Pyrrole thioureas

Figure 6.17 Pyrrole thiourea derivatives evaluated for catalytic activity and selectivity in the asymmetric acetyl-Pictet-Spengler reaction.

With thiourea 52 as the catalyst (5-10 mol%), a range of substituted S-configured tetrahydro- 3-carbolines 1-5 were accessible in good yields (65-81%) and high enantioselectivities (85-95%) as shown in Scheme 6.50. The imine substrates of this two-step procedure were generated in situ by condensation of the tryptamine with the respective aldehyde (1.05 equiv.) and were directly used without further purification (Scheme 6.50). Recovered (by chromatography) pyrrole thiourea catalyst 52 could be reused without the loss of activity or selectivity. 

Chemical antiozonants comprise the second general class of commercial antiozonants. Of the many compounds reported to be chemical antiozonants, nearly all contain nitrogen. Compound classes include derivatives of l,2-dihydro-2,2,4-trimethylquinoline, A/-substituted ureas or thioureas, substituted pyrroles, and nickel or zinc dithiocarbamate salts (see also Antioxidants). The most effective antiozonants, however, are derivatives of -phenylenediamine... 

Pyrrole-containing thiourea derivatives 52 and 53 were developed and optimized for hydrogen-bonding activation of N-acyliminium ions [76] in the acyl-Pictet-Spengler [202, 205] (Schemes 6.50 and 52) and acyl-Mannich reaction [204] (Scheme 6.51). List et al. extended the applicability of this thiourea type to... 

Fortunately, there is now a comprehensive body of knowledge on the metabolic reactions that produce reactive (toxic) intermediates, so the drug designer can be aware of what might occur, and take steps to circumvent the possibility. Nelson (1982) has reviewed the classes and structures of drugs whose toxicities have been linked to metabolic activation. Problem classes include aromatic and some heteroaromatic nitro compounds (which may be reduced to a reactive toxin), and aromatic amines and their N-acylated derivatives (which may be oxidized, before or after hydrolysis, to a toxic hydroxylamine or iminoquinone). These are the most common classes, but others are hydrazines and acyl-hydrazines, haloalkanes, thiols and thioureas, quinones, many alkenes and alkynes, benzenoid aromatics, fused polycyclic aromatic compounds, and electron-rich heteroaromatics such as furans, thiophenes and pyrroles. 

These hosts include simple anion sensor systems containing urea, thiourea, amine, amide, alcohol, and pyrrole groups linked to chromophores. Chro-mophores used in this type of chromogenic anion sensors are mainly organic dyes such as azobenzene, nitrobenzene, indoaniline, and anthraquinone or... 

A new method for the reductive conversion off-keto- -nitroesters to pyrroles involves the use of formamidinesulfinic acid (thiourea S,S-dioxide) as the reducing agent. <95TL9469> The ester substituent seems to be required since no reaction occurred with simple a-nitroketones. 

In the evolution of anion coordination chemistry, it was discovered that neutral molecules can also operate as effective receptors for anions provided that they contain polarised N-H fragments (e.g. of amides [53], ureas [54], thioureas [55] or pyrroles [56]), which act as H-bond donors for anions. These receptors cannot compete for hydrogen bonding with water and alcohols and must be studied in aprotic solvents of varying polarity, e.g. CHCI3, MeCN, DMSO. In this vein,... 

When, on the other hand, organocatalyst 133 (possessing a bulky 2,5-diaryl-pyrrole moiety) is applied, product 134 was selectively formed by a highly diastereo- and enantioselective 1,3-dipolar cycloaddition (11 examples, 56-90%, 60-91% ee). This reaction most likely involves activation of the nitroalkene by the thiourea, via the earlier mentioned doubly hydrogen-bonded interaction, followed by a concerted attack of the in situ formed azomethine ylide (this ylide is not activated by nor coordinated to the organocatalyst, because of the bulky, nonbasic pyrrole group, but is most likely formed via a 1,2-prototropic rearrangement [92]). 

The GN bond—Values for the internuclear distance in the GN bond are given in Table LV. The single G—N bond has a length of 1-47 A in N(GH3)3. In pyrrole, pyridine, pyrazine, urea and thiourea it is decreased, a fact evidenuy connected to the possible contribution of structures containing multiple bonds ... 

A mixture consisting of diastereomeric methyl 2-phenyl-2-(pyrrol-l-yl)cyclopropanecarboxylate and the C H insertion product in a 97 3 ratio (91 mg, 0.38 mmol), was ozonized in CHjClj. After addition of thiourea (30 mg, 0.39 mmol) in MeOH (3 mL) and workup, flash chromatography on silica gel afforded 58.2 mg (0.65 mmol, 73%) of the cyclopropane as a diastereomeric mixture which could be separated into the diastereomers by radial chromatography. 

Polymer-bound 5-(2-bromoacetyl)pyrroles could be converted into thiazolylpyrroles, aminothiazolylpyrroles and selenazolylpyrroles through Flantzsch-type reaction with thioamides, thioureas or selenourea respectively. The reaction conditions that were optimized for the three different types of building blocks are shown in Figure 6.16. 

Dichloromaleimide is converted at or below ambient temperature to a thieno-pyrrole by malononitrile-methoxide, and then hydrogen sulphide [3378]. Several dithienothiophenes are synthesized by oxidative cyclization of the lith-iated intermediates, but when the latter are unstable, the dibromosulphides may be treated with butyllithium and copper(II) chloride [2287]. The chlorodibromo-quinoline (81.2) is efficiently converted into the fused thiophene when heated with thiourea in a protic solvent [2873]. 

Azo compounds Diazo-, diazonium Pyridine-N Imino-N Pyrrole-N Cyanides Isocyanides Guanidines Sulfonamides Thioamides Amides Thioureas, ureas j Enamines Anilines Amines... 

Recent efforts pertaining to direct attachment of chiral sidechain to a heteroarene show that enantioselective Pictet-Spengler reaction is achievable in the presence of a multifunctional thiourea An interesting observation is that annulation of pyrroles by this method (with 17) can give different isomers from CC bond formation at an a- or 3-carbon ... 

A similar strategy tvas originally used for the Biginelli condensation reaction to synthesize a set of pyrimidinones (65-95%) in a household MW oven [196], an approach that has been successfully applied to combinatorial synthesis [197]. Yet another example is the convenient synthesis of pyrroles (60-72%) on silica gel using readily available enones, amines, and nitro compounds [198]. Three-component condensation of an aldehyde, urea or thiourea, and a dicarbonyl com-... 

Abstract This review article provides a broad overview to the area of anion coordination by synthetic organic receptors and includes examples of different functional groups used to bind anions. The first section examines neutral anion receptors containing amide-, sulfonamide-, urea- and thiourea-based receptors. Then aromatics such as pyrrole, car-bazole and indole are discussed before concluding the discussion of neutral systems with examples of hydroxy OH donors. A brief overview of charged systems is also provided. 

---