Lithium hexamethyldisilazane enolate formation

Typically, nonstabilized ylides are utilized for the synthesis of (20-alkenes. In 1986, Schlosser published a paper sununarizing the factors that enhance (Z)-selectivity." Salt effects have historically been defined as the response to the presence of soluble lithium salts. Any soluble salt will compromise die (Z)-selectivity of the reaction, and typically this issue has been resolved by the use of sodium amide or so um or p< sium hexamethyldisilazane (NaHMDS or KHMDS) as the base. Solvent effects are also vital to the stereoselectivity. In general, ethereal solvents such as THF, diethyl ether, DME and /-butyl methyl ether are the solvents of choice." In cases where competitive enolate formation is problematic, toluene may be utilized. Protic solvents, such as alcohols, as well as DMSO, should be avoided in attempts to maximize (Z)-selectivity. Finally, the dropwise addition of the carbonyl to the ylide should be carried out at low temperature (-78 C). Recent applications of i osphonium ylides in natural product synthesis have been extensively reviewed by Maryanoff and Reitz. 

---