Under normal conditions, thiophene and furan derivatives are substituted in the α position, and no convenient alternative methods for the preparation of β‐substitution products have been available until now. The present article describes a method that permits the synthesis of many β‐substituted thiophenes and furans. In this method, the carbonyl group in α‐aldehydes or ketones of the thiophene and furan series is blocked by complex formation with an excess of aluminum chloride, so that electrophilic substitution takes place in position 4. In another useful method, the carbonyl group is blocked by acetalization. The acetals can be metalated in the ring by organolithium compounds.
Das analog angegebenem Reaktionsschema darstellbare 3‐Bromfuran (IV) liefert bei der Acetylierung entgegen den Versuchsergebnissen anderer Autoren kein einheitliches Produkt, sondern ein Gemisch der 3‐ (V) und 4‐Brom‐2‐acetylfurane (VI) im Verhältnis 4 :1 (gaschromatographisch bestimmt).
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