Enantioselective Construction of Spiro[chroman-thiazolones]: Bifunctional Phosphonium Salt-Catalyzed [2+4] Annulation between 5-Alkenyl Thiazolones and ortho-Hydroxyphenyl-Substituted para-Quinone Methides

The enantioselective formal [2+4] annulation of 5-alkenyl thiazolones with hydroxyl-substituted para-quinone methides was disclosed by dipeptide-based phosphonium salt catalysts. A wide range of functionalized spiro-chroman-thiazolone molecules bearing three contiguous 3 degrees and/or 4 degrees stereocenters were readily constructed in high yields with excellent stereoselectivities (>20:1 dr and up to >99.9% ee) under low catalyst loading and mild reaction conditions. The practicality and utility of this protocol were demonstrated by the scaled-up preparation and elaborations of product.