Stereoselective oxa-Michael addition of tyrosine to propargyl aldehyde/esters: formation of benzofurans and flavones

The steroselective oxa-Michael addition of the phenol moiety present in tyrosine and 3-iodotyrosine to different propargyl aldehydes delivered products with predominantly Z stereochemistry, as evidenced by X-ray crystallography analysis. When ethyl ropiolate was used as the propargyl ester source, the products were achieved with exclusively E stereochemistry with yields ranging from 17% to 91%. The oxa-Michael addition compounds served as substrates in the synthesis of 5- and 6-membered heterocyclic compounds. The atmosphere applied to the reaction medium directly influenced the formation of the products. When an inert atmosphere of nitrogen was applied, a 2-aryl-3-formyl-5-alanylbenzofuran core was selectively obtained via a Heck intramolecular reaction, while the reactions carried out under a carbon monoxide atmosphere led exclusively to 6-alanyl-2-arylflavone derivatives via reductive intramolecular acylation.
Keywords
Flavone;  Heck reaction;  oxa-Michael addition;  Tyrosine

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Vasconcelos SN.S., Oliveira IM, Shamim A, Zukerman-Schpector J, Pimenta DC, Stefani HA.. Stereoselective oxa-Michael addition of tyrosine to propargyl aldehyde/esters: formation of benzofurans and flavones. Adv. Synth. Catal.. 2019 Jul;361:4243-4254. doi:10.1002/adsc.201900564.
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