Five-membered ring systems reactions

Ring expansion of five- to six-membered rings such as oxazole —> pyridine derivatives via a Diels-Alder reaction is a well-established procedure. However, the conversion of a six-membered heterocycle into a five-membered ring system has not been exploited to any great extent, and those systems that have been studied usually involve a cationic species. 

Phosphorus heterocyclic compounds, 1, 493-538 five-membered ring systems, 1, 513-523 nomenclature, 1, 496 six-membered ring systems, 1, 497-513 Photoaromatization oxirenes from, 7, 125-126 Photobleaching chromenes in, 3, 880 Photochemical reactions heterocyclic compound synthesis from, 5, 159 reviews, 1, 56 heterocyclic compounds reviews, 1, 71, 72... 

Three skeleton atoms of a hetero-l,3-diene are incorporated into a five-membered ring system on reaction with 1,2 [n,n] compounds (e.g., hydrazines, reaction type 4, equation 10). On further heating in the presence of azobisiso-butyronitrile (AIBN), the bis(trifluoromethyl)-substituted 1,2,4-triazolines are transformed into 5-tnfluoromethyl-l,2,4-triazoles in high yield [t02 (equation 22). 

Five-membered ring systems can be obtained from hetero-l,3-dienes on reaction with oxiranes and thiiranes. To avoid competition from a possible 1,4-addition, the nucleophilic attack of the terminal heteroatom of the diene has to be sterically or electronically hindered by incorporation of the heteroatom into... 

Formation of five-membered ring systems (1,2-addition) can compete with formation of the seven-membered heterocycles (1,4-addition). If the first step of the reaction sequence, namely the nucleophilic attack of the terminal heteroatoin of the diene, is hindered by steric or electronic effects, the five-membered ring product is formed exclusively. 

To account for the course of this reaction theoretical calculations of the coordination of ketomalonate 37 to copper(II) and zinc(II) have revealed that the six-membered ring system is slightly more stable than the five-membered ring system (Scheme 4.30). The coordination of 37 to catalyst (l )-39 shows that the six-membered intermediate is C2-symmetric with no obvious face-shielding of the carbonyl functionality (top), while for the five-membered intermediate (bottom) the carbonyl is shielded by the phenyl substituent. Calculations of the transition-state energy for the reaction of the two intermediates with 1,3-cyclohexadiene leads to the lowest energy for the five-membered intermediate this approach is in agreement with the experimental results [45]. 

These model reactions were of great value in the extension of the C-glycosylation reaction with malonic esters to five-membered ring-systems. Treatment of 2,3 5,6-di-O-isopropylidene-a-D-manno-furanosyl bromide122,123 (150) with diethyl sodiomalonate led to an anomeric mixture of C-glycosyl compounds that could be separated by column chromatography, with the a (151) and /B (152) anomers in... 

Scheme 2-6. Reaction for a five-membered ring system.

Most of the reactions involving nucleophilic attack at a carbon atom of the ring result in cleavage of the five-membered ring system (which is in most cases either an oxazolidine or an oxazolidinone). Basic hydrolysis of... 

Heteroaromatic compounds do not undergo the same variety of photocycloadditions with alkenes as do their carbocyclic counterparts. There are very few reports of this type of reaction for six-membered ring compounds such as pyridines, but five-membered ring systems such as furans do give 1,2-cycloadducts with a range of alkenes (e.g. 357). 

Indolones have both an azomethine and carbonyl group carbon atom within the same five-membered ring system. The azomethine group is the more reactive center and readily takes part in reactions typical of that group.120... 

The five-membered ring systems can also behave as electron-rich dienophiles in [n4 + 2] reactions. Indole and 1,2,4,5-tetrazines yield indolopyridazines (262) via [ 4 + 2] cycloaddition followed by extrusion of nitrogen and aromatization, whilst the dihydro adduct,... 

In this section the known or reported reactions of A,B-diheteropentaIenes will be presented. In general, the compounds for which fully classical structures (la)-(lc) can be written behave like the simple five-membered ring systems which they are composed of. Not surprisingly the fully conjugated furanofurans (la-lc X = Y = 0) are unknown only reduced furanofurans are reported. This is probably a consequence of the high reactivity of the furan system compared to pyrrole, thiophene and selenophene. 

