Understanding the Mechanism of SN2′ vs. SN2 in Cascade Reaction of β‐Naphthol and Nitrostyrene Derived MBH Acetates

The reaction of β‐naphthol with nitrostyrene derived primary MBH acetates in presence of Cs2CO3 as base resulted in the formation of 3‐nitro‐4‐phenyl‐3,4‐dihydro‐2H‐naphthopyran as the major isomer via SN2′ process. Due to the bis‐electrophilic nature of MBH acetates, the minor product 3‐nitro‐2‐phenyl‐3,4‐dihydro‐2H‐naphthopyran was also obtained presumably due to SN2 process with γ ‐attack on MBH acetates. The state of the art density functional theory (DFT) calculations were carried out to account for these competitive pathways towards the formation of major and minor products. A [3+3] annulation protocol for the construction of functionalized 3,4‐dihydro‐2H‐naphthopyrans utilizing nitrostyrene derived MBH acetate and β‐naphthol was developed at room temperature. The bis‐electrophilicity of MBH acetate is accompanied by a cascade process of C−C and C−O bond formation resulting in major and minor products. The two products via SN2’ and SN2 reaction pathways was reasonably justified by DFT calculations. A reasonable reaction mechanism where an initial Friedel‐Craft's type alkylation with subsequent Oxa‐Michael ring closure via 6‐endo‐trig cyclization was justified towards synthesis of functionalized naphthopyran derivatives.