Die Aminopentadienal-Umlagerung

Contrary to the rearrangement of 3-amino-3-X-propenals, which easily gives 3-X-propenamides at low temperature, the postulated rearrangement (Scheme 1) of the vinylogous 5-amino-5-X-pentadienals 2 normally stops at the level of (2H-pyran-2-ylidene)ammonium salts 4 . The main reason is that salts of type 4 are highly delocalized low-energy, charged species which makes addition 4 → 5 of weak nucleophiles difficult. In this paper, the first examples of the so-called `aminopentadienal' rearrangement are reported. Ring-opening 4 → 6 is facilitated by nucleophilic counter ions like X=PhO (see Scheme 4) or by adding an excess of `nucleophilic auxiliaries' such as Et₃N or EtOH (see Scheme 2). In a quite interesting sequence of steps, 5-phenoxy-5-(pyrrol-1-yl)penta-2,4-dienal ( 2g ; X=PhO) is easily transformed into 5H-pyrrolo[1,2-a]azepin-5-one ( 9 ) (Scheme 5).     

Säure-Additionen an ‘Push-Pull’-Enine. Erste Beobachtung einer Umlagerung von 5-Chloro-5-(dialkylamino)pentadienalen

Reaction of ‘Push-Pull’ Enynes with Acids. First Observation of a Rearrangement of 5-Chloro-5-(dialkylamino)pentadienals ‘Push-pull’-enynes 7 react easily with HCl as well as with AcOH to give 5-amino-5-chloropentadienals 8 and 5-(acetoxy)-5-aminopentadienals 13 as well as the corresponding ketones. In view of a postulated rearrangement of compounds 8 and 13 (Scheme 2), both types of compounds have been treated with traces of acid. While no definite reaction is observed in case of 13 , HCl-addition products 8 easily and quantitatively rearrange to give 2H-pyran-2-iminium chlorides 10 which are the postulated intermediates of the rearrangement 8 → 12 (Scheme 2).     

Rearrangement of 5-Substituted 5-Aminopentadienals

Strong nucleophiles are needed for inducing ring opening 3 → 4 since the pyrylium salt intermediates 3 , formed from 5-X-substituted 5-aminopenta-2,4-dienals 2 upon treatment with acid, are quite unreactive due to “aromatic” stabilization. However, this allows an easy access to a variety of 2-aminopyrylium salts 3 from “push–pull” enynes 1 . X=OAc, F, Cl, Br, I, OPh.     

Verallgemeinerung der Aminopentadienal‐Umlagerung

Generalization of the Aminopentadienal Rearrangement Contrary to the rearrangement of 3‐amino‐3‐X‐prop‐2‐enals 2 (R=H), which easily give 3‐X‐prop‐2‐enamides 3 at low temperature, the postulated rearrangement (Scheme 1) of the vinylogous 5‐amino‐5‐X‐penta‐2,4‐dienals 6 (R=H) normally stops at the level of 2‐aminopyrylium salts 7 . The main reason is that the charge in salts of type 7 is highly delocalized, leading to low‐energy species, which make addition of weak nucleophiles difficult. In this paper, two concepts for increasing the chances of the `aminopentadienal rearrangement' 6 →→ 8 are presented and substantiated by typical experiments. On one side, the easily available 2‐aminopyrylium chlorides 7 (X=Cl) are reacted with a twofold excess of secondary amines (Scheme 2) to give 5‐(dialkylamino)penta‐2,4‐dienamides of type 9 and 10 . On the other hand, after replacing the amino groups of 6 by PhO and EtO groups, the corresponding 5‐chloro‐5‐phenoxy‐ ( 13b ) and 5‐chloro‐5‐ethoxypenta‐2,4‐dienals ( 13a ) easily rearrange at low temperature to give 5‐chloropenta‐2,4‐diene‐1‐carboxylates 18a and 18b , respectively, which are now obviously lower in energy than the corresponding pyrylium‐salt intermediates 16 (Scheme 4).     

