Subject pyrrole precursor

Possibly, the most common protocols used in the generation of azomethine ylides are those based on the in situ, fluorine-mediated desilyation of cyanoami-nosilanes developed by Padwa et al. (2). Typically, treatment of precursor 1 with AgF, in the presence of dimethyl acetylenedicarboxylate (DMAD), led to the formation of the intermediate cycloadduct 2, which was subjected to immediate DDQ oxidation to give pyrrole 3. The mechanistic rationale invokes fluoride-mediated desilyation to form the intermediate anion 4, which then undergoes loss of cyanide furnishing the corresponding azomethine yhde (Scheme 3.1). 

In the laboratory of D.F. Taber, the large-scale preparation of a tetrasubstituted pyrrole, a key precursor for the preparation of hemes and porphyrins, was achieved. The 1,4-dicarbonyl substrate was generated from a ketal via hydrolysis and was immediately subjected to the Paal-Knorr pyrrole synthesis by heating it with ammonium carbonate in DMF. The resulting 1/-/-pyrrole was formylated with trimethyl orthoformate in trifluoroacetic acid. 

The synthesis of 4-vinylindoles has also been accomplished using an enyne metathesis approach. Using the same ideas as those shown in Scheme 17.18, Yoshida and Yanagisawa were able to easily synthesize a range of the precursor pyrroles containing an acetylene and alkene functionality on adjacent carbons, namely, compound 207 [68], When 207 was subjected to the 2-Ru catalyst under an atmosphere of ethylene gas, the intermediate 208 was formed. When treated with TsOH, the benzene ring of the 4-vinylindole 209 was subsequently formed in an excellent yield of 99% (Scheme 17.42). Other examples of this useful approach are illustrated in this paper [68],... 

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