Figure 1 The screen of chiral monoenes
With the optimal chiral monoene 5a in hand, various reaction conditions were further optimized. When the reaction temperature was lowered from 40 °C to 30 °C, the conversion was diminished dramatically from 99% to 54% with a little improved ee value (Table 1, entries 1 vs 2). Further increasing the reaction concentration could enhance the reactivity (Table 1, entry 3). Several solvents were also examined for this reaction, and benzene proved to be the optimal choice (Table 1, entries 3-9). Subsequent adjustments of temperature or concentration did not afford better results (Table 1, entries 10 and 11). When the reaction was shortened to 16 h, a quantitative conversion was also obtained along with 80% ee (Table 1, entry 12).
Table 1 Optimization of the Reaction Conditionsa .