2.3- Dihydropyrrols, formation

Non-enolizable imines such as 9-fluorene imines react with alkynylcarbene complexes to afford mixtures of mesoionic pyrrolium carbonyltungstates and dihydropyrrole derivatives [68] (Scheme 23). Although both compounds can be considered as [3C+2S] cycloadducts, formation of each of them follows a very different pathway. However, the first intermediate of the reaction is common for both compounds and supposes the conjugated addition of the imine to the alkynylcarbene complex to form a zwitterionic intermediate. A cyclisation... 

The same principle of sequential cyclopentene-opening RCM resulting in the formation of a dihydropyrrole ring was the key step in Blechert s novel approach to the polyhydroxylated indolizine alkaloid (-)-swainsonine (378) via RRM of 375 (Scheme 73) [157]. 

The tricyclic core of spirotryprostatin B can be formed via formation of the dihydropyrrole 325 <2000AGE4596>. Removal of the silyl protecting group of 322, followed by Dess-Martin oxidation, and reaction of the resultant aldehyde with the potassium salt of the diketopiperazine phosphonate 323 led to formation of the enamide 324. 

In 1979, Claesson et al. observed the formation of the dihydropyrrole 125 and the pyrrole 126 when trying to purify the amine 124 by GLC [85]. They suspected that an initial cycloisomerization first leads to 125 and a subsequent dehydrogenation then delivers 126. Guided by other intramolecular nucleophilic additions to alkynes that are catalyzed by AgBF4, they discovered that this catalyst efficiently allowed the transformation of 124 to 125 (Scheme 15.38). Reissig et al. found that with enantio-merically pure substrates of that kind a cyclization without racemization is possible with Ag(I) catalysts [86],... 

Under Dakin-West reaction conditions (trifluoroacetic anhydride-MeCN/80 °C/5 h), At-methoxycarbonylproline (128 R = Me) yielded Af-methoxycarbonyl-4-trifluoro-acetyl-2,3-dihydropyrrole (129 R = Me) and none of the expected Dakin-West product, the trifluoromethyl ketone (127). A possible mechanism proposed by the authors involves initial formation of a mesoionic l,3-oxazolium-5-olate (130 R = Me), but the pathway to the iV-methoxycarbonyl-2,3-dihydropyrrole (131 R = Me) and thence the final product (129 R = Me) was unexplained. ... 

Additional heterocyclic ring systems, such as benzofurans [125], dihydropyrroles and dihydroazepines [41], piperidines and dihydropyrimidines 36 [126], and fused oxazole derivatives [127], have been described (Eq. 7). The formation of epoxides and aziri-dines, formally emanating from ylides, was recently reported by Doyle et al. [77]. Rho-dium(II)-catalyzed isomiinchnone cycioaddition followed by Lewis acid-mediated ring opening has been used as an entry into the protoberberine azapolycyclic ring structure [128]. 

However, treatment of the precursor 74, where there is no substitution at C(4) (i.e., R = Me) led to a single [3+2] cycloadduct 75 with methyl acrylate. The unstable oxazolines 75, are considered to open spontaneously to their valence bond, 1,3-dipole tautomers 76, which are trapped in situ by the dipolarophile. Use of DMAD led to the formation of the expected 2,5-dihydropyrrole (77), but difficulties in isolation required DDQ aromatization to pyrrole 78 (Scheme 3.19). 

Further study with Rh2(OAc)4-catalyzed reaction styryldiazoacetate 188 and cinnamaldehyde derived imine 189 found the formation of dihydropyrrole 190 and dihydroazepine 191 in high yields and with high stereocontrol. No aziridine products were observed in these cases (Equation (29)). ... 

The homologous azirine (143) with a one-atom bridge gave quite different results.70 Photolysis led to the 3,5-fused bicyclic dihydropyrrole (144). The isomeric azirine (145) also led to (144), although the initial products included dihydropyrrole (146) which apparently converted to (144) as photolysis continued. Azirines (143) and (145) were shown to not interconvert and die postulated two discrete azomethine ylides were trapped with methyl trifluoroacetate. Formation of dihydropyrrole (144) was explained based on a two-step cycloaddition process involving a common diradical intermediate. The observation of (146) from photolysis of (145) but not (143) can be explained based on extinction coefficient differences. Azirine (145) has a high extinction coefficient as does (146). The initial product (146) can then be optically pumped to (144) with a low extinction coefficient. Azirine (143) also has a low extinction coefficient and any (146) that formed from it would be optically pumped to (144) before observa-... 

