Thiochromones, synthesis

A one-pot process to form 1,3,4-substituted pyrazoles 25 by Suzuki coupling of arylboronic acids to chromone 24, followed by condensation with hydrazine has been reported <06JCO286>. The synthesis of 3 or 5-o-hydroxyphenol-4-benzylpyrazoles has been accomplished by treatment of 3-benzylchromones, 3-benzylflavones and their 4-thiochromone analogs with hydrazine hydrate in hot pyridine <06EJO2825>. 

Thiophenols undergo base-catalyzed, Michael addition to acetylenic acids and esters to give trans addition products. These vinyl thioethers have been used in the synthesis of thiochromones. " Recently, Undheim and Lie have shown that thiophenol adds to DMAD with concomitant cyclization to give benzo[6]thiophenes. 

The present volume encompasses a wide range of heterocyclic chemistry. Syntheses of heterocycles from thioureas are reviewed by T. S. Criffin, T. S. Woods, and 1). L. Klayman, while S. W. Schneller describes the chemistry of benzothiins and their derivatives (thiochromans, thiochromones, and thio-chromanones). Developments in chrom-3-ene chemistry are reviewed by L. Merlini. F. D. Popp contributes a chapter on the isatins. A discussion of theoretical aspects of the tautomerism of pyrimidines, by J. S. Kwiatkowski and B. Pullman, follows up a corresponding earlier contribution (Vol. 13) on tautomeric purines. In the final chapter P. and D. Cagniant describe the natural occurrence and synthesis of the benzofurans. 

Ring synthesis by y- closure is a somewhat more versatile entry into this class of compound, though even then it is restricted to suitably activated precursors such as (142) (equation 101) (72T5197). Thiochromones and thioxanthones are probably most frequently prepared by this mode of closure, usually via an intramolecular Friedel-Crafts condensation of a carboxylic acid with an aromatic ring (equation 102) (71GEP2006196). 

Acylation of the thiol moiety of thiosalicylic acid is the starting point for a synthesis of 2-substituted thiochromones in which the key step is an intramolecular Wittig reaction (Scheme 203) <2001T9755>. 

Figure 2.4 Ruhemann s synthesis of chromone- or l-thiochromone-2-carboxylic acid...

Tamura K, Ishihara T, Yamanaka H (1994) Synthesis of 2-polyfluoroalkylated thiochromones and chromones. J Fluor Chem 68 25-31... 

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