The reaction of ketones with an excess of (CH,),TiCI, (6) leads to the replacement of the carbonyl oxygen atom by two methyl groups. This mild method of direct geminal dimethylation involves Grignard-type addition followed by formation of tertiary carbocations which are captured by methyltitanium species. Additional functional groups such as primary alkyl chlorides, thioethers, aromatics, ethers, and esters are tolerated, but not thioketals. The procedure has been applied to the synthesis of ( f )-cuparene (44). Similarly, carboxylic acid chlorides are converted to fert-butyl derivatives. The geminal dimethyl structural unit occurs frequently in terpenes, steroids and compounds of theoretical interest. Compounds containing these and other types of quarternary carbon atoms are generally prepared via multistep syntheses using a variety of techniques l). An attractive strategy calls for geminal dimethylation of ketones, i. e., the position specific replacement of the carbonyl oxygen by two methyl groups. This can be accomplished by a three step sequence 1 + 2 + 3 -+ 4 involving Wittig olefination, Simmons-Smith cyclopropanation and hydrogenolysisz). Although this sequence has been applied successfully in several instances z), we sought a direct method. Previously, it had been shown that certain ketones are geminal dimethylated by a 3 -4 fold excess of AI(CH,), in a closed vessel at 120 -180°C3). Direkte geminale Dimethylierung von

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The preparation of chiral Lewis acids (I) ‐ (III) which have bidentate ligands is described.

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