Scheme 3.

Deprotection/cyclization of the selected U-5C-4CR adducts 1 to substituted 2-oxopiperazines 2. If not stated otherwise, the diastereoisomeric mixtures of 1 were used as starting materials. Isolated yields refer to the sum of diastereoisomers. dr values were measured by ¹H analyses of purified products. The colors indicate the origin of atoms from U-5C-4CR: green, isocyanide; red, α-amino aldehyde; blue, α-amino acid; pink, alcohol. ^aSynthesized without purification of the respective U-5C-4CR adducts, yield over 3 steps. ^bA single diastereoisomer of 2k was isolated. ^cCompound 2n was synthesized from the single (2R, 3S)-1n diastereoisomer