Reactions of 3, 4-dihydro-2 H-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 BF3OEt2. High selectivity (89: 11) for products of chelation controlled addition (threo) was observed for ethylcopper reagents in the ...