Pyrroles structure

Reduction of indolenines with sodium and ethanol gives indolines. The pentachloropyr-role, obtained by chlorination of pyrrole with sulfuryl chloride at room temperature in anhydrous ether, was shown by spectroscopic methods to have an a-pyrrolenine (2H-pyrrole) structure (222). It is necessary, however, to postulate that it is in equilibrium with small but finite amounts of the isomeric /3-pyrrolenine form (3//-pyrrole 223), since pentachloropyrrole functions as a 2-aza- rather than as a 1-aza-butadiene in forming a cycloadduct (224) with styrene (80JOC435). Pentachloropyrrole acts as a dienophile in its reaction with cyclopentadiene via its ene moiety (81JOC3036). 

The amino form is usually much more favored in the equilibrium between amino and imino forms than is the hydroxy form in the corresponding keto-enol equilibrium. Grab and XJtzinger suggest that in the case of a-amino- and a-hydroxy-pyrroles, structure 89 increases the mesomeric stabilization and thus offsets the loss of pyrrole resonance energy, but the increase due to structure 90 is not sufficient to offset this loss. Similar reasoning may apply to furans and... 

Nitrogen compounds in crudes may generally be classified into basic and non-basic categories. Basic nitrogen compounds are mainly those having a pyridine ring, and the non-basic compounds have a pyrrole structure. Both pyridine and pyrrole are stable compounds due to their aromatic nature. 

Another series of pyrroles, structurally related to amino acids, was obtained in a microwave-assisted solvent-free condensation of a-amino acid methyl esters with chloroenones, which provided the four-carbon unit of the pyrrole. The reaction was carried out by mixing the reagents on silica gel and irradiating for 2-6 min inside a multimode microwave cavity (Scheme 7). The authors reported higher yields and cleaner products when microwaves were used instead of conventional heating [34],... 

The DSC and TGA plots of the oxidized polymer (VIII) showed that the Tm is 130°C and the weight loss of 20% and 80% was observed at 455°C and 600°C, respectively, compared to 400° and 482°C for the original polymer VII indicating the oxidized polymer was more stable to heat. This observation was consistent with the chemical structure of the oxidized polymer, which consisted of a repeating aromatic pyrrole structure and, therefore, should be more thermodynamically stable. The thermal data of the polymers are tabulated in Table II. 

The Boger pyrrole synthesis based on a heterocyclic azadiene Diels-Alder strategy (1,2,4,5-tetrazine to 2,2-diazine to pyrrole) was employed by the author for the total synthesis of ningalin B . Thus a Diels-Alder reaction of the electron-rich acetylene 52 with the electron deficient 1,2,4,5-tetrazine 53 proceeded to give the desired diazine 54 which underwent subsequent ring contraction to afford the core pyrrole structure 55. 

Oxime 41c gives a compound in 2% yield (87KGS937), which was first assigned a 3//-pyrrole structure, 3aH-3a,5-dimethyl-4,6-diphenylpyrrolo-[3,2-c]piperidine (43) (Scheme 24). The formation of this compound is explained by involvement of the piperidine fragment methine group in the cyclization (see Section IV.C). 

Fig. 1 Basic structure (left side) and the absorption spectra (right side) of some photosensitizers. The absorption spectra of porphyrins (derived from porphins) consists of a Soret band (around 400 nm) and four Q-bands. Upon reduction of one or two double-bonds of the tetra-pyrrole structure or by expanding the number of 7i-electrons (by expanding the ring structure), the outermost Q-band becomes bathochromically shifted and the absorption coefficient increased as indicated on the figure. As described in the text, such chemical modifications are important for improving the therapeutic effect in deeper tissue layers...

The homo- and heterochiral self-association of lH-l,6a-dihydropirrolo[2,3-b] pyrrole derivatives has been studied using density functional theory (DFT) (B3LYP/ 6-31+G ) and ab initio (MP2/6-311+G ) methods [26]. The selected pyrrolo-pyrrole structure (Scheme 3.16) has a unique disposition of theNH group situated in cis to the substituent in position 6a. This simplifies the study, reducing the potential number of conformations, and also maximizes the interaction of the 6a position X groups in the homochiral complexes, as they are forced to point each other. 

Based upon studies involving the interaction of 6-acetylmethylenepenicil-lanic acid (27) with TEM-1 yff-lactamase, as well as its chemical reactivity with sodium methoxide and hydroxylamine, Arisawa and Adam [45] proposed a mechanism for the inhibition of TEM-1 )8-lactamase by (27) Scheme 6.10). They proposed the initial formation of a rather stable inactivated enzyme (55). This was followed by either slow hydrolysis of the acyl bond, releasing active enzyme, or rearrangement to (56). The stability of this intermediate was explained on the grounds of its pyrrolic structure the conjugated ester behaves as a vinylogous urethane, which is known to be resistant to hydrolysis [72]. 

The product obtained on acetylation of clivorine (32) has been shown to be the acetate of the quaternary pyrrolinium derivative (33). The compound (33) could be converted into the chloride using an ion-exchange resin in the chloride form. The pyrrolic structure was strongly indicated by the deep violet colour given on treatment with the Ehrlich reagent. Analogous quaternary derivatives were given by other otonecine ester alkaloids. 

Evidence from NMR spectra also confirmed a 2//-pyrrole structure, a diastereotopic methylene ( ) distinguishing between isomers 9 and 10.17 These workers were unable to find any evidence of 3//-pyrroles among the compounds isolated (but see Section III,B,1). It is interesting that all earlier workers associated a menthol- or camphor-like odor with products from the Grignard reaction,1013-17 since this is reported to be characteristic of the 3H rather than the 2H compounds.8... 

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