Preparation of 5-chloropyrazoles via tandem Meyer–Schuster/von Auwers rearrangements |
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Authors: | Raphaël Dumeunier Simon Jaeckh Rebekka Goebel |
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Institution: | Syngenta Crop Protection, Schaffhauserstrasse, CH-4332 Stein, Switzerland |
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Abstract: | A new, practical preparation of β,β-dichloroenones in three to four steps allows for a straightforward synthesis of 5-chloropyrazoles. Addition of various propargyl anions to commercially available 4-methyl-4-(trichloromethyl)cyclohexa-2,5-dien-1-one gives high yields of the corresponding propargyl alcohols. These are then transformed, in a single step, via two consecutive rearrangements, a Meyer–Schuster and a (sometimes spontaneous) von Auwers rearomatizing rearrangement, to deliver α,α-aryl-trichloromethylketones. After elimination of HCl, cyclization of the β,β-dichloroenones with various hydrazines delivers 5-chloropyrazoles. The four to five step sequence to 5-chloropyrazoles is very atom economical, expelling only water and two molecules of HCl from all the building blocks. |
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Keywords: | Rearomatization Rearrangement von Auwers Semi-benzene Meyer&ndash Schuster |
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