Exploring Coumarins Reduction: NaBH₄/MeOH versus Nickel Boride Generated In Situ.

Abstract

The role of reagents NaBH4/MeOH and nickel boride (Ni2B) generated in situ from NaBH4 and NiCl2, are compared in the reduction process of coumarin and a variety of 3,7‐substituted coumarins bearing electro‐donating (ED‐group) or electro‐withdrawing groups (EW‐group). Coumarins (chromen‐2‐ones) are only reduced by Ni2B to the cyclic chromanones. This provides a useful and very simple reduction method for electron‐rich coumarins, which are resistant to many other reducing methods. DFT calculations underlined the role of substituents electronic effects in the reactivity. Subsequent methanolysis may open the ring to methyl phenylpropanoate esters and alcohols resulting from their reductions can also be produced.

Interactions of Phenylalanine Derivatives with Human Tyrosinase: Lessons from Experimental and Theoretical tudies

“…The phenols of the 3,5‐ and 2,4‐dihydroxybenzaldehyde (Scheme 2) were first protected to avoid competing with coumarin formation (which would be the case with unprotected 2,4‐dihydroxybenzaldehyde) in step 2 of azlactone formation, [39] and to simplify the otherwise tedious purifications. Under these conditions, the condensation of aldehydes ( 1 a, b ) with hippuric acid led to azlactones ( 2 a, b ) in satisfactory yields.…”

“…[28,29] We were inspired by Lambooy's work for the synthesis of these isomers following the classical Erlenmeyer-Plöchl amino acid method which introduces the azlactone precursor motif of the amino acid as a key step. The phenols of the 3,5-and 2,4-dihydroxybenzaldehyde (Scheme 2) were first protected to avoid competing with coumarin formation (which would be the case with unprotected 2,4-dihydroxybenzaldehyde) in step 2 of azlactone formation, [39] and to simplify the otherwise tedious purifications. Under these conditions, the condensation of aldehydes ( Purification was done by HPLC in order to obtain compounds with an optimal purity degree.…”