Abstract
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).
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