Intermolecular approaches to pyrroles

Novel classes of fused pyrroles, dibenzo[2,3 6,7]oxepino[4,5- ]pyrroles and dibenzo[2,3 6,7]thiepino[4,5- ]pyrroles, were prepared by the intermolecular cyclocondenation of dibenzo[2,3 6,7]oxepin-5-ones and dibenzo[2,3 6,7]thiopin-5-ones with (dimethylhydrazono)acetaldehyde 07JHC1129 . 

Cyclocondensation reactions between 1,2-diaza-l,3-butadienes with silyl enol ethers, mediated by zinc chloride and then TFA, led the to the production of 1 -aminopyrroles 07ASC907 . Extensions of this methodology allowed for the preparation of additional 1-aminopyrroles from bis(silyl enol ethers) 07SL2965 and enamines 07T 11055 . 

A direct synthesis of tetraarylpyrroles has been developed 07S3117 . Treatment of symmetrical 1,3-diketones 16 and imines 17 with TiCb/Sm produced 1,2,3,5-tetraarylpyrroles 18. This mechanism likely involves radicals formed by electron transfer from low valent titanium the latter is formed by the reduction of titanium tetrachloride by samarium. 

Named cyclocondensation reactions between primary amines and 1,4-dicarbonyl compounds (Paal-Knorr) 07SC1791 07SC2793 or 2,5-dialkoxytetrahydrofurans 

The Clauson-Kaas reaction between 2,5-dimethoxytetrahydrofuran 19 and phenylsulfonamide performed in the presence of triflic acid (TfOH) led to either pyrrole 20, indole 21, or carbazole 22 depending on the amount of triflic acid used 07TL4047 . The latter two compounds were formed by the annulation of pyrrole or indole by butanedial (generated by the acid-mediated hydrolysis of 19). A double Clauson-Kaas sequence starting with hydrazine allowed for the preparation of 1,1 -bipyrrole 07JOC93 95 . 

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