Tritylketone und Tritylenone. Beiträge zur sterisch erzwungenen Michael‐Addition und zur diastereoselektiven Aldol‐Addition

Tritylketones and Tritylenones. Contributions to the Sterically Enforced Michael Addition and to the Diastereoselective Aldol AdditionTritylketones are prepared from trityllithium and aldehydes, with subsequent CrO3 oxidation (Scheme 1, 2a‐f). Tritylenones are obtained from the saturated ketones and aldehydes or ketones, preferably by (CH3)3Al‐mediated aldol addition with subsequent dehydration (Scheme 2, products 2e, 4b‐e). The carbonyl of the tritylketone group is sterically protected, but electronically effective (see A‐C); thus, amine‐free enolate solutions can be obtained directly with BuLi; also, exclusive conjugate addition of organolithium derivatives occurs with tritylenones (Schemes 3‐5, products 2d, 5‐7, 15 examples). The lithiumenolates of tritylketones add to aldehydes with practically complete stereoselectivity (Scheme 6, products 10, 9 examples): due to the bulkiness of the trityl group, only the (Z)‐enolates 8 are formed, and the approach of the two trigonal centers in the aldol‐addition step is enforced to occur with relative topicity ul. As a first example of an X‐ray structure determination of a silyl enol ether, the crystal structure of (Z)‐2‐(trimethylsilyloxy)‐1, 1, 1‐triphenyl‐2‐butene (9) is reported. Fortunately, the blocking of the carbonyl group in trityl ketones can be run very specifically (without epimerization at the α‐carbonyl center) by lithium triethylborohydride to furnish, after aqueous workup, primary alcohols and Ph3CH (Eqn. 4, products 13–20, 10 examples); the OH‐group of the aldols 10 must be EE‐, MOM‐ or MEM‐protected (Scheme 8) before this C,C‐bond cleavage can be conveyed. Some of the cleavage products are used for the chemical correlation of the aldol configuration and for the demonstration of the synthetic value of the presented method (see 21–29).