 Another transformation of high synthetic utility is asymmetric <unk> . A number of asymmetric <unk> exist : the <unk> <unk> , the <unk> @-@ <unk> <unk> , and the <unk> @-@ Colonna <unk> . These methods require specific functionality in order to achieve selectivity . The <unk> <unk> is specific to <unk> <unk> , the <unk> <unk> requires <unk> @-@ <unk> <unk> <unk> , and the <unk> <unk> requires α @-@ β <unk> <unk> . <unk> <unk> act <unk> on many <unk> <unk> . It has even possible to effect <unk> <unk> <unk> in the <unk> <unk> unit . Further investigation into these reactions may be required before levels of <unk> excess become practical for large scale synthesis . <unk> et al. have investigated asymmetric <unk> with a <unk> <unk> salt using <unk> as the stoichiometric <unk> seen below . 