Versuche zur Synthese von polymergebundenen `Push-Pull'-Acetylenen

In view of possible applications of `push-pull' acetylenes 1 and `push-pull' olefins 2 as potential non-linear optics materials, synthetic attempts towards polymers of type 4 and 6 (Scheme 2) have been undertaken. Two so far unknown `push-pull' acetylenes 1a , b with R=styrene, have been prepared under carefully controlled conditions which quite surprisingly fail to give polymers 4a , b by anionic or radical-induced polymerization. On the other hand, `push-pull' acetylenes may be reacted with commercially available poly(4-vinylphenol) to give polymers 6c , d with a 40 – 70% conversion of phenol units.     

Photoinduced Molecular Transformations. Part 142. One-step syntheses of 1H-benz[f]indole-4,9-diones and 1H-indole-4,7-diones by a new regioselective photoaddition of 2-amino-1,4-naphthoquinones and 2-amino-1,4-benzoquinones with alkenes

The 2,3-dihydro-1H-benz[f]indole-4,9-diones 3a–d , h were formed in a one-step reaction in 13–82% yield by an unprecedented [3 + 2] regioselective photoaddition of 2-amino-1,4-naphthoquinone ( 1 ) with various electronrich alkenes 2 (Scheme 1, Table). The [3 + 2] photoadducts derived from 1 with vinyl ethers and vinyl acetate gave 1H-benz[f]indole-4,9-diones 4e , f , i , in 33–72% yield, by spontaneous loss of the corresponding alcohol or AcOH from the resulting adducts; 4i has a kinamycin skeleton. The [3 + 2] photoaddition also took place on irradiation of the differently substituted amino-1,4-benzoquinones 6 , 7 , and 12 and excess alkenes 2 in benzene, giving 1H-indole-4,7-dione derivatives 13 and 14 (Scheme 3), 15a and 16 (Scheme 4), and 18 (Scheme 4), respectively. The initial products in these photoadditions were proved to be hydroquinones, the air oxidation of which yielded the heterocyclic quinones; 2,3-dihydro-2-methoxy-2-methyl-5-phenyl-1H-indole-1,4,7-triyl triacetate ( 19 ) was isolated after treatment of the crude photoaddition mixture obtained from 2-amino-5-phenyl-1,4-benzoquinone ( 7 ) and 2-methoxyprop-1-ene ( 2f ) with Ac₂O and pyridine under N₂. A pathway leading to the annelated hydroquinones involving ionic intermediates arising from an electron transfer in these photoadditions is proposed (Scheme 5).     

Co(II) halide complexes with 2-amino-3-methylpyridinium and 2-amino-5-methylpyridinium: synthesis,crystal structures,and magnetic properties

Reaction of CoX₂ · nH₂O with either 2-amino-3-methylpyridine (3-MAP) or 2-amino-5-methylpyridine (5-MAP) in aqueous acid gave complexes, (3-MAPH)₂CoX₄ or (5-MAPH)₂CoX₄ (H₂O) _n [n = 0,1; X = Cl, Br; 3-MAPH = 2-amino-3-methylpyridinium, 5-MAPH = 2-amino-5-methylpyridinium]. The 3-MAPH salts are formed in the triclinic crystal system while the 5-MAPH salts are formed in the monoclinic crystal system. Three of these compounds exhibit weak antiferromagnetic interactions along with varying degrees of single-ion anisotropy, however, 1 shows easy-plane anisotropy and exhibits a mixture of ferromagnetic and antiferromagnetic interactions.     

