Selective Michael−Aldol Reaction by Use of Sterically Hindered Aluminum Aryloxides as Lewis Acids:  An Easy Approach to Cyclobutane Amino Acids

A formal [2 + 2] cycloaddition of 2-amidoacrylates with monosubstituted donor olefins, including its asymmetric version, is described. The stereoselectivity of this reaction can be modulated by the use of sterically hindered aluminum aryloxides or methylaluminoxane as Lewis acids. The reaction was applied to the synthesis of both stereoisomers of 2-benzyloxycyclobutane-α-amino acid, which are protected serine analogues c₄Ser(OBn).