5-Alkenylthiazoles as In-Out Dienes in Polar [4+2] Cycloaddition Reactions

5‐Alkenyl‐2‐aminothiazoles react as in‐out dienes with a wide range of electron‐poor dienophiles leading to the corresponding cycloaddition products in good to excellent yields. The [4+2] cycloadditions of 5‐alkenyl‐2‐aminothiazoles can be classified as site‐selective because only the diene moiety i...

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Bibliographic Details
Published in:European journal of organic chemistry 2013-11, Vol.2013 (33), p.7500-7511
Main Authors: Alajarin, Mateo, Cabrera, Jose, Sanchez-Andrada, Pilar, Bautista, Delia, Pastor, Aurelia
Format: Article
Language:English
Subjects:
Cyclization
Cycloaddition
Nitrogen heterocycles
Reaction mechanisms
Synthetic methods
Zwitterions
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Summary:5‐Alkenyl‐2‐aminothiazoles react as in‐out dienes with a wide range of electron‐poor dienophiles leading to the corresponding cycloaddition products in good to excellent yields. The [4+2] cycloadditions of 5‐alkenyl‐2‐aminothiazoles can be classified as site‐selective because only the diene moiety incorporating the formal C–C double bond of the heterocycle and that of the side‐chain is involved. Calculations of the HOMO energy values of representative 5‐alkenyl‐2‐aminothiazoles are disclosed. The cycloadditions are endo‐selective with N‐phenylmaleimide or maleic anhydride and regioselective when the reactions are conducted with nonsymmetrical dienophiles. Completely oxidized cycloadducts are obtained in the reactions of 5‐alkenyl‐2‐aminothiazoles with naphthoquinone or dimethyl acetylenedicarboxylate (DMAD). Unexpectedly, the reactions with 4‐phenyl‐1,2,4‐triazoline‐3,5‐dione (PTAD) are not stereospecific. A mechanism placed at the concerted/stepwise boundary is proposed. 5‐Alkenyl‐2‐aminothiazoles behave as active in‐out dienes in [4+2] cycloadditions with a wide range of dienophiles. These processes are endo‐selective with certain dienophiles of cyclic nature. Additionally, the reactions with nonsymmetric dienophiles are regioselective. The results point to a mechanism that proceeds at the boundary between a highly asynchronous concerted mechanism and a stepwise process.
ISSN:1434-193X
1099-0690
DOI:10.1002/ejoc.201300925