Catalytic Asymmetric Cascade Vinylogous Mukaiyama 1,6-Michael/Michael Addition of 2-Silyloxyfurans with Azoalkenes: Direct Approach to Fused Butyrolactones.

An unprecedented cascade vinylogous Mukaiyama 1,6-MA/MA of 2-silyloxyfurans and azoalkenes was realized with a Cu(II)/(t)Bu-Box complex. An array of fused butyrolactones containing multiple stereocenters was generally obtained in good yield (up to 90% yield) with exclusive diastereoselectivity (>20:1 dr) and excellent enantioselectivity (up to 99% ee). Carbon isotope effects measured by (13)C NMR revealed a stepwise mechanism for this annulation process.