Bipyrrole 1//-pyrrole

ANTIBIOTICS - NUCLEOSIDES AND NUCLEOTIDES] (Vol 3) 5-(2H-Pyrrol-2-ylidenemethyl)-2,2y-bipyrrole [22187-69-5]... 

The synthesis is straightforward because for the preparation of the tetrapyrranc a bipyrrole can also be used as a central unit to which a benzyl 5-(acetoxymethyl)pyrrole-2-carboxylate can be attached as the terminal pyrrole rings. Debenzylation by catalytic hydrogenation then gives the desired tetrapyrrane building block 68. 

A Sml2-induced reductive cyclization of (V-(alkylketo)pyrroles provided an entry into medium ring 1,2-annelated pyrroles <06EJO4989>. An oxidative radical alkylation of pyrroles with xanthates promoted by triethylborane provided access to a-(pyrrol-2-yl)carboxylic acid derivatives <06TL2517>. An oxidative coupling of pyrroles promoted by a hypervalent iodine(III) reagent provided bipyrroles directly <060L2007>. 

The pyrrole ring is widely distributed in nature. It occurs in both terrestrial and marine plants and animals [1-3]. Examples of simple pyrroles include the Pseudomonas metabolite pyrrolnitrin, a recently discovered seabird hexahalogenated bipyrrole [4], and an ant trail pheromone. An illustration of the abundant complex natural pyrroles is konbu acidin A, a sponge metabolite that inhibits cyclin-dependent kinase 4. The enormous reactivity of pyrrole in electrophilic substitution reactions explains the occurrence of more than 100 naturally occurring halogenated pyrroles [2, 3]. 

Although V-protected 2-lithiopyrroles are readily generated and many types are known [6, 26], these intermediates have not generally been employed to synthesize halogenated pyrroles. One exception is the synthesis of the two natural seabird hexahalogenated bipyrroles 15 and 17,... 

There is a case where the resnlts of the electrode and chemical oxidation coincide (see Scheme 2.20). Calix[8]pyrrole was prepared from 3,3, 4,4 -tetraethyl-2,2 -bipyrrole during oxidative electrolysis in dichloromethane nsing platinum anode, tetrabutylammonium hydrogen sulfate as the... 

Bis(2-pyrrolyl) sulfides, prepared from the reaction of the pyrrole with sulfur dichloride, are useful precursors in the synthesis of corroles and related compounds, as it is possible to cause the extrusion of the sulfur atom with the consequent formation of a bipyrrolic unit within the macrocycle <72JCS(P1)1124). 

Sanchez-Garcia (09OL77) has reported a one-pot procedure (Scheme 21) for the synthesis of 4,4 -diaryl-4,4 -diheteroaryl-substituted-2,2 -bipyrrole 45 based on the oxidative coupling of 2-trimethylstannylated pyrrole 44. 

Another class of extended porphycene, containing an extra sp2 hybridized link between 2 and 2 positions of the bipyrrole subunit has been synthesized by hydrogenation of 80 resulting in the formation of 22jt-electron conjugated porphycene homologue 87 (Scheme 38), also synthesized from pyrrole dialdehyde 86 by reductive coupling under McMurry conditions. 

Other syntheses of sapphyrins include (i) a one-pot reaction of 3,4-dimethyl pyrrole with pyrrole dialdehyde 86 (1983JA6429) (ii) acid-catalyzed reaction of pyrrole, bipyrrole dialdehyde 128 (Scheme 52), and benzaldehyde (1987JOC4394) and (iii) reaction of benzaldehyde and pyrrole in 1 3 molar ratios under oxidative acid catalysis (1995CEJ68). These sequences furnish sapphyrins 127,129, and 130, respectively (1983JA6429,... 

Sessler and coworkers reported a new "3+1+1" route for sapphyrins, without using a bipyrrolic moiety (01TL2447). Tripyrrane dialdehyde 133 with two equivalents of a pyrrole under acidic conditions, followed by oxidation with DDQ furnished sapphyrins 134 bearing different substituents in yields lower than by the "3+2" approach (Scheme 51), but the need of a bipyrrole synthesis could be obviated. This method (Scheme 55) allowed the synthesis of otherwise not known sapphyrins that contained various symmetric bipyrrolic subunits. 

In a later synthetic effort, a doubly N-confused sapphyrin 113 was obtained, wherein both confused rings belong to the bipyrrolic fragment of the macrocycle [255], In this system, a methoxy group is attached to one of the external a-pyrrolic positions, creating a peripheral sp3 center. This structural feature, which is presumably due to spontaneous addition of a methanol molecule during the synthesis,... 

Other interesting new developments in pyrrole chemistry encompass the synthesis of calix[6]pyrrole and calix[ ]furan[m]pyrroles, where m + n = 6 <02CEJ3148>, and the formation of cyclo[8]pyrroles from 2,2 -bipyrrole fragments <02AGE1422>. 

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