Chelation vs. non-chelation control in addition reactions of ethylmetallic reagents to acrolein dimer

Abstract

Reactions of 3,4-dihydro-2H-pyran-2-carboxaldehyde (acrolein dimer) with ethylmetallics based on lithium, titanium, and copper were examined for stereoselectivity. High selectivity (92:8) was found for non-chelation controlled addition yielding the erythro product when ethyllithium was employed in the presence of BF3•OEt2. High selectivity (89:11) for products of chelation controlled addition (threo) was observed for ethylcopper reagents in the presence of magnesium salts. Titanium reagents yielded mixtures rich in products of either chelation or non-chelation controlled addition depending on the titanium substituents. In no cases were stereoselectivities obtained with titanium reagents better than those obtained by use of the aforementioned lithium and copper reagents.