Ethylidenetetronic acid and its derivatives. Condensations with carbonyl compounds, leading to reassignment of ramigenic acid from Penicillium charlesii
Condensation reactions between ethylidenetetronic acid (6) and aryl aldehydes in the presence of weak bases, lead to the 2 : 1 adducts (8); with 3,4-dimethoxybenzaldehyde the 1 : 1 adduct (10) is produced concurrently. Reaction between (6) and acetone initially leads to a similar 2 : 1 adduct i.e. (12), but on further heating in acetone this adduct disporportionates giving the novel dihydropyran (13). The structure of (13) was solved by single crystal X-ray determination. Comparison of spectral data, and synthesis show that the structure of the analogous compound ‘ramigenic acid’ isolated from Penicillium charlesii should be reassigned as shown in formula (17b).