Lombardo reagent

Hydroboration occurred from the less hindered top face of rac-29 and resulted in the formation of alcohol rac-30. After a three-step sequence which included oxidation with tetrapropylammonium perruthenate (TPAP), methyl lithium addition and repeated oxidation with TPAP, ketone rac-31 was isolated. Finally, epimerization of the stereogenic center at C-7 to the correct configuration and methylenation with the Lombardo reagent led to the formation of racemic kelsoene (rac-1). 

Notes Original report L. Lombardo, Tetrahedron Letters 1982, 23, 4293 Sometimes called the Oshima-Lombardo Reagent. 

The advantage of the Takai-Lombardo reagents is the possibility of transferring substituted alky-... 

Scheme 4. Takai-Lombardo reagents TMEDA= N.iV,Af, N -tetramethyl-ethylenediamine.

Another reagent for the methylenation of aldehydes or ketones is the Oshima-Lombardo reagent TiCl4-Zn-CH2l2 (or CH2Br2). This reagent is non-basic and can therefore be advantageous for base-sensitive substrates (2.105). 

The Lombardo reagent is usually prepared by addition of TiCU to a precooled suspension of zinc dust in a solution of dibromomethane in dry THF at very low temperature (e.g., -40°C), and aged for 2 days at 4°C before the methylenation." Different from the Wittig Reaction, the Lombardo methylenation is highly selective and occurs under nonbasic conditions. This reaction is especially valuable for the methylenation of keto ester, for which the Wittig Reaction fails, " and for the olefination of /3,y-unsaturated enones without enolization. Thus the Lombardo methylenation is an alternative olefination method when Wittig Reaction or Tebbe Olefination is not applicable. However, it also... 

A 10 mL 1.0 M solution of TiCU in CH2CI2 (10 mmol) was added dropwise over 10 min to a vigorously stirred suspension of 2.87 g activated zinc dust (44.0 mmol) and dibromethane (1.01 mL, 2.43 g, 14.0 mmol) in 25 mL THF at —40°C. The mixture was warmed to 0°C, and then stirred in a cold room at 0°C for 18 h. A portion (0.5 mL) of this Lombardo reagent was added dropwise over 5 min to a solution of 10 mg (5b, 13a)- and (5a,13a)-8-methyl-18-nor-16-oxaandrostan-4,17-dione (0.0344 mmol) in 1 mL CH2CI2 at 0°C, and the reaction was then stirred at 0°C for 30 min. The mixture was poured into 15 mL saturated aqueous NaHCOs and then extracted with 20 mL Et20. The separated organic phase was washed with 20 mL water, and both separated aqueous layers were then... 

---