Pyrrole 2,3-dihydro- from

Furan, 2,5-dialkoxy-2-( 1 -hydroxyalkyl)-2,5-dihydro-ring expansion, 1, 425 Furan, 2,5-dialkoxytetrahydro-pyrrole synthesis from, 4, 330 Furan, 2,4-dialkyl-synthesis, 4, 661, 685 Furan, 2,5-dialkyl-... 

The rearrangement of cyclic sulfoxide 130a, which could be obtained from the corresponding thiolsulfinate 163 (Section III.C.l), was studied in different experimental conditions. The ci3-sulfoxide refluxed in toluene for 24 h gave a 1 1 mixture of the pyrrole 131 and the dihydro-1,4-thiazine 132. 

Chlorophyll a, the green photosynthesis pigment, is the prototype of the chlorin (2,3-dihydro-porphyrin) class of products. It was first isolated by Willstatter1 at the turn of the century. The common structural unit in this class is the chlorin framework named after chlorophyll. The chromophore with a partially saturated pyrrole ring, which is formally derived from the completely unsaturated porphyrin, is less symmetric than the latter and systematically named according to IUPAC nomenclature as 2,3-dihydroporphyrin. 

The pyrrolotriazines 32 were prepared from 3,4-dihydro-5-ethoxy-2//-pyrrole 31 by sequential reaction with hydrazine hydrate and a-oxoesters (85GEP3340026). They are useful as selective herbicides (Scheme 10). 

Methyl 3-acyl-l-diphenylmethyleneamino-4,5-dioxo-4,5-dihydro-l//-pyrrole-2-carboxylates 489 are formed from 488 and oxalyl chloride in good yields. Preparative thermolysis of these compounds at 130-140°C gives mixtures of dipyrazolo[l,2- l,2- [l,2,4,5]tetrazines 491 as major products and pyrazoles 492 as minor hydrolytic by-products. The intermediacy of mesoionic compound 490 is expected (Scheme 83) <2004T5319>. 

It is interesting to note that these compounds attract mainly the males of the respective species. Therefore, these compounds might be pheromone mimics. Protein hydrolysates, on the other hand, attract both sexes of most Tephritids and are widely used as food baits. However, volatiles from protein hydrolysates may not be simply food attractants. The cyclic imine, 3,4-dihydro-2H-pyrrole, has been identified in protein hydrolysate volatiles (12), and this compound has been identified by Baker and coworkers (11) as the sex pheromone Which attracts virgin females to the sexually mature male Medflies. Why the protein hydrolysate volatiles attract the males and other species remains unanswered. 

Disubstituted thiophenes 96 and 1,2,5-tri- and 2,5-disubstituted pyrrole derivatives 97 are available readily from m-3,6-disubstituted-3,6-dihydro-l,2-dioxins in a one-pot synthesis. The reaction proceeds by an initial Kornblum-de la Mare rearrangement of the 3,6-dihydro-l,2-dioxin to its isomeric 1,4-diketone followed by the condensation with Lawesson s reagent, ammonium carbonate, or a primary amine (Scheme 21) <2002TL3199>. 

Erba et al. (102) observed a novel formation of pyrrole imines 181 from the 1,3-dipolar cycloaddition of mtinchnones 179 and 5-amino-l-aryl-4,5-dihydro-4-methylene-1,2,3-triazoles 180 (Table 10.4). Treatment with benzaldehyde yielded 3-formylpyrroles (182). The reaction presumably involves loss of carbon dioxide, nitrogen, and morpholine from the initial cycloadduct. Unsymmetrical mtinchnones behave regioselectively and furnish products derived from bonding between C(2) of... 

The allylically activated chiral methanimidamides are more reactive and can be prepared from 2,5-dihydro-l//-pyrrole or 1,2,5,6-tetrahydropyridine by heating with a chiral auxiliary substituted methanimidamide in toluene. Deprotonation of the more acidic 1 -iminomethy 1-2,5-dihydro-1//-pyrrole with butyllithium was complete after a few minutes, even at — 100 °C41. Alkylation afforded a mixture of regioisomers, 2-alkylated 2,5-dihydro-l //-pyrrole 1 (n = 1) and 3-alkylated 2,3-dihydro-l//-pyrrole 2 (n = 1), the former strongly predominating (about 92 8). During hydrazinolysis of the 2-substituted 2,5-dihydro-1 //-pyrrole 1 (n = 1) the minor product decomposed, thus separation of the regioisomers was unnecessary. About 80-85% of the chiral auxiliary (S)-l- m-butoxy-3-methyl-2-butanamine was recovered after hydrazinolysis. 

