Efficient synthesis of 4-sulfanylcoumarins from 3-bromo-coumarins via a highly selective DABCO-mediated one-pot thia-Michael addition/elimination process

A facile and efficient protocol for the highly selective direct sulfanylation of 3-bromocoumarins under DABCO promotion, was developed. The transformation took place with aromatic and aliphatic thiols as well as with alpha,omega -dithiols, affording the expected products in very good to excellent yields. Simple and convenient ways to access 4-((omega -mercaptoalkyl) thio)coumarins and the dimeric 4,4 '-(alkane-1,4-diylbis(sulfanediyl))bis(coumarins) were also devised with the use of alpha,omega -alkanedithiols in different ratios with regards to the starting 3-bromocoumarin. The transformation seems to proceed through the DABCO-mediated thia-Michael stereoselective addition of the thiolate anion to the alpha,beta -unsaturated carbonyl system of the coumarin, followed by a DABCO-assisted stereoselective dehydrobromination of the resulting alpha -bromo carbonyl intermediate.