Thiazol-2-ylidenes, synthesis

Enders D, Muller-Huwen A (2004) Asymmetric synthesis of 2-amino-1,3-diols and D-erythro-sphinganine. Eur J Org Chem 2004 1732 Enders D, Niemeier O (2004) Thiazol-2-ylidene catalysis in intramolecular crossed aldehyde-ketone benzoin reactions. Synlett 2004 2111-2114... 

How close the stability of the monomeric 3-(2,6-diisopropylphenyl)thiazol-2-ylidene actually is at its dimerisation limit was demonstrated by Arduengo et al. in an attempt to synthesise the less bulky 3-(mesityl)thiazol-2-ylidene. When the monomer was generated at low temperatures using Bu OK as the base, it dimerised at temperatures above 0°C in solution, whereas synthesis at ambient temperature using KH [same reaction conditions as for 3-(2,6-diisopropylphenyl)thiazol-2-ylidene] resulted in the dimer exclusively [2]. With a methyl group on the nitrogen atom, the intermediate carbene monomer could no longer be unequivocally detected even at low temperatures. 

Figure 6.14 Synthesis of copper(l) thiazol-2-ylidene complexes.

The thiazolium-mediated three-component reaction of thiazolium salts 201, aryl aldehydes and dimethyl acetylenedicarboxylate provides a facile synthesis of 2-amino-2-arylfurans 202 <05OL1343>. The reaction pathway may involve the sequential nucleophilic addition of thiazol-2-ylidene 203 with the aldehyde and DMAD to form the spirocyclic intermediate 204 through the simultaneous formation of two C-C bonds and a C-O bond Selective ring opening of the spirocyclic intermediate 204 followed by hydrolysis leads to 3-aminofuran 202 via 205. 

Scheme 1.14 Synthesis and dimerization of thiazol-2-ylidene and benzothiazolium salt 105.

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