Diastereoselective Lewis Acid Mediated Reductions of a-Alkyl-b-Functionalized Carbonyl Compounds

A high stereochemical induction has often been observed in the reduction of ketones with stereogenic centres close to the carbonyl function. The level of stereoselectivity is particularly high when “hard” donor groups such as nitrogen or oxygen atoms are present in the molecule. These results have been commonly explained in terms of a chelate-controlled process. However, this interpretation has recently been challenged, and in many cases an open-chain mechanism can better account for the observed results. Thus, Lewis-acid-mediated chelation and nonchelation control is one of the most fundamental and practical concepts in transferring a nucleophile moiety to carbonyl compounds. In order to clarify the role of a hard Lewis acid in controlling the stereochemical outcome of a given reaction, the Lewis acid mediated reduction of a series of α -alkyl-β -functionalized carbonyl compounds with metallic hydrides in various solvents was recently investigated. In particular, the results derived from the reductions in the presence of dry CeCl3 were compared to those obtained under the same experimental conditions but in the presence of TiCl4 , whose ability to form chelation com-plexes is well established. These investigations show a stereochemical outcome that is fully consistent with a chelation-controlled pathway in the case of titanium, andan open-chain-controlled pathway in the case of CeCl3 . The strongly chelating TiCl4 led to the syn isomer in high diastereomeric excess in noncoordinating solvents at –78 °C with borane–Lewis base complex reducing agents, while nonchelating CeCl3 produced a high excess of the anti isomer in coordinat-ing solvents at the same temperature with metal borohyd-rides as reducing agent. Therefore, our method based on the choice of CeCl3 or TiCl4 as a Lewis acid under the reported conditions represents a significant contribution to the development of new stereoselective reductions of α -alkyl-β -func-tionalized ketones. These procedures are one of the best known ways to obtain an alcoholic moiety.