Radikalische Cyclisierungen von Alkenyl-substituierten 4,5-Dihydro-1,3-thiazol-5-thiolen

Radical Cyclizations of Alkenyl-Substituted 4,5-Dihydro-1,3-thiazole-5-thiols Heating of 5-alkenyl- or 5-alkinyl-4,5-dihydro-1,3-thiazole-5-thiols of type 5 in the presence of a radical initiator gave dithiaspirobicycles in fair-to-excellent yield (Scheme 3). Under analogous conditions, the 4,5-dihydro-4-vinyl-1,3-thiazole-5-thiol 5d underwent a cyclization to give the annellated dithiabicycle 7 (Scheme 4). In this reaction, a minor product 8 was formed by an unknown reaction mechanism. The structure of 8 was established by X-ray crystallography. The starting 1,3-thiazole-5-thiols 5 have been synthesized by carbophilic alkylation of me C?S group of 1,3-thiazole-5(4H)-thiones with Grignard-reagents or alkylcuprates. The thiazolethiones were obtained by the reaction of 3-amino-2H-azirines with thiobenzoic acid followed by sulfurization and cyclization. The 4-benzyl derivative 1b was thermally rearranged via 1,3-benzyl migration to yield the benzyl (1,3-thiazol-5-yl) sulfide 11 (Scheme 5).     

Synthese von gestapelten `Push-Pull'-Acetylenen

Synthesis of Stacked `Push-Pull' Acetylenes. In view of possible solid-state polymerizations of crystalline stacked `push-pull'-acetylenes 1 , a series of compounds 1 has been synthesized (Scheme 3), and the results of X-ray analyses of `push-pull'-acetylenes 1d , e , f are discussed. Of these three compounds, methyl 2-morpholinoacetylene carboxylate ( 1d ) is by far the best candidate giving crystals with nicely stacked molecules (Fig. 3). Even in this case, however, stacking parameters d=4,12 Å and α=31,6° are too large for allowing solid-state polymerizations.     

Preparation and Structures of 2‐Substituted 5‐Benzyl‐3‐methylimidazolidin‐4‐one‐Derived Iminium Salts,Reactive Intermediates in Organocatalytic Transformations Involving α,β‐Unsaturated Aldehydes

Preparations of the title compounds, 5 – 7 (Scheme 1 and Table 1), of their ammonium salts, 9 – 11 (Scheme 2 and Table 2), and of the corresponding cinnamaldehyde‐derived iminium salts 12 – 14 (Scheme 3 and Table 3) are reported. The X‐ray crystal structures of 15 cinnamyliminium PF₆ salts have been determined (Table 4). Selected ¹H‐NMR data (Table 5) of the ammonium and iminium salts are discussed, and structures in solution are compared with those in the solid state.     

Zur Synthese sulfonierter Derivate von 2,3-Dimethylanilin und 3,4-Dimethylanilin

On the Synthesis of Sulfonated Derivatives of 2,3-Dimethylaniline and 3,4-Dimethylaniline Baking the hydrogensulfate salt of 2,3-dimethylaniline ( 1 ) or of 3,4-dimethylaniline ( 2 ) led to 4-amino-2,3-dimethylbenzenesulfonic acid ( 4 ) and 2-amino-4,5-dimethylbenzenesulfonic acid ( 5 ), respectively (Scheme 1). The sulfonic acid 5 was also obtained by treatment of 2 with sulfuric acid or by reaction of 2 with amidosulfuric acid. 3-Amino-4,5-dimethylbenzenesulfonic acid ( 3 ) and 5-Amino-2,3-dimethylbenzenesulfonic acid ( 6 ) were prepared by sulfonation of 1,2-dimethyl-3-nitrobenzene ( 9 ) to 3,4-dimethyl-5-nitrobenzenesulfonic acid ( 11 ) and of 1,2-dimethyl-4-nitrobenzene ( 10 ) to 2,3-dimethyl-5-nitrobenzenesulfonic acid ( 12 ), respectively, with subsequent Béchamp reduction (Scheme 1). Preparations of 2-amino-3,4-dimethylbenzenesulfonic acid ( 7 ) and of 6-amino-2,3-dimethylbenzenesulfonic acid ( 8 ) were achieved by the sulfur dioxide treatment of the diazonium chlorides derived from 3,4-dimethyl-2-nitroaniline ( 24 ) and from 2,3-dimethyl-6-nitroaniline ( 31 ) to 3,4-dimethyl-2-nitrobenzenesulfonyl chloride ( 29 ) and 2,3-dimethyl-6-nitrobenzenesulfonyl chloride ( 32 ), respectively, followed by hydrolysis to 3,4-dimethyl-2-nitrobenzenesulfonic acid ( 30 ) and 2,3-dimethyl-6-nitrobenzenesulfonic acid ( 33 ), and final reduction (Scheme 3). Compound 7 was also synthesized by reaction of 4-chloro-2,3-dimethylaniline ( 23 ) with amidosulfuric acid to 2-amino-5-chloro-3,4-dimethylbenzenesulfonic acid ( 20 ) and subsequent hydrogenolysis (Scheme 2). 4′-Bromo-2′, 3′-dimethyl-acetanilide ( 13 ) and 4′-chloro-2′, 3′-dimethyl-acetanilide ( 14 ) on treatment with oleum yielded 5-acetylamino-2-bromo-3,4-dimethylbenzenesulfonic acid ( 17 ) and 5-acetylamino-2-chloro-3,4-dimethylbenzenesulfonic acid ( 18 ), respectively. Their structures were proven by hydrolysis to 5-amino-2-bromo-3,4-dimethylbenzenesulfonic acid ( 21 ) and 5-amino-2-chloro-3,4-dimethylbenzenesulfonic acid ( 22 ), followed by reductive dehalogenation to 3 .     

