Sunscreen Mediated Selectivity – An Original Concept in Organic Photochemistry

We have recently demonstrated a novel type of chromatic selectivity by exploiting differences in molar absorption coefficients in the presence of UV absorbing molecules as filters or "sunscreens". The methodology was employed in the field of photolabile protecting group (PPGs) where 2-nitrobenzyl protected alcohols were selectively cleaved in the presence of supersilyl ethers upon irradiation with 254 nm with an internal or external solution of phenanthrene as sunscreen.^[1] Recently we reported the extension of the concept to the UV-A region with 1-pyrene carboxaldehyde as a sunscreen, to selectively activate photoswitchable ruthenium olefin metathesis catalyst, and carry out olefin cross metathesis (CM) reactions of 2-nitrobenzyl protected 2-vinyl phenols without removal of the PPG.^[2] This was materialized in a bichromatic two-step synthesis of coumarins where the first step is a UV-A induced CM reaction of 2-nitrobenzyl protected vinyl phenols with acrylates with an external 1-pyrene carboxaldehyde solution as sunscreen. Subsequent removal of the external filter solution and irradiation with UV-C light yielded various coumarin derivatives. Additionally, sunscreens were used to manipulate selectivity in a two-step divergent photochemical synthesis of butenolides and levulinates.^[3] The first step, UV-A induced CM of simple allyl alcohols and acrylates is followed by irradiation of UV-C light in the presence or absence of phenanthrene. The absence of phenanthrene allows a photochemical [1,5- H] shift to obtain levulinates. When phenanthrene is present the [1,5- H] shift is suppressed allowing only double bond isomerization and a subsequent cyclization to afford butenolides.

[1] O. Eivgi, E. Levin, N. G. Lemcoff, Org. Lett. 2015, 17, 740-743.

[2] O. Eivgi, R. Sutar, O. Reany, N. G. Lemcoff, Adv. Synth. Catal. 2017, 359, 2352-2357

[3] R. Sutar, O. Eivgi, S. Sen, I. Schapiro, O. Reany, N. G. Lemcoff, Submitted