As already shown (see Section IV.A), pyrolysis of 0-vinyloximes in the absence of KOH/DMSO does not lead to 4//-2-hydroxy-2,3-dihydropyrroles. It has been assumed (84MI1) that the rearrangement is of specific anionic character and does not necessarily involve the formation of neutral 0, A-divinylhydroxylamines (87,90,93). The intermediate anions (96) are likely to be more active in this case (Scheme 46). 

The authors suggested a stepwise mechanism in which precomplexation of the sulfoxide to the lithium azaenolate would take place, thus allowing conjugate addition to follow. This notion was based on Posner s and Paquette s earlier work on conjugate additions of nucleophiles to 57. As before, thermal elimination of toluene sulfenic acid led to the conjugated products (not shown) in 92-93% yields for R2 = methyl.30 However, for R2 = H thermal elimination of arylsulfenic acid did not afford any dihydropyrrole product but rather led to the formation of pyrroles. Treating 58 or 59 with Raney nickel at low temperature led to the rapid formation of 60 and 61, respectively, in 82-86% yield. 

In the presence of Mn(OAc)3 or a Ce(iv) salt, 1,3-dicarbonyl compounds and /3-carbonyl imines react with allylsilanes to give silylmethylated dihydrofurans and dihydropyrroles, respectively.199,200,20011 A proposed mechanism involves the formation of a /3-silylcarbenium ion intermediate via two-electron oxidation and subsequent intramolecular nucleophilic attack (Equation (51)). a... 

Electrophile-mediated cyclization reactions of alkynes tethered to pendant heteroatom nucleophiles is an emerging strategy for the synthesis of heterocycles. This methodology has now been applied to the synthesis of pyrroles. The iodocyclization of 3-aminoalkynes 1 led to the formation of dihydropyrrole 2 <07TL7906>. Treatment of the latter with mesyl chloride in the presence of triethylamine then gave (i-iodopyrrolcs 3. 

Among ions, the opening of a cyclopropyl anion is exemplified by the reactions of the trans and cis aziridines 6.55 and 6.58, which are isoelectronic with the cyclopropyl anion. They open in the conrotatory sense to give the W- and sickle-shaped ylids 6.56 and 6.59, respectively, which are isoelectronic with the corresponding allyl anions. This step is an unfavourable equilibrium, which can be detected by the 1,3-dipolar cycloaddition of the ylids to dimethyl acetylenedicarboxylate, which takes place suprafacially on both components to give the cis and trans dihydropyrroles 6.57 and 6.60. The conrotatory closing of a pentadienyl cation can be followed in the NMR spectra of the ions 6.61 and 6.62, and the disrotatory closing of a pentadienyl anion can be seen in what is probably the oldest known pericyclic reaction, the formation of amarine 6.64 from the anion 6.63. 

The formation of pyrrole derivatives has been reported earlier. The triazoline (392) was converted by heating at its melting point into the 4-oxazoline (393) which in turn gives the 2,5-dihydropyrrole (394) on reaction with ethyl acetylenedicarboxylate (70CR(C)(27i)9S8). 

The ambident 1,3-dipolar 2-arylthiocarbamoyl imidazolium salts 859 are readily prepared from the corresponding carbenes and phenyl isothiocyanate as crystalline solids. Reaction of 859 with DMAD leads to the formation of spiro[imidazoline-2,3 -dihydrothiophenes] 860, indicating that 859 acts as a C-C-S dipole. In contrast, the reaction of 859 with ethyl propiolate proceeds slowly to form spiro [imidazoline-2,3 -dihydropyrroles] 861, indicating a C-C-N dipolar reaction being thermodynamically driven (Scheme 211) <2006CC1215>. 

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