Stereoselective Routes to Conjugated Enynes

To a solution of TBDMSCHgCtaCSiMej (2mmol, preparation given, but not detailed) m THF (6ml), cooled to -78°C, was added dropwise, with stirring, a solution of t-BuLi in pentane (2 mmol, 1.8 m). After 1 h at -78 °C, a freshly prepared solution of MgBr2 (2.2 mmol, from Mg and 

BrCH2CH2Br) in ether (6 ml) was added, and stirring was continued for a further 15 min. The carbonyl compound was then added, and after 5 min the solution was warmed slowly to 50 °C and stirred at this temperature for 2-3 h to complete elimination. After cooling, water was added, the solution was extracted repeatedly with ether, and the combined extracts were washed with dilute aqueous HC1 and water, and dried. Concentration followed by chromatography on silica gel gave the product (65-90%). 

TIPSCHjC CTIPS (preparation given, but not detailed) was metallated (21) by treatment with n-BuLi in hexane (1 eq.) in THF at —20 °C for 15 min. Addition of cyclohexane carboxaldehyde (1 eq.) to the anion in THF at -78 °C was followed by gradual warming to ambient temperature over 6h. Normal extraction procedures followed by chromatography on silica gel afforded the (Z)-enyne, 71%, 20 1 (Z) (E). 

Heptanal (57%) and 2,2-dimethylpropanal (79%) give similar stereoselectivity. 

Alternatively (21), when the same procedure was followed using THF containing 5eq. HMPA (CAUTION—CANCER SUSPECT AGENT) (relative to the aldehyde) at — 78 °C for 15-20 s, followed by quenching at —78 °C and isolation, the (E)-enynes were obtained, (E) (Z) 20 1 to 10 1, 60-64%. 

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