Add a solution of 2,2-Difluoro-9-oxo-6,7,8,9-tetrahydro-1,3-dioxa-5-aza-cyclohepta[f]indene-5-carboxylic acid tert-butyl ester (2.2 mmol) in THF (20 mL), to a cooled (−78° C.) solution of Borane-methyl sulfide (2.6 mmol) and (R)-2-methyl-CBS-oxazaborolidine (3.3 mmol) in THF (20 mL). After warming slowly to 0° C. for 1 h, quench the reaction with methanol (2 mL). Remove solvent under vacuum and chromatograph using ethyl acetate/hexane (2-35%) to elute. This gives the title compound as a colorless solid.
