Synthesis of Pterocarpans by Means of a "Disfavored" 5-endo-trig Radical Cyclization Reaction

• Published in: European journal of organic chemistry Vol. 2001; no. 18; pp. 3461 - 3466
• Main Authors: Santhosh, Kalpathy Chidambareswaran, Gopalsamy, Ariamala, Balasubramanian, Kalputtu Kuppuswamy
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag GmbH 01.09.2001; WILEY‐VCH Verlag GmbH; Wiley
• Subjects: Chemistry; Chemistry, Organic; Cyclizations; Heterocycles; Natural products; Physical Sciences; Radical reactions; Reaction mechanisms; Science & Technology; cyclizations; REARRANGEMENT; natural products; MITSUNOBU REACTION; heterocycles; radical reactions; reaction mechanisms; PHENYLACETATES; 2H-CHROMENES; ALDOL CONDENSATION; BENZALDEHYDES
• ISSN: 1434-193X; 1099-0690
• DOI: 10.1002/1099-0690(200109)2001:18<3461::AID-EJOC3461>3.0.CO;2-9

A successful synthesis of pterocarpans 1, based on a “disfavored” 5‐endo‐trig radical cyclization reaction, has been accomplished. The radical precursor 4‐(2′‐bromoaryloxy)‐2H‐chromene 8 was synthesized in six steps, starting from aryl propynyl ether 2. On treatment with tributyltin hydride in refluxing benzene, aryl enol ether 8 underwent radical cyclization to furnish the pterocarpan 1. A deuterium label study was performed to prove the occurrence of 5‐endo‐trig radical cyclization. This methodology was extended further to synthesize a class of hitherto unknown pterocarpans 13. A novel and stereoselective route towards the synthesis of aryl ethers of bromohydrins by means of the Mitsunobu reaction is also described.