Pyrroles cycloisomerization into

A novel s3mthesis of [fc]-fused pyrroles involved an intramolecular cyclization of enaminones (Knorr-t) e p3rrole synthesis) <0583152>. Treatment of Weinreb amide 20 with two equivalents with thienyUithium led to intermediate ketone 21 which cycloisomerized into pyrrole 22 upon heating. 

Cycloisomerization represents another approach for the construction of cyclic compounds from acyclic substrates, with iridium complexes functioning as efficient catalysts. The reaction of enynes has been widely studied for example, Chatani et al. reported the transformation of 1,6-enynes into 1-vinylcyclopentenes using [lrCl(CO)3]n (Scheme 11.26) [39]. In contrast, when 1,6-enynes were submitted in the presence of [lrCl(cod)]2 and AcOH, cyclopentanes with two exo-olefin moieties were obtained (Scheme 11.27) [39]. Interestingly, however, when the Ir-DPPF complex was used, the geometry of olefinic moiety in the product was opposite (Scheme 11.28) [17]. The Ir-catalyzed cycloisomerization was efficiently utilized in a tandem reaction along with a Cu(l)-catalyzed three-component coupling, Diels-Alder reaction, and dehydrogenation for the synthesis of polycyclic pyrroles [40]. 

While furans have been the main focus of palladium-catalyzed allene cycloisomerization, pyrroles can also be generated via reaction of allenyl-substituted amines. A number of metal catalysts have been reported to mediate the cyclization of these substrates to pyrrolines, however, the use of palladium catalysis can allow the concomitant incorporation of aryl functionality into the 3-position, as shown in Scheme 6.27. At elevated temperatures, oxidation of the pyrroline occurs to afford pyrroles [37]. 

Analogously to the precedent transition metal-catalyzed cycloisomerization of (Z)-pent-2-en-4-yn-l-ols like 113 into furan compounds (Scheme 8.45), Gabriele disclosed a general and a very convenient route leading to pyrroles 218 from nitrogen analogs of these reactive precursors (Scheme 8.81) [269]. Thus, di-, tri-, and tetra-substituted pyrroles 218 with different substitution patterns could be readily synthesized via the Cu(I)- or Cu(II)-catalyzed [270] cycloisomerization of (Z)-(2-en-4-ynyl)amines 217. The mechanism of this transformation is similar to that proposed for the transition metal-catalyzed synthesis of furans (Scheme 8.46). 

In a related way, Rh(II)-imino carbenes (97), easily formed from alkynyl A -tosyl triazoles (98), have been suggested to cycloisomerize via a metathesis mechanism to Rh(II)-carbenoid intermediates (99) which are converted into the corresponding fused pyrroles (100), ° ... 

An efficient Cu-catalyzed cycloisomerization of alkynyl imines into the 2-monosubstituted and 2,5-disubstituted pyrroles was reported by Gevorgyan and coworkers (Scheme 8.3). This methodology was efficient to the synthesis of various types of fused heteroaromatic compounds and even synthesis of ( ) monomorine. A C(sp )-H of the substrate was activated during this copper-catalyzed pyrrole formation [12]. Moreover, they also realized the... 

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