Metal- free and benign approach for the synthesis of dihydro-5' H- spiro[ benzo[ c] chromene8,4'- oxazole]-5', 6( 7H)- dione scaffolds as masked amino acids plus circle plus

An eco-friendly, straightforward, and three-component condensation/cascade reaction of 4-hydroxycoumarins and (Z)-azlactones to construct diversified dihydro-5H-spiro[benzo[c]chromene-8,4-oxazole]-5,6(7H)-diones as new masked amino acid derivatives has been developed with high to excellent yields and regio- and diastereoselectivity. This metal-free reaction proceeds via a one-pot cascade Michael addition/lactonization/decarboxylation reaction utilizing reusable propylene carbonate as a green solvent. The scale-up examination was also performed, showing high atom economy under the reaction conditions. Moreover, the mechanism of the reaction was further investigated using isotope-labeling, LC-MS monitoring, and TLC-MALDI-MS.