Unkatalysierte sigmatrope 1,5-Wanderung von Acylgruppen bei der Thermolyse von 5-Acyl-5-methyl-1,3-cyclohexadienen

Uncatalyzed Sigmatropic 1,5-Shift of Acyl Groups in the Thermolysis of 5-Acyl-5-methyl-1,3-cyclohexadienes Four different 5-acyl-5-methyl-1,3-cyclohexadienes 1a–d (R = COOCH₃, COCH₃, COC₆H₅, CHO) have been shown to yield mixtures of 1,3-disubstituted cyclohexadienes 2–7 and 1,3-disubstituted aromatic product 8 upon thermolysis at 150–300° in solution and at 350–500° in the gas phase in a flow system. Two reaction pathways (A and B in Scheme 2) are considered for the rearrangement of the C-Skeleton. For the ester 1a ¹³C-isotopic substitution shows that products arise to 75–86% through a 1,5-sigmatropic shift of the methoxycarbonyl group ( A in Scheme 2) and to 14–25% through a sequence of reaction steps involving a 1,7-H-shift reaction in an acyclic intermediate ( B in Scheme 2). For the more reactive compounds 1b–d isomerization is assumed to follow the 1,5-sigmatropic pathway exclusively ( A in Scheme 2). A kinetic study yields the following sequence for the migration tendency of acyl groups toward sigmatropic 1,5-shift: COOCH₃ < COCH₃ < COC₆H₅ < CHO.     

Reactions of 3-cyclopropyl-3-oxopropionitrile anion generated by electroreduction of 5-cyclopropylisoxazole

3-Cyclopropyl-3-oxopropionitrile anion obtained by cathodic reduction of 5-cyclopropylisoxazole in an aprotic medium was used as an example to demonstrate that cyano ketone anions show a dual reactivity. The reaction of acetyl chloride with the electrogenerated tetrabutylammonium salt of 3-cyclopropyl-3-oxopropionitrile gave O-acylation products, whereas the reaction with its sodium salt gives C-acylation products. The reactions of these salts with hydroxylamine hydrochloride follow a different route: in the case of the tetrabutylammonium salt, resinification takes place, while in the case of the sodium salt, 5-amino-3-cyclopropylisoxazole is formed. The condensation of this product with 4,4,4-trifluoro-1-(2-thienyl)butane-1,3-dione in glacial AcOH affords 3-cyclopropyl-6-(2-thienyl)-4-(trifluoromethyl)isoxazolo[5,4-b]pyridine in 85% yield. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2110–2114, November, 2007.     

