A new synthetic route to 5-substituted 2-pyrones is described. Alkylation of enamine 1 with methyl acrylate, cyclization of carboxylic acid 2b employing p-toluenesulfonic acid in benzene, and dehydrogenation of enol lactone 3 with a palladium catalyst or by a N-bromosuccinimide sequence comprised the key steps to a-pyrone S. Other methods (utilizing derivatives of acetic acid) investigated for the enol lactonization reaction gave mainly lactone 4. Application to bufadienolide chemistry was studied by transformation of dehydroepiandrosterone via intermediates 6-8 to 3β-acetoxy-14α-bufa-5,20 (21)-dienolide (9). Similar conversion of epiandrosterone acetate (10) led to bufenolide 16, which was dehydrogenated to bufadienolide 17 using sulfur. Other dehydrogenation methods, including those quite useful with enol lactone 3, were unsatisfactory.
ASJC Scopus subject areas
- Organic Chemistry