Simple and fast ruthenium catalyzed direct aldol reaction: scope and limitations

An asymmetric aldol reaction is one of the most common methods for carbon-carbon bond formation in organic molecules. It is extensively applied in the synthesis of carbohydrates, amino sugares, steroids, and other valuable chiral organic compounds. It provides an atom-economic approach to β-hydroxyl carbonyls, which make up a large family of chiral intermediates for the synthesis of biologically active substances and natural products. In continuation of our research in environmentally benign and microwave assisted conditions, we now report an operationally simple and fast method for the preparation of aldol products by one-pot condition reaction. In this research, the direct aldol reaction of ketones with aromatic aldehydes using catalytic amount of RuCl₃.nH₂O and chiral ligand proceeded smoothly under extremely mild conditions to give the corresponding aldol adducts in good yields with moderate to good diastereoselectivity. The system can efficiently proceed at room temperature with a small excess of ketone. From a practical and economic point of view this reaction condition is convenient with respect to most of other protocols where a large excess of ketone is used. Also, microwave technique offers simple, clean, fast, efficient, and economic for the synthesis of a large number of aldol adducts. All the synthesized compounds are confirmed by spectroscopy methods such as ¹H-NMR, ¹³C-NMR and IR. Important advantages of this thechnology include highly accelerated rate of the reaction, reduction in reaction time with an improvement in the yield and quality of the products.