Provided is a salt including a compound represented by a formula (4) and a compound represented by a formula (5). A method for producing the salt includes reacting a compound represented by a formula (2) with a compound represented by a formula (3) in the presence of a quaternary onium salt and a base to form the compound represented by the formula (4); and mixing the compound represented by the formula (4) with the compound represented by the formula (5). A method for producing a compound represented by a formula (6) includes removing, from the salt including the compounds of formulae (4) and (5), a protecting group P1. In the following formulae, P1 and P2 are both protecting groups.