A Novel Route to Isoannulated Heteroaromatic Compounds. Part 2. The Dipole Route to Furo/Thieno Pyrroles and α-Pyridones, and Their Benzo Derivatives.-A new methodology, involving 1,7-dipolar cyclizations of conjugated dipoles followed by a multistep rearrangement affording ring-contracted products is employed for the synthesis of various examples of the title compounds. As demonstrated for (IV) and (V) the reaction starts with the alkynylcarbaldehydes (I) which are converted into nitrones such as (III). In some cases beside the Z isomers 3-10% of the E isomers are obtained. Under different thermolysis conditions the nitrones are converted into the pyrroles and α-pyridones. The product distribution is strongly influenced by the nature of the terminal alkynyl substituent. The derivatives (V) are not stable. They react with oxygen to the imides (VI). The benzo-analogues are more stable, some Diels-Alder reactions of them are demonstrated. -(BUSSENIUS, J.; LABER, N.; MUELLER, T.; EBERBACH, W.; Chem.

Photochemical excitation of the 2,3‐dihydroisoxazoles 19−21 resulted in the formation of the stable azomethine ylides 22, 25, and 28, representing the first isolable examples of such species bearing stabilizing groups only at one end of the 1,3‐dipole. The UV and NMR spectroscopic data of the photoproducts clearly indicate that the iminium and the anionic parts of the azomethine ylide systems are not planar. This conclusion is unambiguously confirmed by crystal structure analysis of 25b, in which a twist angle of 73° was determined between the two polar moieties. In the case of 25d the rotation barrier around the central CN bond amounts to 16 kcal/mol at 333 K. In line with the unusual stability of the ylides is their low reactivity against dipolarophiles. Only the highly reactive N‐methyltriazolidinedione gives formation of the products 33a, 35a−c, and 41, but these are the result not of an initial 1,3‐dipolar cycloaddition but rather of a formal [4+2] addition with involvement of 34a, 36a−c, and 41 as unstable primary products. On heating in refluxing toluene the azomethine ylides are transformed into annulated pyrrole systems (42/43, 46/47, 48/49). In contrast to other examples, the direct thermal transformation of the annulated 2,3‐dihydroisoxazoles into pyrroles is not successful, but the reaction can be achieved by simultaneous thermal and photochemical activation, as shown for the conversion of 19 into the pyrroles 42/43. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Summary.A comparative photoelectron spectroscopical investigation of the title compound (11) and its 3,4-and 7,8-dihydro derivatives (9 and 10) indicates that a considerable 'through bond' interaction exists between the n-orbitals in 11. The PE. spectra of the 3,4-diaza-analogue of 10 and 11, which contain a cis azo group in a four-membered ring, yield a splitting A ,

