| Abstract
| - The addition of enolates derived from indole-3-acetic esters1−3 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]indoles5−7, 25−27, which havebeen subsequently elaborated into 4E-ethylidene(or4α-ethyl)-hexahydro-1,5-methanoazocino[3,4-b]indoles.Closure of the six-memberedC ring of akuammiline alkaloids by formation of C-6/C-7 bond fromappropriatelyN(b)-substitutedderivatives of these tetracyclic ABDE substructures has beenextensively investigated. In theN-unsubstituted indole series, both cyclization of thioniumions generated either by Pummererreaction from sulfoxide 16 or by DMTSF treatment ofdithioacetal 36 and photocyclization ofchloroacetamide 47 occur upon the indole nitrogen to givepentacycles 18, 38, and 49,respectively.When the indole nitrogen is blocked by a substituent, the thioniumions derived from sulfoxides 17and 43 and dithioacetals 37 and 44 donot cyclize and lead to different products depending onthereaction conditions, whereas chloroacetamides 48 and51 undergo a reductive photodehalogenation.Attempted radical cyclization of seleno derivatives 53,55, and 56 under a variety ofconditionsgives the corresponding reduced products. Finally, attemptedphotoisomerization of 1-acylindole62 leads to theN(b)-methyl tetracycle63.
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