A NOVEL TWO-STEP PHOTOCHEMICAL SYNTHESIS OF COUMARINS

We have recently demonstrated a novel type of chromatic selectivity by exploiting differences in molar absorption coefficients. Thus, selective removal of photolabile protecting groups (PPGs) using internal or external "sunscreens" was achieved using a single light source.¹

In this current work we apply the sunscreen methodology for a novel two-step photochemical synthesis of coumarin derivatives starting from vinyl phenols and acrylates. The first step is a the light triggered cross metathesis (CM) reaction of methyl acrylate and protected 2-vinyl phenols using the UV switchable sulfur chelated ruthenium olefin metathesis catalysts developed by our group.^2,3. 2-Vinyl phenols are considered impractical for olefin metathesis reactions due to their tendency to form a stable chelate with the ruthenium catalyst.^4-6 By protecting 2-vinyl phenols with a 2-nitrobenzyl PPG we exploited the large difference in the molar absorption coefficients of the catalyst and the 2-nitrobenzyl chromophore in the UVA region to selectively activate the ruthenium catalyst and perform CM with methyl acrylate, without removing the 2-nitrobenzyl in the presence of pyrene carboxaldehyde as a sunscreen. Subsequent irradiation after removal of the sunscreen triggered a chain reaction consisting of deprotection of the 2-nitrobenzyl PPG, double bond isomerization and cyclization to form the desired coumarin derivatives.

References:

(1) Eivgi et. al Org. Lett. 2015, 17, 740.

(2) Ginzburg et. al Organometallics 2011, 30, 3430.

(3) Ben-Asuly et. al Organometallics 2009, 28, 4652.

(4) Kozłowska et. al Chemistry – A European Journal 2014, 20, 14120.

(5) Garber et. al J. Am. Chem. Soc. 2000, 122, 8168.

(6) Kingsbury et. al. J. Am. Chem. Soc. 1999, 121, 791.