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N-Allyl-2-(het)arylethynyl-3,4,5,6-tetrahydropyridinium triflates 1c,d,e and N-allyl-2-(het)aryl-4,5,6,7-tetrahydro-3H-azepinium triflates 1g,h undergo a thermal isomerization reaction leading to derivatives of [a,f]-annulated isoindolium salts 2 in good yields. Similarly, N-allyl-2-phenylethynyl-pyridinium triflate 4 is transformed into the condensed pyridinium salt 5. An intramolecular [4 + 2] cycloaddition reaction, in which the (het)arylethynyl moiety acts as the 4pi component, is considered as the key step of this transformation. In contrast, the related N-allyl-4,5-dihydro-3H-pyrrolium salts 1a,b and N-homoallyl-3,4,5,6-tetrahydropyridinium salt 1f undergo unspecific decomposition under thermal impact.

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Palladium-catalyzed cross-coupling of (1-methylindol-2-yl)zinc chloride (1) with 2-(bromoethenyl)arenes 2a-e provides 2-(2-arylethenyl)-1-methylindoles 3a-e in good yields. The alkenes (Z)-2b,c,e are coupled with retention of stereochemistry. With (Z)-ß-bromo-4-nitrostyrene [(Z)-2d], only (E)-3d is obtained whose structure has been confirmed by single-crystal X-ray diffraction.As compared to 3-vinylindoles, 2-vinylindoles have only recently been recognized as useful building blocks in the synthesis of annullated or otherwise functionalized indoles as well as alkaloids. 1-3 In the course of a project directed towards the synthesis of bioisosters of antitumoractive pyridocarbazoles, we needed 2-(2-arylethenyl)-1-methylindoles with Z-configuration at the olefinic C=C bond. It appears that only a few such compounds are known. They were synthesized by intermolecular Wittig reaction, 4,5 by intramolecular Peterson olefination, 6 by the reaction of 2-methylindole-3-carboxylates with ketones or aldehydes, 7 by the reaction of (1-methylindol-2-yl)cyanocuprate with a ß-styryliodonium salt, 8 and in low yield by a palladium-catalyzed cross-coupling reaction of triethyl-(1-methylindol-2-yl)borate with ß-bromostyrene. 9 Leaving aside the last two procedures (no stereochemistry reported 10 ), the E-alkene dominates or is formed exclusively in these preparations. Only for 2-[2-(2-or 4-pyridyl)ethenyl]indoles, the Z-and E-diastereomers have been separated and characterized spectroscopically. 4,11 Palladium-catalyzed cross-coupling reactions of metalated arenes with vinyl halides occur in general with retention of configuration of the olefinic component. 12 Since efficient coupling reactions involving p-excessive heteroaromatics closely related to 1-methylindole under Neghishi conditions 13 have been reported (e.g. arylvinylation of HetZnCl, Het = imidazol-4-yl; 14 arylation of HetZnCl, Het = 1-methylpyrrol-2-yl 15 ), we generated (1-methylindol-2-yl)zinc chloride (1) by lithiation of 1-methylindole and transmetalation with ZnCl 2 . Its Pd(0)-catalyzed cross-coupling with commercial ß-bromostyrene (2a, E/Z = 90:10) gave the desired 2-vinylindole 3a with complete retention of stereochemistry (E/Z = 89:11). After chromatographic workup only the (E)-isomer was isolated (Scheme 1). Scheme 1The alkenes (Z)-2b-e were readily prepared from the known 1-aryl-2,2-dibromoalkenes 4 16 (Scheme 2) in analogy to a published procedure. 17 They were obtained diastereomerically pure ( 1 H NMR), and the cis-relationship was established by the value of the 3 J(H,H) coupling constant (8.0-8.3 Hz). Scheme 2The cross-coupling of 1 with bromoalkenes (Z)-2b-e was achieved with catalytic amounts of (Ph 3 P) 4 Pd (2.2 mol%) in THF at 65 o C and gave the expected 2-arylvinyl-1-methylindoles 3 in 64-91% yield (Table). While the coupling reaction leading to 3b,c,e occurred with retention of stereochemistry and furnished the desired (Z)-isomers, we were surprised to obtain exclusively (E)-3d from (Z)-ß-bromo-4-nitrostyrene [(Z)-2d]. The diastereomers can be di...