2 pyrroles preparation

Pyrroles prepared by the Barton-Zard reaction are very important as precursors of porphyrins and also of conducting polymers. The ester group at the 2-position is readily removed on heating with KOH in ethylene glycol at 170 °C to give a-free pyrroles, which are useful for preparing porphyrins (Eq. 10.42)47 or polypyrroles (Eq. 10.43).38a,4S... 

Representatives of Pyrroles Prepared from Dialkyl Ketoximes... 

Representatives of Condensed Pyrroles Prepared from Oximes of Cyclic Ketones... 

In further studies of the synthesis of porphobilinogen and isoporphobilinogen, a-free pyrroles (prepared by Knorr type syntheses) were aminomethylated by the Tschemiak-Einhom reaction, for example, treatment with A -hydroxy-methylchloroacetamide in hot ethanolic hydrogen chloride. The resulting chloro-acetylamino derivatives (14) could be reduced catalytically to the acetylamino-methyl pyrroles (15). In related studies an acetylaminomethyl pyrrole (16b) was also prepared by treatment of 2,4-dimethylpyirole with acetyl thiocyanate followed by Raney nickel reduction of the intermediate acetyl thiocarbamide (16a). 

Pyrrole carbinols were identified as new base-labile protecting groups for aldehydes. The optimal bases for the preparation of a pyrrole carbinol via the reaction of metal pyrrolates with iso-butyraldehyde were sought. Alkali metal pyrrolates performed better than their alkaline earth counterparts. Lithium and sodium pyrrolates prepared via the deprotonation of pyrrole with n-BuLi, LHMDS, andNaHMDS afforded the corresponding pyrrole carbinol in 90%, 85% and 85% )fields, respectively, whereas potassium... 

Another type of pyrrole preparation via allylic sp C—H bond cleavage has been reported by Glorius et al. (Scheme 25.39) [29],... 

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