Annelation Friedel-Crafts acylation

The electron-rich thiophene ring system can be elaborated into complex, fused thiophenes by acid-mediated intramolecular annelation reactions. For example, treatment of alcohol 96 with trimethylsilyl triflate promoted a Friedel-Crafts acylation and subsequent dehydration giving benzo[b]thiophene 97, a potential analgesic <00JMC765>. Treatment of ketone 98 with p-toluenesulfonic acid resulted in the formation of fused benzo[b]thiophene 99 <00T8153>. Another variant involved the cyclization of epoxide 100 to fused benzo[f>]thiophene 101 mediated by boron trifluoride-etherate . 

The electron-rich thiophene ring system can be annelated by intramolecular Friedel-Crafts acylation reactions to give fused thiophenes <99IJC648, 99JMC2774>. The synthesis of a thiophene isostere of ninhydrin involved an intramolecular Friedel-Crafts acylation <99SL1450>. Specifically, treatment of thiophene 86 with thionyl chloride followed by aluminum chloride gave annelated thiophene 87. The synthesis of isothianinhydrin 88 was then accomplished in six steps from 87. 

Annelations via Friedel-Crafts acylation were applied to racemic syntheses of [njhelicenes with n up to 6. The 12-step synthesis of [6]helicene (1) by Newman and Lednicer employed stepwise diannelation, followed by oxidative aromatiza-tion and then resolution with the complex of a chiral n-acceptor, a-2,4,5,7-tetrani-tro-9-fhiorohdeneaminooxypropionic acid (TAPA) and 1 [25]. The one-step diannelation was ubiquitously applied to syntheses of triarylamine [4]helicenes,... 

Vinylsilanes (8, 491-492) aHylic alcohols (9, 340). Details are available for conversion of a ketone to a vinylsilane in which the C—Si bond has replaced the C==0 group (enesilylation). The reaction affords the less substituted vinylsilane in the case of unsymmetrical ketones. The paper includes details for use of vinylsilanes for cyclopentenone annelation by Friedel-Crafts acylation with acryloyl chlorides and subsequent cyclization of penladienyl cations (9,498-499).1... 

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