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To a solution of (S,Z)-N-(7-bromo-3-chloro-5H-chromeno[2,3-c]pyridin-5-ylidene)-2-methylpropane-2-sulfinamide (3000 mg, 7.25 mmol) in THF (25 mL) was slowly added 2-tert-butoxy-2-oxoethylzinc chloride (0.5 M in diethyl ether, 36.3 mL, 18.13 mmol) in an ice bath. The mixture was stirred at 0° C. for 25 mins, then 80 mL of saturated NH4Cl aqueous solution was added to quench the reaction. The reaction mixture was extracted with EtOAc (2×35 mL). The organic extract was washed with saturated NaCl aqueous solution, dried over MgSO4, filtered and concentrated to give the crude material as an orange solid. The crude material was absorbed onto a plug of silica gel and purified by chromatography through a Redi-Sep pre-packed silica gel column (80 g), eluting with a gradient of 0% to 20% to 50% DCM/MeOH (90/10) in DCM. The fractions which contain mixture were repurified using 40 g Redi-Sep pre-packed silica gel column and eluting with a gradient of 0% to 20% to 50% DCM/MeOH (90/10) in DCM. The desired fractions were collected and concentrated to provide tert-butyl 2-((S)-7-bromo-3-chloro-5-((S)-1,1-dimethylethylsulfinamido)-5H-chromeno[2,3-c]pyridin-5-yl)acetate (isomer A, high Rf), and tert-butyl 2-((R)-7-bromo-3-chloro-5-((S)-1,1-dimethylethylsulfinamido)-5H-chromeno[2,3-c]pyridin-5-yl)acetate (isomer B, low Rf) as off-white solid. Isomer A: 1H NMR (400 MHz, DMSO-d6) δ ppm 0.97 (s, 9 H) 1.03 (s, 9 H) 3.42 (s, 2 H) 6.41 (s, 1 H) 7.15 (d, J=8.71 Hz, 1 H) 7.52 (dd, J=8.80, 2.35 Hz, 1 H) 7.79 (s, 1 H) 7.98 (d, J=2.45 Hz, 1 H) 8.37 (s, 1 H). Isomer B: 1H NMR (400 MHz, DMSO-d6) δ ppm 0.97 (s, 9 H) 1.04 (s, 9 H) 3.34 (s, 1 H) 3.42 (s, 1 H) 6.35 (s, 1 H) 7.16 (d, J=8.80 Hz, 1 H) 7.54 (dd, J=8.80, 2.45 Hz, 1 H) 7.77 (d, J=2.35 Hz, 1 H) 7.95 (s, 1 H) 8.35 (s, 1 H).

1. customto quench
2. customthe reaction
3. extractionThe reaction mixture was extracted with EtOAc (2×35 mL)
4. extractionThe organic extract
5. washwas washed with saturated NaCl aqueous solution
6. dry with materialdried over MgSO4
7. filtrationfiltered
8. concentrationconcentrated
9. customto give the crude material as an orange solid
10. customThe crude material was absorbed onto a plug of silica gel
11. custompurified by chromatography through a Redi-Sep pre-packed silica gel column (80 g)
12. washeluting with a gradient of 0% to 20% to 50% DCM/MeOH (90/10) in DCM
13. customwere repurified
14. washeluting with a gradient of 0% to 20% to 50% DCM/MeOH (90/10) in DCM
15. customThe desired fractions were collected
16. concentrationconcentrated