873, 84, 764 cm ; Anal. Calcd for C H 8 O 3 : C, 70.2; H, Found: C, 69.99; H, S8

19 S9

20 S20

21 S2

22 S22

23 S23

24 S24

25 S25

26 S26

27 S27

28 S28

29 S29

30 S30

31 S3

32 S32

33 S33

34 S34