Enantioselective Synthesis of Oxazocines via MQ-Phos Enabled Palladium-Catalyzed Asymmetric Formal [4+4]-Cycloadditions.

ABSTRACT

Oxazocines are key structural intermediates in the synthesis of natural products and pharmaceutical molecules. However, the synthesis of oxazocines especially in a highly enantioselective manner, is a long-standing formidable challenge due to unfavorable energetics involved in cyclization. Herein, a series of new PNP-Ligand P-chiral stereocenter is first designed and synthesized, called MQ-Phos, and successfully applied it in the Pd-catalyzed enantioselective higher-order formal [4+4]-cycloaddition of α, ß-unsaturated imines with 2-(hydroxymethyl)-1-arylallyl carbonates. The reaction features mild conditions, excellent regio- and enantiocontrol and a broad substrate scope (54 examples). Various medium-sized rings can be afforded in moderate to excellent yields (up to 92%) and excellent enantioselectivity (up to 99% ee). The newly developed MQ-Phos is critical for synthesis of the medium-sized ring in excellent catalytic reactivity and enantioselectivity.