Dihydropyrrole, substituted

The hexahydropyrrolo[l,2- ]imidazole chloro cycloadducts 57a-c, as a 1/1 mixture of stereoisomers, underwent an elimination on treatment with l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in dimethyl sulfoxide (DMSO) at 100°C. It was not possible to isolate the desired dihydropyrroles 58a-c and a second elimination occurred to form the T-substituted pyrroles 59a-c respectively (Scheme 4) <1998J(P1)2061>. 

In analogy with a-hydroxy-substituted alkoxyallene adducts, the corresponding allenylamines, e.g. 109 in Scheme 8.30 and 85 in Eq. 8.20, can be cyclized to dihydropyrrole derivatives either under basic conditions [43, 74, 75] or by treatment with catalytic amounts of AgN03 in acetone or acetonitrile [43, 73, 74],... 

Dihydropyrroles and y-Lactams via Allylic Substitution and Ring-Closing Metathesis... 

Products from reactions with diacylamines or nosylamines can be very easily deprotected to give primary aUylamines. These were used as nucleophiles in allyhc substitutions to give secondary amines, which were transformed into unsymmetri-cally 2,5-disubstituted 2,5-dihydropyrroles (Scheme 9.28) [28aj. Thus, the allylic... 

Scheme 9.28 2,5-Disubstituted 2,5-dihydropyrroles via allylic substitution in combination with ring-closing metathesis.

Methoxyethyltosylamide also participates in the [3 -I- 2] addition reaction with 102, although it does not give any of the expected dihydropyrrole derivative 104. Instead, the major product was found to be pyrrole 105, which presumably results through ready elimination of methanol from the putative intermediate 104. Thus, this addition holds promise for the synthesis of 2-substituted tosylpyrroles (Scheme 29). In addition to 105, a minor product 106 (12%) is also formed in this reaction. 

The McMurry reaction also gave new multiphotochromic dithieny-lethenes 145 and 146 containing an N-substituted dihydropyrrole bridge... 

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). 

The fluorination of 1-substituted indole 32 with F-Teda BF4 (6) in acetonitrile or acetonitrile/ methanol gives 3-fluoro-2-methoxy-l-tosyl-2,3-dihydroindole (33) in 48% isolated yield.87 Relative stereochemistry about the 2,3-bond in the dihydropyrrole ring was confirmed as trans by X-ray crystallographic analysis. F-Teda BF4 (6) reacts with 2- and 3-trimethylstannyl-sub-stituted 1-tosylindoles to give the corresponding fluoroindoles in 40 and 21 % yield, respectively.108... 

A similar intermolecular cyclization was recently utilized in the synthesis of highly substituted dihydropyrrole derivatives [133 -135]. In a specific example, the addition of pentadienyltosylamide derivatives 177 to propynyl(phenyl)iodo-nium triflate initiates a sequence of transformations that furnishes the complex, highly functionalized cyclopentene-annelated dihydropyrrole products 178 in moderate yields with complete stereoselection (Scheme 66). Under similar reaction conditions, the isomeric isoprene-derived tosylamide 179 reacts with propynyl(phenyl)iodonium triflate to give azabicyclo[3.1.0]hexane 180 as the final product [134]. 

To extend the synthetic scope of this new reaction, we have carried out the functionalization of the new 1,4-dilithiated derivatives 321 with different electrophiles. The obtained dihydropyrrole derivatives 322 or 323 could be easily oxidized to the corresponding 3,4-difunctionalized pyrroles, which present a pattern of substitution difficult to achieve by conventional methods (Scheme 82)141. 

Silver(I)-catalyzed cyclizations of substituted allenes to heterocydic ring systems induding 2,5-dihydropyrroles have been described previously [4,10]. Moreover, sil-ver(I) salts are known to form stable rr-complexes with terminal acetylenes [11]. On the other hand, on treatment with silver nitrate silylacetylenes were reported to afford silver acetylides [12]. Based on these considerations and additional experimental evidence [5,13], the following mechanism has been proposed forthe sUver(I)-mediated oxidative cydization of homopropargylamines to pyrroles ]5] (Scheme 15.3). 

Cyclization of 3-Hetero-Substituted w-Azido-1,3-Dienes to Fused Dihydropyrroles General Procedure148 ... 

Bach, T., Brummerhop, H., Harms, K. The synthesis of (+)-preussin and related pyrrolidinols by diastereoselective Paterno-Buchi reactions of chiral 2-substituted 2,3-dihydropyrroles. Chem.— Eur. J. 2000, 6, 3838-3848. 

Scheme 6.18 Asymmetric hydrogenation of 2 substituted N alkyl 4,5 dihydropyrroles by an iridium catalyst.

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