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1 Introduction [2+2] Cycloadditions of electron-poor acetylenes to push–pull systems, such as 3-(dimethylamino)propenoates, acyclic and cyclic enamines, and some heterocyclic enaminones, have been described recently [ 1 ]. Polysubstituted butadienes have been prepared by microwave-assisted [2+2] cycloadditions of enaminones to electron-poor acetylenes [ 2 ], [ 3 ]. Polysubstituted aminobutadienes prepared by this procedure are suitable for the preparation of polysubstituted pyridine derivatives. They also represent a group of isomeric intermediates in regard to

,N-dimethylformamide dimethyl acetal (DMFDMA), and propyne iminium salts as electron-poor acetylenes, is described. Key words: Enaminones, Propyne Iminium Salts, [2+2] Cycloadditions, 2,4,5-Trisubstituted Pyridines, 2,4,5-Trisubstituted Pyridine N-Oxides Introduction Pyridines and their N-oxides with aromatic or het- eroaromatic substituents constitute an important class of compounds, which have found applications in medicinal chemistry [1]. Many of them are biologi- cally active [2], such as streptonigrin [3], glivec [4] and rosuvastatin [5], and various natural products [6, 7]. They are

) present [2+2] cycloadditions of electron-poor acetylenes with conjugated double bonds. The group of R. Wilhelm (Paderborn) investigates protic ionic liquids as catalysts for the activation of imines in a three-component oxidative dehydrogenation tandem reaction to obtain a quinoline derivative. In a review, N. Müller (Wallhausen) is closing this issue with an overview of the many important iminium salt structures occurring during the growing of grapes and wine production, leading to the vital aroma compounds – and off-odors – of wine. In conclusion the contributions to

1 Introduction 3-Dimethylaminopropenoates and related enaminones have been demonstrated to exhibit a broad applicability in heterocyclic synthesis [ 1 – 5 ], including the preparation of natural products and their analogues, such as aplysinopsins [ 6 , 7 ], meridianines [ 8 , 9 ], and dipodazines [ 10 – 12 ]. Recently, polysubstituted butadienes have been prepared by microwave-assisted [2+2] cycloadditions of enaminones to electron-poor acetylenes [ 13 , 14 ]. Polysubstituted aminobutadienes prepared by this procedure are suitable for the preparation of

structure as the carbohydrate fragment Tetrahedron Lett. 2011 52 3614 3617 [9] Gracia-Ruiz, V.; Martin-Otero, L. E.; Puyet, A. Transformation of thiodiglycol by resting cells of alcaligenes xylosoxydans PGH10. Biotechnol. Prog. 2002 , 18 , 252–256. 11934292 10.1021/bp010190x Gracia-Ruiz V. Martin-Otero L. E. Puyet A. Transformation of thiodiglycol by resting cells of alcaligenes xylosoxydans PGH10 Biotechnol. Prog. 2002 18 252 256 [10] Stanovnik, B. [2+2] Cycloaddition of electron-poor acetylenes to enaminones, enamino esters and related systems. Rearragements and ring

auf 2-Hydroxyacetophenon . Liebigs Ann. Chem . 1973 , 1 , 134–140. [12] Fondjo, S. E.; Döpp, D.; Henkel, G. Reactions of some annelated 2-aminothiophenes with electron poor acetylenes. Tetrahedron 2006 , 62 , 7121–7131. [13] Fogue, S. P.; Lunga, K. P.; Sopbue, F. E.; Tamokou, J. D.; Thaddée, B.; Tsemeugne, J.; Tienga, T. A.; Kuiate, J. R. Substituted 2-aminothiophenes: antifungal activities and effect on Microsporum gypseum protein profile. Mycoses Diagn. Ther. Prophyl. Fung. Dis. 2012 , 55 , 310–317. [14] Gewald, K. Zur Reaktion von α-Oxo-mercaptanen mit

approaches to stereo- specific syntheses of such ligands via diphosphina- tion of alkynes have recently been reported. Thus, Os- hima et al. [7] prepared symmetrical E-1,2-bis(diphen- ylphosphino)ethenes via the radical-promoted addition of tetraphenyldiphosphine to alkynes, and Pringle et al. [8] synthesized both symmetrical and unsymmetri- cal Z-1,2-bis(phosphinyl)ethenes through the addition of diphosphines to electron-poor acetylene mono- and 64 S. Burck et al. · Activation of Polarized Phosphorus-Phosphorus Bonds by Alkynes Scheme 2. dicarboxylates. The latter

internal acetylenic iminium salts leading to iminium-functionalized 1,2,3-triazoles. In preceding papers we have shown that the propyne iminium system, in which an iminium group is directly attached to a C≡C bond, resembles the acetylenic ketones under various aspects of reactivity, but features an even more electron poor acetylenic bond than the latter due to the presence of the cationic iminium function. As far as cycloaddition reactions are concerned, this could be confirmed for [4+2] cycloaddition reactions of propyne iminium salts with dienes (Diels-Alder reactions

that the chelating phosphine ligand inhibits a potential [2+2+2] cyclization reaction and allows the simple coordination of an alkyne ( Scheme 27 ). Starting from 65 , the coordination of electron-poor acetylenes like in 106 was possible just as the exchange of the ethylene for a terminal acetylene at low temperatures (0°C) as proven by the isolated complex 107 (Butenschön et al. 1992, Kettenbach et al. 1993, Foerstner et al. 1996a). The reaction of 65 with acetylene or t -butylacetylene led, presumably via the intermediacy of the π-coordinated alkynes, to