Imination of N-methylpyridinium salts by liquid ammonia-potassium permanganate. A new synthesis of nudiflorine

Reaction of substituted 1-methyl(benzyl)pyridinium salts ( 1 ) with liquid ammonia/potassium permanganate leads to introduction of the imino group at the carbon adjacent to the nitrogen. The regiospecificity of the reaction strongly depends on substituent X: at C-6 for X = H, CONH₂, C₆H₅ and at C-2 for X = CH₃. 3-Aminocarbonyl-1-t-butylpyridinium iodide ( 5 ) on treatment with liquid ammonia/potassium permanganate exclusively gives the 4-imino compound 8 ; ¹H nmr spectroscopy shows that 5 in liquid ammonia gives a mixture of the σ-adducts 4-amino-1,4-dihydro- and 6-amino-1,6-dihydro-3-pyridinecarbonamide ( 6 and 7 ). Surprisingly, an oxodemethylation reaction is observed on treatment of 3-aminocarbonyl-1,6-dimethylpyridinium iodide ( 13 ) with liquid ammonia/potassium permanganate, 1,6-dihydro-1-methyl-6-oxo-3-pyridinecarboxamide ( 14 ) being obtained. This compound can easily be converted by phosphorus oxychloride into the alkaloid nudiflorine ( 15 ).     

Exceptional Products of the Desulfurization of N,2-Diaryl-5-(arylimino)- 2,5-dihydro-4-nitroisothiazol-3-amines

The desulfurization of several N,2-diaryl-5-(arylimino)-2,5-dihydro-4-nitroisothiazol-3-amines 5 with Ph₃P led to complex mixtures of products in low yields. For instance, quinoxaline-2-carboxamide 1-oxides of type 6 (Scheme 2) and, in some cases, also 3-nitroquinolines of type 7 (Scheme 5) were isolated. By the desulfurization of the substituted derivatives 5b – e , a rearrangement of the intermediates yielded 6 and 7 with a different substitution pattern from that expected from the starting materials (Scheme 3). The additional formation of two isomeric 1,2,5-oxadiazole-3-carboxamides 8 was observed only in the case of 5d (R¹=R²=F) (Scheme 6). Under the same reaction conditions, the major product of the desulfurization of 5c was the quinoxaline-2-carboxamide 1-oxide 9 (Scheme 7). Reaction mechanisms involving intermediate ketene imines and O transfer from the NO₂ group to the neighboring ketene imine are proposed. The structures of 6a , 6e , 6k , 7b , and 8d were established by X-ray crystallography, while the structure of 9 was elucidated by 2D-NMR spectroscopy and corroborated by X-ray crystallography.     

THE SOLVOLYSIS AND NUCLEOPHILIC SUBSTITUTION IN SERIES OF CYCLIC HALOGENALKYLISOTHIOUREAS

Abstract

Previously we reported 1, that solvolytic rearrangement of salts and bases of 2-amino-5-X-5,6-dihydro-4-H-1,3-thiazines \I a,b, where a X=C1, b X=Br\ proceeds with a contraction of cycle and results in salts and bases of 2-amino-5-X-methyl-2-thiazolines \II a-c, where a X=C1, b X=Br, c X=OH\. The reversibility of this reaction is shown now. It was found by means of radiochromatography that hydrolysis of ³⁵S-labelled IIb bromide gives rise to rearranged Ib besides hydrolytic product lie. It was shown, also, that the solvolysis of salts IIa,b in 50% aqueous ethanol, when excess of Na³⁶Cl is present, gives both ordinary substitution products and certain amount of rearranged Ia. In the course of ³⁵-labelled IIb bromide solvolysis in the same conditions the return of bromide ions takes place. It was found that the addition of salts affects the composition of Ib bromide hydrolysis products. This testifies that the composition of reaction products depends on the episulphonium III form present in the medium \ scheme\. It is possible, that the formation of covalent system I is most probably due to internal return from IIIa, but covalent form II results from the other steps of the dissociation. This is confirmed by absence 2-amino-5-hydroxy-5,6-dihydro-4-H-1,3-thiazine in reaction mixture. The addition of NaC10₄ and NaNO₃ supresses bromine ions return owing to ion exchange resulting in new external pair IIId, what leads to the increase of the relative amount of IIc. On the other hand the addition of KBr and NaCl decrease the amount of IIc, due to the increase of the degree of ions return and the probability of halogen ions attack to episulphonium III.     

