Cerium enolates synthesis

Aldol Additions to Ketones. Traditionally, cerium enolates or the Reformatsky-type reaction have been employed to achieve high-yielding aldol additions to enolizable ketones. In this regard, methyl trichlorosilyl ketene acetal provides a reliable alternative for the synthesis of tertiary -hydroxy esters. In the absence of a Lewis base promoter, the aldol additions of 1 to ketones are too slow to be synthetically useful. On the contrary, with pyridine A-oxide as catalyst, methyl trichlorosilyl ketene acetal reacts smoothly with nearly all classes of ketones (7) (Scheme 1). Good yields of the tertiary alcohol products (8) are obtained (eq 4), table 2 from aromatic (entries 1-2 and 4—6), hetereoaromatic (entry 3), olefinic (entries 7-8), acetylenic (entries 9-10), and aliphatic (entries 11-14) ketones. The only poorly performing substrate is 2-tetralone (7o), which affords a 45% yield of the addition product and returns 45% of unreacted starting material, most likely from competitive enolization. 

Salts of other transition metals including vanadium, cerium, chromium and manganese have been used for a-oxygenation, although rarely applied in synthesis. Manganese triacetate has been used for the efficient a -oxidation of enones (Section 2.3.2.2.1.i), but appears not to have been used for the a-hydrox-ylation of saturated ketones des]Hte its known ability to form the corresponding a-keto radicals. Similarly the use of Lewis acid assisted enolization in the oxidative process appears to have been limited to the LTA-mediated examples. 

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