Combined theoretical and experimental study on high diastereoselective chirality transfer based on...

We report a paracyclophane N-Me thioamide chiral reagent for the asymmetric thio-Claisen rearrangement with high diasteroselectivity. Comparisons between candidate chiral reagent N-phenyl-N-([2.2]paracyclophan-4-yl)amide, N-methyl amide, N-phenyl thioamide, and N-methyl thioamide are made both by experiment and theoretical calculations to clarify the principle behind the high diasteroselectivity. Dynamic (1)H NMR phenomenon tested by varying temperature (VT) experiments has proved that N-Ph amides have triple splitting peaks, while N-Ph thioamide would reduce the number to two, further substituting the Ph to Me made dynamic phenomenon disappear. So the side chain is thought to be the most rigid in N-Me thioamide, which accounts for a structure prerequisite favoring high efficient chirality transfer. This is confirmed by theoretical calculation: remarkable energy difference exists between the Re and Si faces of the chiral molecule. To further clarify the possible pathways for thio-Claisen rearrangement, theoretical prediction is adopted. The result implies that the cisoid pathways will dominate the process. Further experiment confirmed this: with N-Me thioamide, the asymmetrical reaction affords γ-unsaturated thioamides in good yields and high diastereoselectivities up to 98%. After removing the thioamide auxiliaries under hydrolysis conditions, product β,γ-substituted chiral alcohols reached high enantiopurity of 98% ee.