Herstellung und Reaktionen der valenzpolaromeren Verbindung (4,4-Dimethyl-2-thiazolin-5-dimethyliminium)-2-thiolat ⇌ (1-Dimethylthiocarbamoyl-1-methyl-äthyl)-isothiocyanat aus 3-Dimethyl-amino-2,2-dimethyl-2H-azirin und Schwefelkohlenstoff

Synthesis and reactions of the valence polaromeric compound (4,4-dimethyl-2-thiazoline-5-dimethyliminium)-2-thiolate ? 1-dimethylthiocarbamoyl-1-methyl-ethyl isothiocyanate from 3-dimethylamino-2,2-dimethyl-2H-azirine and carbon disulfide. 3-Dimethylamino-2,2-dimethyl-2H-azirine ( 1 ) reacts with carbon disulfide to give crystals which have the dipolar structure 3a [(4,4-dimethyl-2-thiazoline-5-dimethyliminium)-2-thiolate, Scheme 1]. In solution, the non-dipolar (charge-free) isomeric form 3b (1-Dimethyl-thiocarbamoyl-1-methyl-ethyl isothiocyanate) is almost exclusively populated. Reaction products are derived from both forms: Derivatives of 3a are the hydrolysis product 6 , the sodium borohydride reduction product 7 and the methylation products 9 and 10 , respectively (Scheme 2). The isothiocyanate form 3b is responsible for the various reaction products with amines (Scheme 3). One of the reaction products with ammonia, namely 20 , is also obtained by the reaction of 1 with thiocyanic acid. Thermolysis of the azirine/carbon disulfide adduct 3 leads to 2-dimethylamino-4,4-dimethyl-2-thiazoline-5-thione ( 17 ) in high yield. A possible mechanism is outlined in Scheme 4. The same compound is also formed by rearrangement of 3 under the catalytic influence of dimethylamine. Its structure has been established by X-ray crystallography (section 4). Again a rearrangement is involved in the reductive (NaBH₄) conversion of 17 to 7 , the direct reduction product of the dipolar species 3a (Scheme 5). The isothiocyanate form 3b is able to react with a second molecule of 3-dimethylamino-2,2-dimethyl-2H-azirine ( 1 ) to yield compound 25 , which in the crystalline or dissolved state appears to be almost entirely populated by the carbodiimide form with structure 25b (Scheme 7), though all reaction products of 25 (reduction with sodium borohydride, addition of water or hydrogen sulfide, Schemes 7 and 8) are derived from the dipolar form 25a , not detectable as such; here again therefore there is a dynamic equilibrium 25a ? 25b . The two forms of adduct 3 , namely 3a and 3b , are obviously very easily interconverted at room temperature and therefore can be considered as valence polaromeric forms (section 5). A classification of the dipolar (zwitterionic) form is given, which allows a comparison of various dipolar species and gives as indication of charge stabilization by delocalization. The versatile reactivity of the 3-dimethylamino-2,2-dimethyl-2H-azirine/carbon disulfide adduct is demonstrated by the fact that with simple reagents approximately 25 derivatives have been obtained, most of them being new heterocyclic compounds.     