Die thermische Isomerisierung der Vinyloxirane 8 -13, 15 und 17 -20 zu den Dihydrofuranen 21 -27 wird beschrieben. Je nach Art der Substitution lassen sich die Ringerweiterungsreaktionen bevorzugt in Losung oder in der Gasphase (Kurzzeitpyrolyse) mit Ausbeuten von 50-95% durchfuhren. Wahrend 8 -13, 15, 17 und 19 unabhangig von der Konfiguration des Oxiranrings stereoselektiv zu cis-2,3-Dihydrofuranen (21 -23 und 25 -27) umgewandelt werden, fuhrt die Thermolyse des Diesters 18 zu einem 3:2-Gemisch der cis-trans-Isomeren 24c und 24t. Als Mechanismus der Vinyloxirau-Dihydrofuran-Isomerisierung wird eine zwei-oder mehrstufige Reaktionssequenz iiber 2-Oxapentadienyl-Dipole als Zwischenstufen vorgeschlagen, wobei der jeweils erste (C/C-Spaltung des Oxiranrings) und letzte Schritt (Cyclisierung des 1,5-Dipols) stereochemisch einheitlich erfolgen (die nur geringe Stereoselektivitat bei der Reaktion von 18 wird auf einen erhohten Diradikalcharakter der geoffneten Spezies zuruckgefuhrt). Im Falle der Diastereomeren-Paare 8/9, 10/11 und 12/13 ist neben der Umwandlung zu Dihydrofuranen eine vorgelagerte cis F? trans-Isomerisierung zu beobachten. Die Aktivierungsparameter der Umwandlungen 12/13 + 23 und 15 + 26 werden bestimmt und im Zusammenhang mit denen der Grundkorperreaktion diskutiert. Cycloadditionsexperimente mit 8 -13 liefern 1 : I-Addukte mit vollstandiger oder weit iiberwiegender ,,Inversion" der Konfiguration von Aryl-und Vinylrest: cis(trans)-Vinyloxirane fuhren zu 2,5-trans(cis)-Dihydro-bzw. Tetrahydrofuranen. Das Dinitril 20 als das thermisch labilste Vinyloxiran in dieser Reihe reagiert mit En-Komponenten nur unter drastischen Bedingungen und in besonders niedrigen Ausbeuten; ebenso wie in den anderen Beispielen fungiert hierbei das intermediare 6-Elektronensystem ausschliel3lich als 1,3-Dipol The thermal conversion of the vinyloxiranes 8 -13, 15, and 17 -20 to dihydrofurans (21 -27) is described. Depending on the substitution pattern the ring expansion reactions are performed preferentially in solution or in the gas phase (short-time pyrolysis) with yields in the range of 50 to 95%. Contrasting with the behavior of compounds 8-13, 15, 17, and 19, which -regardless of the configuration of the oxirane ring -lead stereoselectively to cis-2, 3-dihydrofurans (21 -23, 25-27), thermolysis of the diester 18 gives rise to the formation of a 3 : 2-mixture of the cis/transisomers 24c and 24 t. The mechanism proposed for the vinyloxirane dihydrofuran isomerisation suggests a two or multistep reaction sequence with 2-oxapentadienyl dipoles as intermediates in which the first (C/C-cleavage of the oxirane ring) and the last step (cyclisation of the 1,5-dipole) take place in a stereospecific manner (the low stereoselectivity for the transformation of 18 to 24 may be explained in terms of increased diradical character of the ring-opened species). In the case of the diastereomeric pairs 8/9, lO/ll, and 12/13 the formation of dihydrofurans is preceded by cis P trans isomerisation. The activation parameters for the transformati...

Der Substituenteneinfluo auf die einzelnen Schritte der 30-3x-Route zu 1 H-Azepinen wird an Hand mehrerer neu synthetisierter Edukte niiher definiert: Das C-unsubstituierte 7-Azanorbornadien 2a, das 2,3-Dichlorderivat 2 b, der 5,6-Dichlor-2,3-dicarbonester 2c und die an C-l/N-7 mit einem dipolarophilen Rest versehenen 2,3-Dicarbonester 26, e lassen sich durch sensibilisierte/direkte Lichtanregung mehr oder weniger einheitlich in die zum Teil hochlabilen Azaquadricyclane 29 ae isomerisieren. Fur die thermische Umwandlung des Grundgeriistes (N-Tos) 29a werden die kinetischen Parameter bestimmt (Benzol): E, =28.0 f 0.2 kcal/mol, lg A = 15.7; AH* = 27.3 f 0.2 kcal/mol, AS* = 11.1 f 0.7 e.u.Diese Barriere wird durch die Halogenreste in 29b(c) noch starker als durch die Esterreste (290 herabgesetztwobei die +M-im Gegensatz zu den ---Resten ausschlieBlich die Spaltung der gegeniiberliegenden Cyclopropanbindungen bewirken. Die intermediaren Azomethinylide konnen je nach Substitution mit dipolarophilen Reagentien unterschiedlich gut abgefangen werden. Im Fall von 29d (28d) ist die intramolekulare Addition der nichtaktivierten In-Seitenkette (37) bei -30°C so rasch, daB Azepinbildung fast vollstandig unterdriickt wird (Cn2 + 02 + 02], 36?). Das Azepin/Benzolimin-Gleichgewichtsgemisch 31c P 32c (ca. 90: 10) kristallisiert als 31 c (Rontgenstrukturanalyse). Photochemical Transformations, 65 ') The 3a-+fx-Route to lH-Azepines/Benzene IminesWith several newly prepared substrates the influence of substituents upon the individual steps in the 30+3x-route to Iff-azepines is more precisely defined: The C-unsubstituted 7-azanorbornadiene 2a, its 2,3-dichloro derivative 2 b, the dimethyl 5,6-dichloro-2,3-dicarboxylate 2c, and the diesters 2d,e with dipolarophilic groups at C-1/N-7 are selectively isomerized by sensitized/direct photoexcitation into the azaquadricyclanes 29a -e, some of which are highly unstable. For the thermal conversion of the basic skeleton (N-Tos)29a the kinetic parameters have been determined (benzene): E, = 28.0 f 0.2 kcal/mol, Ig A = 15.7; AH* = 27.3 f 0.2 kcal/mol, AS* = 11.1 f 0.7 e. u. This barrier is lowered more efficiently by the chloro (29b,c) than by the methoxycarbonyl substituents (290, with the former (latter) causing exclusive scission of the opposite (neighbouring) cyclopropane bonds. The intermediate azomethine ylides are captured with dipolarophilic reagents more or less efficiently 0 VCH Verlagsgesellschaft mbH, D-

