We initially screened several solvents and additives with (S)-proline as catalyst (30 mol%) at room temperature to find appropriate reaction conditions for the cyclization of the aldehyde-ketone 4a [17] to favor the desired aldol reaction to the dihydrobenzofuranol 5a over the competing condensation reaction. DMF (0.1 M) as solvent gave the best results (91% yield) in combination with an excellent diastereoselectivity of 94% and an enantiomeric excess of 76%. The diastereo- and ...
