Copper (II) Triflate/ Vitamin-C Promoted Modified Morita-Baylis-Hillman Annulation: A Facile Route to Fluorescent Cyclopentene Derivatives.

Herein we report a rapid and efficient method for the synthesis of fused cyclopentenoids via Intramolecular Morita-Baylis-Hillman (IMBH) annulation. In this approach, copper triflate (Cu(OTf)2) serves as the promoter, favoring cyclization and aromatization over the conventional MBH addition product. A characteristic feature among most of the cyclized product contain two non-separable isomeric mixture of cyclopentene derivatives which has been readily interconverted via 1,5-H shift. This sigmatropic shift has been proved via deuterium labelling experiment using the first synthesized 1-bromo-6-methoxy-3,4-dihydronaphthalene-2-carbaldehyde-d. The equilibration of two cyclopentadiene ring via sigmatropic shift has also been characterized by 2D NMR experiment. The scale-up experiment in the modified IMBH reaction also flourish our protocol significantly. Additionally, the same compounds have also been synthesized in solvent-free conditions using organic acids, which reduces the use of hazardous substances, providing an alternate pathway for the IMBH reaction avoiding conventional aqueous work-up. Moreover, this novel IMBH reaction has been thoroughly investigated and elucidated using Density Functional Theory (DFT). The construction of such fluorescent derivatives and the evaluation of their unique photophysical properties revealed acid-base interactions with active methylene groups, facilitating their use as sensors of hazardous amine derivatives via fluorescence quenching mechanism.