Base treatment of the pyridinium bromides 11a-e gives rise to the formation of the dihydropyridoazepines 14a-e as the only monomolecular products. The reaction takes place by initial deprotonation to the ylides 12, which undergo 8pi-electrocyclization affording the seven-membered-ring systems; no products of a dipolar 6pi-cyclization were detected. On the basis of quantum mechanical calculations a rationalization of the periselectivity of the electrocyclization process is given.

8 241 A Novel Route to Isoannulated Heteroaroamtic Compounds, 21'1. -The Dipole Route to Furo/Thieno Pyrroles and a -Pyridones, and Their Benzo DerivativesA new methodology involving 1 ,?-dipolar cyclizations of con-pyrrole systems or Wolff rearrangement leading to conjujugated dipoles followed by a multistep rearrangement affor-gated ketenes, the precursors of the observed a-pyridones ding ring-contracted products has been employed for the (see Scheme 1). Diels-Alder reactions were performed with synthesis of various examples of the title compounds. Under benzofuropyridone 40a, napthopyridone 62 as well as with different thermolysis conditions the annulated alkynyl nitro-the corresponding dihydronaphtho compounds 64a, b. Whenes 15-19, 45, and 46 are converted into isoannulated pyrro-reas the tert-butyl derivatives 22b, 23b and 41b are rather les and a-pyridones, respectively, with yields up to 90% (ge-unreactive against dienophiles, they are easily transformed 3 Qb c a = N R ( c = H, Alkyl, Aryl, TMS ) 6 b = O Fur Carbonylylide als Dipol-Vorstufen (3, a = 0, b = CRR') hat sich diese Umwandlung als ein alternativer, ergiebiger Zugang fur 2-Vinylfurane erwiesen (5, a = 0, b = CRR'); dabei wurden nicht nur hochsubstituierte und verbruckte Vertreter hergestellt[16~1s], sondern es konnten auch spezielle, z.T. noch unbekannte Systeme wie Dioxa-, Oxaaza-und Oxathiapentalene[]l sowie 2,5-Furanophane zuglnglich gemacht werden[I9].

A Novel Route to Isoannulated Heteroaromatic Compounds, 1. -Synthesis and Reactions of Furofurans, Thienofurans, Furobenzofurans, and BenzothienofuransA general method for the synthesis of furo-and thienofurans 4-7 has been developed. The reaction principle is based on the thermal transformation of suitably structured epoxyhexenynes (15 -19) following the general reaction sequence 1 --+ 2 --+ 3 (annulation type A). Derivatives of the so far unknown diheteropentalenes furo [3,4-b] I II III Iv V