L -Prolinal Dithioacetal: A Highly Effective Organocatalyst for the Direct Nitro-Michael Addition to Selected Cyclic and Aromatic Ketones

The synthesis of novel l -prolinal dithioacetal and its application as an organocatalyst for the direct Michael addition of cyclic ketones and acetophenone derivatives to trans -β-nitrostyrene and related compounds is described. The prolinal dithioacetal acts as effective catalyst in the case of cyclic ketones of different ring size, in particular five- and six-membered examples, as well as larger and smalle r ring systems. High enantioselectivity and diastereoselectivity is observed for different substrates and trans -β-nitrostyrenes. Also, the first asymmetric syntheses of selected 2-methyl-4-nitro-1,3-diphenylbutan-1-one derivatives by application of the obtained organocatalyst is presented.