The use of (–)-sparteine/organolithium reagents for the enantioselective lithiation of 7,8-dipropyltetrathia[7]helicene: single and double kinetic resolution procedures

Julien Doulcet, G. Richard Stephenson

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7 Citations (Scopus)

Abstract

The effect of organolithium reagent (RLi: R = n-Bu, i-Pr, s-Bu, t-Bu), solvent (diethyl ether, diethyl ether/THF and MTBE), and stoichiometry on the (–)-sparteine-mediated silylation of 7,8-dipropyltetrathia[7]helicene shows that, unusually, substantially more than 0.5 eq of RLi (R = i-Pr, s-Bu, t-Bu) and a large excess of (–)-sparteine (R = n-Bu, s-Bu) is often needed to achieve substantial conversions and good e.e.s. With n-BuLi, however, just 1 equivalent of the organolithium reagent is sufficient. Our best results were obtained using the convenient t-BuLi/(–)-sparteine adduct with which the need for a high (–)-sparteine:RLi ratio can be avoided. Single- and double-KR procedures give enantiopure samples of 2-trimethylsilyl- and 2,13-di(trimethylsilyl)-7,8-dipropyltetrathia-[7]helicene and two-step double-KR combining (–)-sparteine/s-BuLi and chiral formamides affords the synthetically valuable 2-formyl-7,8-dipropyltetrathia[7]helicene. This is the first use of (–)-sparteine for the enantioselective lithiation of helicenes and the first report of t-BuLi outperforming s-BuLi in a (–)-sparteine-mediated procedure.
Original languageEnglish
Pages (from-to)18677-18689
Number of pages13
JournalChemistry - A European Journal
Volume21
Issue number51
Early online date12 Nov 2015
DOIs
Publication statusPublished - 14 Dec 2015

Keywords

  • Tetrathia[7]helicenes
  • Asymmetric Synthesis
  • (–)-Sparteine-mediated lithiation
  • Kinetic Resolution
  • Double-KR

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