A mixture of silver(I) fluoride and 2,2 -bipyridine replaces chlorine with fluorine in 1-chloro-octane in 86% yield after 45 minutes at 130°C.28 Some heterocyclic polychloro derivatives are also fluorinatcd by silver(I) fluoride. Thus, perfluoro(2,3,4,5-tetrahydropyridine) (1) and per-fluoro(3,4-dihydro-2//-pyrrole) are prepared from 2,2,6-trichloro-3,3,4,4,5,5-hexafluoro-2,3.4,5-tetrahydropyridine and 2,2,5-trichloro-3,3,4.4-tetrafluoro-3,4-dihydro-2//-pyrrole by reaction with silver(l) fluoride at room temperature.31 Since the fluorination of 2,2,5-trichloro-3,3,4,4-tetrafluoro-3,4-dihydro-2//-pyrrole with silver(l) fluoride is slow, recycling is necessary.31... 

The relative importance of through-bond (hyperconjugative) and through-space (homoconjugative) interactions between the heteroatom and the double bond in 2,5-dihydro-furan, -thiophene and -pyrrole has been the subject of a CNDO/S study (76ZN(A)215). This analysis concluded that the proportion of through-space interaction increased from 19% in the dihydrofuran and 20% for dihydrothiophene to 83% for the dihydropyrrole (cf. Section 2.3.3.9). 

A second type of route from pyrroles to indoles involves construction of a vinylpyrrole suitable for [4 + 2] cycloaddition leading to a dihydro- or tetrahydro-indole (Scheme 19) (80JOC4515, 81T1597). The dihydro systems can be easily aromatized. 2-Nitrovinylpyrroles, which are readily available by condensation of pyrrole-2-carbaldehyde with nitromethane, give fully aromatic indoles on reaction with dienophiles (equation 139) (73JCS(P1)2450). The aromatization results from elimination of nitrous acid and a further dehydrogenation. 

Preparative details for passing from dihydro- to tetrahydro-1,3-oxazines followed by ring opening have been given.241,242 The methods have been used to synthesize systems related to natural products243 including those with a pyrrole ring.244-248... 

Although 2,5-dimethyl-1-hydroxypyrrole was synthesized in 1886 by Knorr, the parent 1-hydroxypyrrole was obtained only 90 years later. These compounds have not been widely investigated. The corresponding radicals, the pyrrole 1-oxyls, their 2,5-dihydro derivatives, and particularly the pyrrolidine 1-oxyls, if the adjacent positions to the nitrogen atom are substituted, can be stable. Many of these pyrrolidines have been prepared and used as spin labels. Only one 1-hydroxypyrrole, an antitumor antibiotic, has so far been isolated from living systems. 

The hypothesis about the formation of compounds like B was also proven by other authors [35, 36]. For example, the Padmavathi et al. [36] studied the reactions of heterocyclic analogues of chalcones 14 obtained from the appropriate diaroylacetones 13 and diazomethane (Scheme 2.5). It was shown that this treatment gives a solid which was identified as (4 -aryl-4/,5 -dihydro-l/Ff-pyrazol-3 -yl)-(4-aryl-l/f-pyrrol-3-yl)methanone 15. 

To a cooled solution of 2 (5 mmol) in dichloromethane (20 ml), an ethereal solution of diazomethane (40 ml, 0.4 M) and triethylamine (0.12 g) was added (Scheme A.9). The reaction mixture was kept at —20 to — 15 °C for 40-48 h. The solvent was removed under reduced pressure. The resulting solid, (4 -aryl-4, 5 -dihydro-l i/-pyrazol-3 -yl)-(4-aryl-l//-pyrrol-3-yl)methanone 3, was purified by recrystallization from methanol. 

In CHEC-II(1996), several applications of dihydro-2//-l,2,3-diazaphospholes, generated in situ from a ketone hydrazone and PC13, as useful intermediates for the synthesis of indoles, pyrroles, pyrrolylacetates, and 1,2-di-hydro-2-alkenyl-37/-pyrazol-3-ones are listed <1996CHEC-II(4)771>. 

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