Remarkable Effects of Lone Pair-Lone Pair Interactions on the Extremely Stereoselective Cycloaddition of Azidoketene to Chiral 3-Imino-β-lactams

Article abstract of DOI:10.1021/ja00240a033

The stereoselective <2 + 2> cycloaddition of azidoketene to cis-3-imino-β-lactams 1, 2, and 3, trans-3-imino-β-lactam 4, and cis-3-iminoazetidine 5 were carried out.It was found that the reactions of 1, 2, and 3 proceeded with > 99.5 percent stereoselectivity to give bis-β-lactams 9, 14, and 19, respectively.The reaction of 4 gave a bis-β-lactam 20 with 62 percent de, and the reaction of 5 gave a β-lactamazetidine 21 with 32 percent de including the asymmetry in the opposite direction.These results clearly indicate that, in addition to the conventional steric effects, the lone pair-lone pair interaction of β-lactam oxygen with the intermediate betaine's oxygen is the crucial factor for the extremely stereoselective <2 + 2> cycloaddition of azidoketene to 1, 2, and 3.Possible mechanisms for these stereoselective <2 + 2> cycloadditions are proposed.The X-ray crystal structure of 3a and the calculated energy-minimum conformations of 3a, 4, and 5 with MODEL-MM2-ROTOCHEM programs are provided.