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.
490Rontgenkontrastmittel Auf Kurvenbild 1 zeigen U n i b a r y t und R o n t y u m fast den gleichen Kurvenverlauf. Nur etwa 10% sind groBer als 2 ,~. waherend rund 90% eine KorngroBe unter 2 p haben. Etwas unterhalb dieser Kurven liegt die Feinheitskennlinie der beiden Lactobaryt-Praparate (s. Bild 2).Wesentlich tiefer liegen die Kurven fur die beiden C i t o b a r y u mund fur beide R o e b a r y t-Praparate. Aus der Tabelle 11 ging schon hervor, daB etwa die Halfte dieser RontgenlKontrastmittel in den ersten fiinf Minuten sedimentiert ist. Bei etwa 50% ist aZso die Korngr6Be uber 15p. Sonst zeigen aber die K,urven einen mehr geradlinigen Verlauf, was besagt, dla8 bei ihnen die KorngroBen in ziemlich gleichmaBiger Mischung vorliegen.
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