Systems which can react by either 5-exo or 6-endo cyclization normally produce the five-membered ring system. Exceptions result when equilibration of the initially formed five-membered ring is facile and substitution electronically favors the 6-endo mode of cyclization. Several examples have been found in amidoselenation reactions.41158 216 216 232 For example, halocyclization of thioimidate (17) produced only the pyrrolidone product,217b 217c 233 while selenocyclization of amide (18) produced only the piperi-done product.232 Note that the cyclization of (18) to a piperidone also involves regioselectivity in the cyclization of an amide functionality to a lactam rather than an imidate. Both the ring size and type of product can be explained by equilibration. 

The TBS protecting group is selectively cleaved in the first step. The released propargylic alcohol is then reduced with Red-AI to an allylic alcohol. Reduction of the propargylic alcohol occurs selectively trims. Since during this reaction a five-membered ring system 41 is created through coordination of aluminum with the alkox-ide, it is assured that aluminum is located m the 3-position relative to the alkoxide. 

The characteristic feature of the aforementioned oxazaborolidine catalyst system consists of a-sulfonamide carboxylic acid ligand for boron reagent, where the five-membered ring system seems to be the major structural feature for the active catalyst. Accordingly, tartaric acid-derived chiral (acyloxy)borane (CAB) complexes can also catalyze the asymmetric Diels-Alder reaction of a,P-unsaturated aldehydes with a high level of asymmetric induction [10] (Eq. 8A.4). Similarly, a chiral tartrate-derived dioxaborolidine has been introduced as a catalyst for enantioselective Diels-Alder reaction of 2-bromoacrolein [11] (Eq. 8A.5). 

The synthesis of five-membered ring systems can be achieved by a formal [4+1]- or a [2 + 2 + 1]-cycloaddition process essentially depending on the point of view with which one chooses to consider the reaction. Here we discuss the progress which has been made with iron complexes as catalysts for such transformations. 

The photochromic properties of DHA systems depend strongly upon the substituents on the five-membered ring, the reaction media, and the temperature. For example,1201 to obtain a steady state equilibrium mixture of 22a and 22b (Scheme 3), the 2,4-dinitrophenyl derivative of the DHA 22a had to be irradiated (in acetonitrile, 366 nm irradiation wavelength) at low temperature (200 K). In contrast to this, arene derivatives 21a and 23a, on irradiation at 250 K, were quantitatively converted to the corresponding VHFs 21b and 23b, respectively. In the case of the DHA 24a, a stationary equilibrium between 24a and 24b could be observed at room temperature. Thus, in general, it was observed that the presence of electron-withdrawing substitu-... 

Indole (2) undergoes electrophilic substitution preferentially at the b(C3)-position whereas pyrrole (1) reacts predominantly at the a(C2)-position [15]. The positional selectivity in these five-membered ring systems is well explained by the stability of the Wheland intermediates for electrophilic substitution. The intermediate cations from 3 (for indole, 2) and a (for pyrrole, 1) are the more stabilized. Pyrrole compounds can also participate in cycloaddition (Diels-Alder) reactions under certain conditions, such as Lewis acid catalysis, heating, or high pressure [15]. However, calculations of the frontier electron population for indole and pyrrole show that the HOMO of indole exhibits high electron density at the C3 while the HOMO of pyrrole is high at the C2 position [25-28] (Scheme 3). 

There are only a few reported examples of the formation of fluorinated five-membered ring systems by cycloaddition reactions. These are generally [3 + 2] cycloadditions of a fluoroalkene, fluoroalkyne or fluoroallene to a 1,3-dipolar ion. ... 

Most diazotized heterocyclic amines undergo normal coupling reactions with common aromatic coupling components, as well as with some C—H acidic compounds . This pertains for five-membered ring systems such as pyrrols 46 pyrazoles imidazoles 45 , l,2,4-(49) and 1,2,3-triazoles 50... 

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