2- pyrroles, conformational

As illustrated in Scheme 8.1, both 2-vinylpyrroles and 3-vinylpyiroles are potential precursors of 4,5,6,7-tetrahydroindolcs via Diels-Alder cyclizations. Vinylpyrroles are relatively reactive dienes. However, they are also rather sensitive compounds and this has tended to restrict their synthetic application. While l-methyl-2-vinylpyrrole gives a good yield of an indole with dimethyl acetylenedicarboxylate, ot-substitiients on the vinyl group result in direct electrophilic attack at C5 of the pyrrole ring. This has been attributed to the stenc restriction on access to the necessary cisoid conformation of the 2-vinyl substituent[l]. 

Iron Porphyrins. Porphyrias (15—17) are aromatic cycHc compouads that coasist of four pyrrole units linked at the a-positions by methine carbons. The extended TT-systems of these compounds give rise to intense absorption bands in the uv/vis region of the spectmm. The most intense absorption, which is called the Soret band, falls neat 400 nm and has 10. The TT-system is also responsible for the notable ring current effect observed in H-nmr spectra, the preference for planar conformations, the prevalence of electrophilic substitution reactions, and the redox chemistry of these compounds. Porphyrins obtained from natural sources have a variety of peripheral substituents and substitution patterns. Two important types of synthetic porphyrins are the meso-tetraaryl porphyrins, such as 5,10,15,20-tetraphenylporphine [917-23-7] (H2(TPP)) (7) and P-octaalkylporphyrins, such as 2,3,7,8,12,13,17,18-octaethylporphine [2683-82-1] (H2(OEP)) (8). Both types can be prepared by condensation of pyrroles and aldehydes (qv). 

Although the same theoretical studies indicate very small energy differences between the syn and anti conformers of the 3-carbaldehydes of furan, thiophene and pyrrole with a slight preference for the syn conformer, in chloroform solution the furan- and thiophene-3-carbaldehydes adopt the anti conformers to the extent of 100 and 80% respectively (82X3245). However, A-substituted 3-(trifluoroacetyl)pyrroles exist in solution as mixtures of rotational isomers (80JCR(S)42). 

Beckmann rearrangement, 4, 292 phototransposition, 4, 204 synthesis, 4, 223 Wittig reaetion, 4, 294 Wolff-Kishner reduetion, 4, 291 Pyrrole, 1-aeyl-barrier to rotation, 4, 193 IR speetra, 4, 21, 181 rearrangement, 4, 41 synthesis, 4, 82 thermal rearrangement, 4, 202 Pyrrole, 2-aeyl-aeidity, 4, 297 cleavage, 4, 289 conformation, 4, 33... 

Pyrrole, 4-ethynyl-2-formyl-3-methyl-synthesis, 4, 222 Pyrrole, formyl-oxidation, 4, 289 reactions, 4, 292 with sulfoxides, 4, 293 synthesis, 4, 223, 274, 287 Pyrrole, 1-formyl-barrier to rotation, 4, 193 Pyrrole, 2-formyl-benzoylation, 4, 220 conformation, 2, 107 4, 193 diacetoxythallium derivative iodination, 4, 216 dipole moment, 4, 194 ketals, 4, 290 protonation, 4, 47 reactions... 

N-alkylation, 4, 236 Pyrrole, 2-formyl-3,4-diiodo-synthesis, 4, 216 Pyrrole, 2-formyl-1-methyl-conformation, 4, 193 Pyrrole, 2-formyl-5-nitro-conformation, 4, 193 Pyrrole, furyl-rotamers, 4, 546 Pyrrole, 2-(2-furyl)-conformation, 4, 32 Pyrrole, 2-halo-reactions, 4, 78 Pyrrole, 3-halo-reactions, 4, 78 Pyrrole, 2-halomethyl-nucleophilic substitution, 4, 274 reactions, 4, 275 Pyrrole, hydroxy-synthesis, 4, 97 Pyrrole, 1-hydroxy-cycloaddition reactions, 4, 303 deoxygenation, 4, 304 synthesis, 4, 126, 363 tautomerism, 4, 35, 197 Pyrrole, 2-hydroxy-reactions, 4, 76 tautomerism, 4, 36, 198... 

Pyrrole-2-carboxamide, N,N-dimethyl-conformation, 4, 194 Pyrrole-3-carboxamide, N,N-dimethyl-conformation, 4, 194 Pyrrolecarboxamides synthesis, 4, 242 Pyrrole-2-carboxamides synthesis, 4, 148, 360 Pyrrolecarboxylhydrazides Curtius degradation, 4, 362 Pyrrole-2-carboxylic acid, l-benzyl-3-hydroxy-ethyl ester... 

Michael addition reactions, 4, 302 reactions with ally halides, 4, 301 Pyrrole-2-carboxylic acid, 1-methyl-conformation, 4, 194 esters... 

Pentadienyl radical, 240 Perturbation theory, 11, 46 Propane, 16, 165 n-Propyi anion conformation, 34 n-Propyl cation, 48, 163 rotational barrier, 34 Propylene, 16, 139 Protonated methane, 72 Pyrazine, 266 orbital ordering, 30 through-bond interactions, 27 Pyridine, 263 Pyrrole, 231... 

Silver(I) complexes with macrocyclic nitrogen ligands are also very numerous. Mono- or homodi-nuclear silver-containing molecular clefts can be synthesized from the cyclocondensation of functionalized alkanediamines or triamines with 2,6-diacetylpyridine, pyridine-2,6-dicarbalde-hyde, thiophene-2,5-dicarbaldehyde, furan-2,5-dicarbaldehyde, or pyrrole-2,5-dicarbaldehyde in the presence of silver(I).486 97 The clefts are derived from bibracchial tetraimine Schiff base macrocycles and have been used, via transmetallation reactions, to complex other metal centers. The incorporation of a range of functionalized triamines has provided the conformational flexibility to vary the homodinuclear intermetallic separation from ca. 3 A to an excess of 6 A, and also to incorporate anions as intermetallic spacers. Some examples of the silver(I) complexes obtained are shown in Figure 5. 

More success was obtained in the WL, NZ-diazasesquinorbomane series where it was possible to lock the N-Z isomers in conformations that were stable in solution to well above 100 °C. Thus, reaction of the N-Z 7-azanorbomadiene-2,3-anhydride 165 with N-Z pyrrole 166 yielded a single stereoisomer 167 in which the N-Z bridges were syn-facially related (Scheme 30). H NMR spectroscopy indicated that the N-Z groups were undergoing rotation in solution at room temperature and provided evidence for syn- and anri-atropisomers being present at lower temperature. 

Similarly, in a 1,3-dipolar cycloaddition of DMAD to the conformationally locked cyclic a-alkoxycarbonylnitrone (727), bicyclic ring systems, containing a nitrogen atom at the bridgehead position have been synthesized. A mechanistic interpretation of the origin of the fused pyrroles (729) includes the intermediate formation of the aziridine ring in (728) (Scheme 2.303) (820). 

W. Rettig, F. Marschner, Molecular conformation and biradicaloid charge transfer states in substituted N-pheny 1-pyrroles, NewJ. Chem. 74,819(1990). 

Antibodies also well catalyze the reactions where a molecule should he forced to adopt a particular and reactive conformation, thanks to privileged interactions with the amino acids of the binding site. For example, ahzymes with a ferrochelatase activity, 7G12, force the mesoporphyrin IX ring to adopt a distorted conformation favorable to the insertion of a Cu ion in the center of the macrocycle,thanks to an interaction with the HlOOc methionine which constrains one of the pyrrole rings to be left outside the plane of porphyrin (Figure 24). 

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