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Addition of the enolate derived from 2-acetylindole 1a to pyridinium salt 2 followed by in situ trapping of the initially formed 1,4-dihydropyridine 3a with Eschenmoser's salt gives tetracycle 5a. Subsequent elaboration of the exocyclic methylene and E-ethylidene substituents leads to N a-methylervitsine (17a). A similar sequence from the N a-protected 2-acetylindole 1c establishes the first total synthesis of the 2-acylindole alkaloid ervitsine. Alternatively, dihydropyridine 3a is trapped with BrSePh to give the tetracyclic selenide 7a, which is then converted to N a-methylervitsine by way of selenoxide 20. The synthesis of the alkaloids of the ervatamine group starts with the addition of the enolate derived from 2-acetyl-1-benzylindole (1g) to pyridinium salt 24 and the conversion of the resulting 1,4-dihydropyridine to 3,5-diacylated dihydropyridine 26g. Chemoselective reduction of the vinylogous amide moiety of 26g, followed by deprotection of the indole ring and LiEt3BH reduction leads to diol 37b. On sequential treatment with Eschenmoser's salt, methyl iodide, NaCNBH3, and MnO2, 37b is converted to the tetracyclic 2-acylindole 39, from which the first total synthesis of 19,20-didehydroervatamine and 20-epiervatamine is accomplished by manipulation of the 1-hydroxyethyl chain. The above syntheses can be considered as biomimetic, as cyclization of the key intermediates I and II mimics the key steps of the biosynthesis of the title alkaloids.

[structure: see text] A high-yielding totally regioselective intramolecular homolytic acylation of a quinoline ring constitutes the key step in a new synthesis of the pentacyclic indolo[3,2-j]phenanthridine alkaloid calothrixin B.

The dimethyltitanocene methylenation of N-acylamides derived from ortho-vinylanilines, ortho-allylaniline, and ortho-vinylbenzylamine provides the corresponding enamides, which upon exposure to the second generation Grubbs ruthenium catalyst give access to indoles, 1,4-dihydroquinolines, and 1,2-dihydroisoquinolines, respectively. This sequential protocol also allows the synthesis of dihydrobenzoazepines, although the ring-closing metathesis (RCM) step is complicated by the alkene isomerization processes. From certain substrates, the direct annulation is observed in the titanium-mediated step, which is likely to occur through an olefin metathesis-intramolecular olefination sequence.

[reaction: see text] 2-Indolylacyl radicals generated from the corresponding selenoesters under hexabutylditin-hnu conditions undergo regioselective intramolecular reaction with unprotonated pyridines to give polycyclic indolylpyridyl ketones. For substrates bearing a (3-pyridyl)methyl moiety connected to the 3-position of the indole ring, the cyclization provides easy access to ellipticine quinones.

The addition of enolates derived from indole-3-acetic esters 1−3 to pyridinium salts 4, 23, and 24, followed by acid cyclization of the resulting 1,4-dihydropyridines, leads to tetrahydro-1,5-methanoazocino[3,4-b]indoles 5−7, 25−27, which have been subsequently elaborated into 4E-ethylidene(or 4α-ethyl)-hexahydro-1,5-methanoazocino[3,4-b]indoles. Closure of the six-membered C ring of akuammiline alkaloids by formation of C-6/C-7 bond from appropriately N (b) -substituted derivatives of these tetracyclic ABDE substructures has been extensively investigated. In the N-unsubstituted indole series, both cyclization of thionium ions generated either by Pummerer reaction from sulfoxide 16 or by DMTSF treatment of dithioacetal 36 and photocyclization of chloroacetamide 47 occur upon the indole nitrogen to give pentacycles 18, 38, and 49, respectively. When the indole nitrogen is blocked by a substituent, the thionium ions derived from sulfoxides 17 and 43 and dithioacetals 37 and 44 do not cyclize and lead to different products depending on the reaction conditions, whereas chloroacetamides 48 and 51 undergo a reductive photodehalogenation. Attempted radical cyclization of seleno derivatives 53, 55, and 56 under a variety of conditions gives the corresponding reduced products. Finally, attempted photoisomerization of 1-acylindole 62 leads to the N (b) -methyl tetracycle 63.

An indole-templated ring-closing metathesis has been used to create the central nine-membered ring of the cleavamine-type alkaloids. A subsequent intramolecular vinyl halide Heck reaction upon the resulting azacyclononene ring completes the assembly of the strained 1-azabicyclo[6.3.1]dodecane framework of the alkaloids. The usefulness of the approach is illustrated with the synthesis of (±)-cleavamine and (±)-dihydrocleavamine.

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