Abstract: A method for selectively producing stable alpha form crystals of sapropterin hydrochloride is provided. In this method, the alpha form crystal of sapropterin hydrochloride is produced by dissolving sapropterin hydrochloride with hydrochloric acid at a concentration of not less than 4 mol/L at not less than 70 degrees C.; adding heated ethanol to the solution; and cooling the solution at a cooling rate of not faster than 3 degrees C./min to a temperature of 40 to 55 degrees C. to precipitate the crystals.

Abstract: A method for selectively producing stable alpha form crystals of sapropterin hydrochloride is provided. In this method, the alpha form crystal of sapropterin hydrochloride is produced by dissolving sapropterin hydrochloride with hydrochloric acid at a concentration of not less than 4 mol/L at not less than 70 degrees C.; adding heated ethanol to the solution; and cooling the solution at a cooling rate of not faster than 3 degrees C./min to a temperature of 40 to 55 degrees C. to precipitate the crystals.

Abstract: To provide a method for producing L-biopterin on a large industrial scale by using a reagent which is inexpensive and easy to handle, without requiring a use of any particular equipment or plants. A method for porducing a biopterin derivative represented by the formula (6): wherein R1 and R2, which are the same or different from each other, each represents a hydrogen atom, an alkyl group, or an aryl group, comprising: reacting a compound belonging to triacetoxy-5-deoxy-L-arabinose phenylhydrazones represented by the formula (4): wherein R1 and R2 are the same as defined above, with 6-hydroxy-2,4,5-triaminopyrimidine (5) under catalytic influence of a Lewis acid in an aqueous solvent.

Abstract: To provide a method for producing L-biopterin on a large industrial scale by using a reagent which is inexpensive and easy to handle, without requiring a use of any particular equipment or plants.