D-(-)-Pantoyl lactone could be produced through the following synthetic routes.

A. Preparation of catalyst solution. A 250-mL, round-bottomed flask fitted with a septum and magnetic stirring bar is charged with 486.9–488.2 mg (0.985–0.990 × 10-3 mol) of chloro(1,5-cyclooctadiene)rhodium(I) dimer and, under argon, with 1.20 g (2.15 × 10^-3 ml) of (2S, 4S)-N-tert-butyloxycarbonyl-4-diphenylphosphino-2-diphenylphosphinomethylpyrrolidine, (S,S)-BPPM. The sealed flask is charged by cannula, under argon, with 150 mL of degassed benzene and stirred under argon for 15 min at room temperature. The catalyst is transferred by cannula, under argon, into the autoclave (see below).

B. Asymmetric hydrogenation. A stainless steel stirred autoclave with a total volume of 500 mL is charged with 25.6 g (0.2 mol) of ketopantoyl lactone. The autoclave is flushed with argon and the catalyst solution (see above) is added by cannula, under argon. The autoclave is sealed and hydrogenation is carried out at 40°C, 750-psig hydrogen and 950–1050 rpm for 48 hr. Care should be taken to flush all the lines before connecting to the autoclave. After the autoclave is cooled to room temperature, it is vented and opened. The reaction mixture is then transferred to a 500-mL, round-bottomed flask and most of the solvent is removed by rotary evaporator. Distillation of this reddish solid affords 24–25.6 g (92–98%) of D-(-)-pantoyl lactone: bp 90–110°C (4 cm); [α]_D25 −39.3° to −42.4° (c 2, H₂O) (78 to 84% e.e.).

The pantoyl lactone thus obtained (25.41 g), [α]_D25 −40.8° (80.5% e.e.) is refluxed with 75 mL benzene and 290 mL of UV-grade hexanes. The cloudy solution is stirred briskly overnight as solids form. Filtration of the solids and drying for 3 hr at 0.25 mm, 30°C in a vacuum oven affords 21.51 g of product; [α]_D25 −47.7° (94.27% e.e.). This material is again refluxed and crystallized from 30 mL of benzene and 116 mL of UV-grade hexanes to afford 19.97 g (77%) of product; [α]_D25 −49.87° (98.5% e.e.). Anal. calcd. for C₆H₁₀O₃: C, 55.37; H, 7.75. Found: C, 55.34; H, 7.57.