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α-DMS-esters 1 can be easily prepared from the corresponding esters by deprotonation with LiNi-Pr2 followed by silylation with Me2SiHCl. See:
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Typical Procedure for the Preparation of α-DMS-esters 1
Under N2 atmosphere, n-BuLi (1.61 M in hexane, 62 mL, 100 mmol) was added to a solution of i-Pr2NH (14 mL, 100 mmol) in THF (100 mL) over 5 min at 0 °C. After 10 min, the mixture was cooled to -78 °C. Then, EtOAc (9.3 mL, 95 mmol) was added to the solution of LDA over 5 min. After 2 h, the reaction mixture was treated with chlorodimethyl-silane (12.2 mL, 110 mmol) and gradually warmed to r.t. over 12 h. The resultant mixture was diluted with dry pentane (50 mL) and filtered through Celite®. After evaporation of the filtrate, the residual oil was diluted with dry pentane (50 mL) again, filtered through Celite®, and evaporated. Purification of the crude product by distillation gave 1a (9.2 g, 63 mmol) in 66% yield.
Compound 1a: bp 58-60 °C (180 Torr). IR (neat): 1669 (C=O), 1253, 1205 cm-1. 1H NMR (CDCl3): δ = 0.20 (d, J = 3.6 Hz, 6 H), 1.23 (t, J = 6.9 Hz, 3 H), 1.96 (d, J = 3.3 Hz, 2 H), 4.06 (sept, d, J = 3.6, 3.3 Hz, 1 H), 4.10 (q, J = 6.9 Hz, 2 H). 13C NMR (CDCl3): δ = -4.36 (CH3 × 2), 14.09 (CH3), 24.08 (CH2), 59.91 (CH2), 172.54 (C). Anal. Calcd for C6H14O2Si (%): C, 49.53; H, 9.69. Found: C, 49.27; H, 9.65.
12
General Procedure for the Aldol Reaction of 1 with Aldehydes 2
Under the atmosphere, dry LiCl (5.5 mg, 0.13 mmol) was added to a two-necked, round-bottomed flask (10 mL), which was connected with a nitrogen balloon. After introduction of nitrogen, DMF (1.0 mL) was added to the flask. The mixture was warmed to 30 °C under stirring. After 10 min, 2 (0.50 mmol) and 1 (0.60 mmol) were added to the mixture. After being stirred for 5 h, the reaction mixture was treated with 2 M aq HCl (1 mL) for 5 min and neutralized with sat. aq NaHCO3. The aqueous mixture was extracted with EtOAc (3 × 10 mL). The extract was dried over Na2SO4 and evaporated. The crude product was purified by silica gel column chromatography.
The Reformatsky reaction of ethyl bromoacetate with 5g shows much lower stereoselectivity toward equatorial attack. See:
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18 The bond dissociation energy of Si-Cl in Me3SiCl (472 kJ/mol) is larger than that of Si-Br in Me3SiBr (402 kJ/mol). See: Walsh R. In The Chemistry of Organic Silicon Compounds
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19 As shown in Table
[3]
, the reaction of 1a with 8a proceeded efficiently irrespective of the metal chloride used. This observation may be due to higher coordinating ability (Lewis basicity) of α-enones, which allows carbonyl activation even with less Lewis acidic metal ions. For the coordinating ability of α-enones, see ref. 17a.