Pyrrole dimerisation

Schmuck, C., Heil, M. (2001) Anion-dependent dimerisation of a guanidiniocarbonyl pyrrole cation in DMSO, Org. Lett. 3, 1253-1256. 

Bromination of ketone 3.17 gives 3.18 which can be converted to azide 3.19. Hydrogenation of 3.19 in the presence of hydrochloric acid affords aminoketone hydrochloride salt 3.20. Such aminoketones are often isolated as the corresponding salts because the free aminoketones are prone to dimerisation, having both nucleophilic and electrophilic centres. (For a common alternative preparation of aminoketones, see the Knorr pyrrole synthesis, Chapter 2.) Liberation of the free base of 3.20 in the presence of the acid chloride affords amide 3.21 which is cyclised to oxazole 3.22. Ester hydrolysis then affords the biologically-active carboxylic acid 3.23. 

As with the multi-species components, it is not necessary for hydrogen bonding to be used in order to form dimeric capsules. This is highlighted by the recent synthesis of a molecule which assembles into dimers that are held together by n-interactions [137]. The meso-hexaphenyl calix[6]pyrrole 59 assembles into a dimeric capsule via the interactions between six phenyl groups arranged in a central belt around the capsule. The capsule is capable of holding two molecules of chloroform simultaneously, as seen in the solid state structure (Fig. 51). Similar phenyl interactions have been observed in ap-phenylcalix[5]arene which has been shown to dimerise and include C60 within its cavity (Fig. 52) [138]. 

The photochemistry of a phenyldiacrylic acid derivative has been studied in Langmuir-Blodgett films J The crystal structure of the major dimer formed on perdeuterioacetone-sensitized irradiation of f-butyl-2,5-dihydro-5,5-dimethyl-2-0X0-IH-pyrrole-l-carboxylate has been determined. Styryldicyanopyrazines undergo topochemical dimerisation when they are irradiated in the crystalline phase. ... 

Pyrrole can be 2,2 -dimerised with the hypervalent iodine(lll) reagent phenyliodine bis(trifluoroacetate) via what may be an SET process. ... 

Pyrroles can be dimerised, regiospeciflcally, via a radical cation produced from the pyrrole by reaction with phenyliodine(III) bis(trifluoroacetate). ... 

In one of the rare examples of the use of pyrrolyl radicals, Merz (99AGE1442) achieved dimerisation of 2-iodo-3,4-dimethoxy-5-phenyl pyrrole using high temperatures in the presence of copper. D Auria (97H(45)1775) studied the photochemical behaviour of 2-iodopyrrole derivative 237 in benzene (Scheme 65). 

---