A new synthesis of pyrroles from nitroalkenes

Abstract

Nitroalkenes, or their fi-acetoxynitro precursors, react with a-isocyanoacetate esters in the presence of base to give high yields of 5-unsubstituted pyrroles; some important pyrroles are easily synthesised by this method.We have recently reported1 a convenient method for the synthesis of pyrroles from nitroketones using diphenyl disulphide-tributylphosphine as reductant. We now report another new route to pyrroles unsubstituted in position 5 and, therefore, particularly relevant to the porphyrin field. Central to this method is the ability of the nitro group to depart as nitrite under certain conditions.2 Thus base-catalysed Michael addition of an a-isocyanoacetate (1) to a nitroalkene (2) followed by cyclisation of the nitronate anion (4) onto the isocyano group would lead to the pyrroline ( 5 ) (Scheme 1). Similar isonitrile cyclisations have been used extensively by Schollkopf, Van Leusen, and others for the preparation of various heterocycles.3 Base-catalysed expulsion of nitrite from the pyrroline and double bond rearrangement would finally give the pyrrole (6) (Scheme 1).This conception was easily reduced to practice. Thus, addition of the P-nitrostyrene derivative (2a) to a mixture of t-butyl a-isocyanoacetate (la) and the guanidine base (7)4 in tetrahydrofuran-propan-2-01 at room temperature resulted in a smooth reaction to give the expected pyrrole (6a) in 90%

Synthesis of highly substituted pyrroles using ultrasound in aqueous media

“…The classic methods for the synthesis of pyrroles described in the literature are Knorr (11Á14), Hantzsch (15), PaalÁKnorr (16Á20), and BartonÁZard (21,22) reactions. The PaalÁKnorr reaction, in which 1,4-dicarbonyl compounds react with primary amines under slightly acidic conditions, is a very attractive approach.…”

Five‐Membered Heterocycles: Pyrrole and Related Systems

“…An ensuing isomerization of 110 to 111, followed by elimination of nitrite, provides the 3H-pyrroles 112, which finally tautomerize to the target pyrrole-2-carboxylate derivatives 113 (Scheme 4.32). Sensitive nitroolefins are preferably formed in situ from the corresponding b-nitroacetoxyalkanes [151,152] [153,154]. Alternatively, the nitroolefin components may be replaced by a,b-unsaturated sulfone derivatives [155][156][157] or by acetate precursors bearing a vicinal nitro group [158].…”

Distance Dependence of Photoinduced Electron-Transfer: Syntheses and Properties of Biphenylene-Spacered Porphyrin-Quinone Cyclophanes

“…The rigidly extended biphenylene spaup the wanted biphenylene-spacered porphyrin cyclophanes. In the next step the condensation of the aldehyde cers in the porphyrin-quinone cyclophanes 3, of course, would prevent such a close interaction between the quinone groups of 16 with 2-benzyloxycarbonyl-3-ethyl-4-methylpyrrole [7] , carried out earlier with good results in the syn-part and the zinc ion in the porphyrin centre. Thus, from 3(X‫؍‬H) and 17 with zinc(II) acetate in trichloromethane/ theses of other porphyrine cyclophanes [1] [2] [3] , in this specific case resulted in unsatisfactory yields since the hy-methanol (5:1) the zinc complexes of the biphenylene-spacered cyclophanes 20 and 21 were obtained in about 80% drogenolytic removal of the benzyl groups also led to a partial cleavage of the four-membered rings of the biphenyl-yield.…”