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

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.