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α-Amino nitriles tethered to alkenes through a urea linkage undergo intramolecular C-alkenylation on treatment with base by attack of the lithionitrile derivatives on the N′-alkenyl group. A geometry-retentive alkene shift affords stereospecifically the E or Z isomer of the 5-alkenyl-4-iminohydantoin products from the corresponding starting E- or Z- N′-alkenyl urea, each of which may be formed from the same N-allyl precursor by stereodivergent alkene isomerization. The reaction, formally a nucleophilic substitution at an sp2 carbon atom, allows the direct regioselective incorporation of mono-, di-, tri-, and tetrasubstituted olefins at the α-carbon of amino acid derivatives. The initially formed 5-alkenyl iminohydantoins may be hydrolyzed and oxidatively deprotected to yield hydantoins and unsaturated α-quaternary amino acids. [ABSTRACT FROM AUTHOR]