Selektive Herstellung von l- oder u-Aldolen aus Äthyl-trityl-keton und aromatischen Aldehyden über Lithium- bzw. Aluminium-Enolate

Selective Preparation of l- or u-Aldols from Ethyl Trityl Ketone and Aromatic Aldehydes through Lithium and Aluminium Enolates, Respectively The ethyl trityl ketone ( 1 ) is deprotonated to the (Z)-enolate 2 which adds to aromatic aldehydes with relative topicity ul to give the aldols 3 (?78°, THF, kinetic control). If, on the other hand, 1 is heated with trimethylaluminium in toluene, the aluminium enolate formed combines with aromatic aldehydes and with cinnamic aldehyde with opposite relative topicity (lk) to the aldols 5 (+20°, toluene, 2 d, precipitation of the aluminium aldolate 4 , thermodynamic control). The mechanism is discussed (cf. Fig. 1). The adducts to benzaldehyde ( 3a , 5a ), furfural ( 3f , 5f ), and cinnamaldehyde ( 3i , 5i ) are O-(2-methoxyethoxy)methyl (MEM) protected and cleaved by lithium triethylborohydride to the 1,3-diol derivatives 9–11 with a free primary and a protected secondary OH-group. Analogous conversions of 1 and other ketones may also be diastereoselective, but turned out to be of less preparative value (see 6–8 , 12 ).