Abstract
Triethyl orthoformate and boron trifluoride–ether complex convert chroman-4-one into 4-ethoxy-2H-chromen-3-carbaldehyde (II), characterised by conversion into trans-3-(4-ethoxy-2H-chromen-3-yl)-1-phenylprop-2-en-1-one (XIV) and thence into the 2-phenyl-5H-pyrano[3,2-c][1]benzopyran-1-ylium cation (XV). Deethylation of (II) with boron trichloride affords a mixture of 3-hydroxymethylenechroman-4-one (XI) and 3-chloromethylchromone, the latter by a ring-substituent exchange reaction. De-ethylation by cold acids gives (XI) only. Hot acids hydrolyse (XI) to chromanone, air oxidises it to (2-carboxyphenoxy)acetic acid, and silica converts it into 2,3-dihydro- 3,3′-methylenebischromen-4-one (XVIII)via another ring-substitutent exchange reaction followed by an alkylation–hydrolysis sequence. Diazomethane converts (XI) into the 3-methoxymethylene compound (X) which is also obtained by the action of methanol and acid on (II) and therefore represents the thermodynamically stable enol.
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