Synthesis of fully functionalised spiropyran pyrazolone skeletons via a formal [4 + 2] cascade process using β-nitro-styrene-derived MBH-alcohols

Abstract

An efficient protocol was established to construct spiro pyrazolone tetrahydropyran scaffolds at ambient temperature under metal-free conditions. The reaction proceeded via formal [4 + 2] cyclisation of trans-β-nitro-styrene-derived Morita–Baylis–Hillman (MBH) alcohol with α-arylidene pyrazolone. The reaction followed an oxa-Michael/Michael cascade pathway, resulting in the formation of new C–C and C–O bonds. Organocatalytic synthesis of spiropyrazolones using quinine-derived catalyst resulted in 94% enantiomeric excess (ee) and excellent (>20 : 1) diastereoselectivity.