O-TBDPS-D-Thr-N-Boc-L-tert-Leu-Diphenylphosphine

Typical procedure (entry 1: R = CH₃CH₂CH₂-): To a dried round bottom flask is added N-butylidene-P,P-diphenylphosphinic amide (27.1 mg, 0.1 mmol), O-TBDPS-D-Thr-N-Boc-L-tert-Leu-diphenylphosphine (3.6 mg, 0.005 mmol) and MS5Å (60 mg) under N₂, followed by the addition of anhydrous Et₂O (1 mL). The reaction mixture is cooled to 0 °C, tert-butyl 2,3-butadienoate (22 micro-L, 0.15 mmol) is then added, and the mixture is stirred at 0 °C for 30 min. The reaction mixture is then filtered (to remove MS5Å) and concentrated under reduced pressure. The residue is purified by column chromatography on silica gel (eluent: hexane/EtOAc = 10:1 to 2:1) to afford the cycloaddition product (37.9 mg, 92% yield, 96% ee) as a colorless oil.