Sulfur-substituted pyrroles

A new azomethine ylide containing a dithiolanyl ring was shown to react with typical dipolarophiles to give sulfur-substituted pyrroles. <95TL9409>... 

The acid-catalysed rearrangement of sulfur substituents from the a-position of kinetically-controlled attack, exemplified in these sulfur-substituted pyrroles, is a useful means for the preparation of P-substituted pyrroles (see also section 13.1.5). ... 

The stable tautomer of indole-2-thiol is the thione, thiooxindole, and this compound and simple derivatives are conveniently prepared from the corresponding oxindoles by reaction with phosphorus pentasulfide (69CPB550). Like the pyrrole analogs, these compounds are quite oxygen sensitive. Spande has reviewed other aspects of the synthesis of sulfur-substituted indoles, including methods for the synthesis of compounds with carbocyclic sulfur substituents (79HC(25-3)199). 

Extrusion of sulfur dioxide from a ring-fused dihydiothiophene derivative provides an exceptionally easy method for the preparation of heterocyclic o-dimethylene compounds. These compounds are valuable intermediates in intermolecular Diels-Alder reactions. This extrusion method was used to prepare the o-dimethylene compounds 44-46 <95CC1349> as well as quinolinone derivative 47 <95TL5983>. TTie dihydrothiophene dioxide moiety also played an important role in the formation of the intramolecular Diels-Alder reaction of N-substituted pyrrole 48 <95CC807>. 

Acid catalyses rearrangement of sulfur substituents from the a-position to give p-substituted pyrroles (see also 16.1.1.5 and 16.12), perhaps initiated by protonation at the sulfur-bearing a-carbon. 

Refluxing one equivalent of S2CI2 or SCI2 with two equivalents of the amino acrylates 133 in chlorobenzene gives pyrroles 137 in good yields, Scheme 34. Initial cyclization gives the 1,4-thiazines 135 from which sulfur is extruded via 136. This method is particularly useful for the preparation of perfluoroalkyl-substituted pyrroles, which are difficult to obtain by the Knorr synthesis (84JOC4780). 

Veisi, H. 2010. Silica sulfuric acid (SSA) as a solid acid heterogeneous catalyst for one-pot synthesis of substituted pyrroles under solvent-free conditions at room temperature. Tetrahedron I tt. 51(16) 2109-2114. 

The range of preparatively useful electrophilic substitution reactions is often limited by the acid sensitivity of the substrates. Whereas thiophene can be successfully sulfonated in 95% sulfuric acid at room temperature, such strongly acidic conditions cannot be used for the sulfonation of furan or pyrrole. Attempts to nitrate thiophene, furan or pyrrole under conditions used to nitrate benzene and its derivatives invariably result in failure. In the... 

A more complex reaction is involved in the cooligomerization of acetylenes and tert-butyl isocyanide using nickel acetate as the catalyst (Scheme 20)43 the nature of intermediate complexes leading to the formation of 2-cyano-5-terf-butylaminopyrroles has not been established. Cocyclization of tert-butyl isocyanide with coordinated hexafluoro-2-butyne gives rise to coordinated cyclopentadienone anils for molybdenum systems,44 hence the nature of acetylene substitutents and of the organometallic catalyst play crucial roles in these processes. The pyrrole products from the former reaction can be decomposed by sulfuric acid and the overall sequence provides a simple synthesis of 5-amino-2-cyanopyrroles (Scheme 20). 

Poisoning and sometimes death from eating (unidentified) mushrooms is well known. In particular, Amanita sp. are particularly dangerous, with much emphasis on the death cap fungus , Amanita phalloides.24 The best known toxins are the amatoxins and phallotoxins, which are complex, bicyclic peptides. An unusual feature relates to sulfur a tryptophan (or substituted tryptophan) unit is linked to a cysteine sulfur at the carbon atom next to the NH group of the pyrrole ring, forming the unit, -CH2-S-C(NH)=C, e.g. in... 

A photocyclization of 3-(2-pyrrolyl)-3-amino-2-alkeneimines under acidic reaction conditions leads to the formation of pyrrolo[3,2-3]pyridine as shown in Equation (22) <1999T14079>. When oxygen or sulfur heteroatoms are substituted for the pyrrole ring nitrogen, the corresponding furo- and thieno[3,2-3]pyridines are formed. All of the photocyclizations require lengthy reaction times and the yields tend to be low, especially for the furopyridine derivatives. 

Complex benzofurans, such as N-substituted 3-oxo-l,3-dihy-drobenzofuro[2,3-c]pyrroles (95),263 or bacteriostatic substituted 5,6,7,8-tetrahydrobenzofuro[2,3-6]pyridines,270 4a-alkoxy-l,2,3,4,4a, 9b-hexahydrobenzofuro[3,2-c]pyridines and polysubstituted benzo-furo[3,2-c]pyridines,271 and other polyheterocyclic compounds with sulfur or nitrogen atoms272,273 have also been synthesized by this method. 

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