Pyrroles analogous transformation

Later, Hiroya reported that an analogous transformation azides 246 lea ng to pyrroles 247 could be achieved with the Pt(IV)-pyndme ligand catalytic system (Scheme 8.91) [282]. Various mono-, di-, and tri-substituted pyrroles were obtained in moderate to high yields under these reaction C(m... 

Fusion of a benzene ring onto the C2/C3 positions of pyrrole formally produces the corresponding benzopyrrole 7.1 known as indole. An analogous theoretical transformation can be envisaged to form benzofuran 7.2 and benzothiophene 7.3. This chapter will concentrate exclusively on indole, by far the most important member of this series. 

The fluorescent chlorophyll catabolites, such as pFCC (10), were observed not to accumulate during chlorophyll breakdown in senescent leaves (24). The indicated further transformation of the FCC chro-mophore to those of non-fluorescent chlorophyll catabolites (NCCs) was suggested to possibly be the result of a non-enzymic isomerization (56, 62). In analogy to the results of studies on the tautomerization chemistry of a range of hydro-porphinoids (91), the isomerization of the chromo-phore of FCCs into that of NCCs was judged to be rather favorable, thermodynamically. The complete de-conjugation of the four pyrrolic units, characteristic of the tetrapyrrolic NCCs, thus may occur in the course of natural chlorophyll breakdown under rather mild and, possibly, even without catalysis by (an) enzyme(s) (56). 

Furan and thiophene have also been utilized in this type of transformation as building blocks. In the same maimer, prerequisite structures prepared by cross-coupling as well as traditional carbanion addition were converted expectedly into benzo- and dibenzofurans and their thiophene analogs (i.e., 47 48) [80]. Likewise, sesquiterpene furanoquinone 51 was synthesized [81], and the total synthesis of the indohzidine alkaloid, septicine 54, was performed with the key step 52 -> 53 for the pyrrole case [82] (Scheme 9). 

Li, H.Y., Drummond, S., DeLucca, I., Boswell, G.A. 1996. Singlet oxygen oxidation of pyrroles Synthesis and chemical transformations of novel 4,4-bis(trifluoromethyl)imidazo-line analogs. Tetrahedron 52(34) 11153-11162. 

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). 

Buchwald elaborated on a novel 4 + 1 Cu(I)substituted pyrroles 283 (Scheme 8.101) (296] utilizing 5-endo-dig cycHzation of aminoenynes 282, analogously to the protocol developed by Dovey (Schemes 8.87 and 8.88). The key reactive intermediates of this transformation. 

Ruthenium-catalyzed oxidative coupling of 2-aryl-substituted indoles and pyrroles with a variety of alkynes to generate structural analogs of bioactive marine alkaloids was reported (Eq. (7.26)) [34]. Ambient air was used as the terminal oxidant together with the addition of cocatalytic amounts of Cu(OAc)2 H2O in this transformation. The annulation reactions favored the electron-deficient alkyne and the more acidic C-H bond. Mechanistic experiments showed that the catalytic process took place via a concerted, acetate-assisted deprotonative metalation step. 

The possibility of transformation of 2-acetylcoumarone oxime, a close structural analog of 2-acetylfuran oxime, to earlier unknown 2-pyrrolylbenzo[ ]furan and its N-vinyl derivative has been demonstrated [234]. Upon short contact of this oxime with acetylene under pressure (70°C, 5 min) in the systan KOH/DMSO, the expected pyrrole is formed in 24% yield, while the intermediate O-vinyl oxime is obtained with 22% yield (Schone 1.59). 

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