Reaction of 3-Amino-2H-azirines with 2-Amino-4,6-dinitrophenol (Picramic Acid): Synthesis of Quinazoline- and 1,3-Benzoxazole Derivatives

The reaction of 3-(dimethylamino)-2H-azirines 1a–c and 2-amino-4,6-dinitrophenol (picramic acid, 2 ) in MeCN at 0° to room temperature leads to a mixture of the corresponding 1,2,3,4-tetrahydroquinazoline-2-one 5 , 3-(dimethylamino)-1,2-dihydroquinazoline 6 , 2-(1-aminoalkyl)-1,3-benzoxazole 7 , and N-[2-(dimethylamino)phenyl]-α-aminocarboxamide 8 (Scheme 3). Under the same conditions, 3-(N-methyl-N-phenyl-amino)-2H-azirines 1d and 1e react with 2 to give exclusively the 1,3-benzoxazole derivative 7 . The structure of the products has been established by X-ray crystallography. Two different reaction mechanisms for the formation of 7 are discussed in Scheme 6. Treatment of 7 with phenyl isocyanate, 4-nitrobenzoyl chloride, tosyl chloride, and HCl leads to a derivatization of the NH₂-group of 7 (Scheme 4). With NaOH or NaOMe as well as with morpholine, 7 is transformed into quinazoline derivatives 5 , 14 , and 15 , respectively, via ring expansion (Scheme 5). In case of the reaction with morpholine, a second product 16 , corresponding to structure 8 , is isolated. With these results, the reaction of 1 and 2 is interpreted as the primary formation of 7 , which, under the reaction conditions, reacts with Me₂NH to yield the secondary products 5 , 6 , and 8 (Scheme 7).     

Chiral Acylsilanes in Organic Synthesis. Part 3. Silicon-directed stereoselective preparation and Ireland ester-enolate rearrangement of O-acyl-substituted α-silylated allyl alcohols

Stereocontrolled addition of alk-1-enylmetal reagents to the chiral (alkoxymethyl)-substituted acylsilanes (±)- 6 gave rise to α-silylated allyl alcohols, which were converted to the corresponding acetates or propionates 11–16 (Scheme 2). Deprotonation and silylation with Me₃SiCl afforded – in an Ireland ester-enolate-accelerated Claisen rearrangement – stereoselectively αδ-silylated γδ-unsaturated carboxylic acids 18–24 (Scheme 4). The Me₃Si groups in α-position to the COOH group of these compounds were removed chemoselectively in presence of the chiral silyl group in δ-position by treatment with Bu₄NF · 3 H₂O or Et₃N · 3 HF (→ 27–32 ; Scheme 5). The reaction sequence allows a novel stereocontrolled access to chiral C-frameworks possessing a vinylsilane moiety with its full reaction potential.     

4-Amino-1,5-dihydro-2H-pyrrol-2-one aus Bortrifluorid-katalysierten Umsetzungen von 3-Amino-2H-azirinen mit Carbonsäure-Derivaten

4-Amino-1,5-dihydro-2H-pyrrol-2-ones from Boron Trifluoride Catalyzed Reactions of 3-Amino-2H-azirines with Carboxylic Acid Derivatives Reaction of 3-amino-2H-azirines 1 with ethyl 2-nitroacetate ( 6a ) in refluxing MeCN affords 4-amino-1,5-dihydro-2H-pyrrol-2-ones 7 and 3,6-diamino-2,5-dihydropyrazines 8 , the dimerization product of 1 (Scheme 2). Thus, 6a reacts with 1 as a CH-acidic compound by C? C bond formation via C-nucleophilic attack of deprotonated 6a onto the amidinium-C-atom of protonated 1 (Scheme 5). The scope of this reaction seems to be rather limited as 1 and 2-substituted 2-nitroacetates do not give any products besides the azirine dimer 8 (see Table 1). Sodium enolates of carboxylic esters and carboxamides 11 react with 1 under BF₃ catalysis to give 4-amino-1,5-dihydro-2H-pyrrol-2-ones 12 in 50–80% yield (Scheme 3, Table 2). In an analogous reaction, 3-amino-2H-pyrrole 13 is formed from 1c and the Li-enolate of acetophenone (Scheme 4). A reaction mechanism for the ring enlargement of 1 involving BF₃ catalysis is proposed in Scheme 6.