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p ‐TSA‐Based DESs as “Active Green Solvents” for Microwave Enhanced Cyclization of 2‐Alkynyl‐(hetero)‐arylcarboxylates: an Alternative Access to 6‐Substituted 3,4‐Fused 2‐Pyranones
Author(s) -
Curti Fabiola,
Tiecco Matteo,
Pirovano Valentina,
Germani Raimondo,
Caselli Alessandro,
Rossi Elisabetta,
Abbiati Giorgio
Publication year - 2019
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/ejoc.201801884
Subject(s) - regioselectivity , chemistry , alkyne , trifluoromethanesulfonate , combinatorial chemistry , catalysis , polar effect , organic chemistry
In this paper, we describe the use of p ‐TSA based Deep Eutectic Solvents (DESs) as alternative environmental‐friendly “active” solvents for the microwave‐mediated synthesis of 6‐substituted 3,4‐fused 2‐pyranones, and in particular isocoumarins, starting from 2‐alkynyl‐(hetero)arylcarboxylates. When the alkyne terminus bears a neutral or an electron‐donating group (EDG), the reactions are fast, clean and highly regioselective, to give the 6‐ endo ‐ dig cyclization products in good to excellent yields. For substrates bearing an electron‐withdrawing group (EWG) on the alkyne end, the regioselectivity can be tuned by adding a small amount of silver(I) triflate as co‐catalyst. DES was demonstrated to be reusable without loss of efficiency in terms of reaction yields. Based on experimental evidence and previous findings, two competitive mechanisms working simultaneously are proposed to explain the outcomes and the regioselectivity issues.