Stereoselective process for preparing substituted polycyclic pyridone derivatives

The present invention provides industrially suitable processes for preparing intermediates in the production of substituted polycyclic pyridone derivatives having a cap-dependent endonuclease inhibitory activity.In the process as shown below, wherein each symbol is as defined in the specification, an optically active substituted tricyclic pyridone derivative of the formula (VII) is obtained in high yield and high enantioselectivity by subjecting a compound of the formula (III) or (VI) to intramolecular cyclization with controlling stereochemistry to obtain a compound of the formula (IV) having a removable functional group on an asymmetric carbon, and then removing the functional group thereof.