N-[N-[3-(2-hydroxy-substituted phenyl)propyl]-L-α-aspartyl]-L-phenylalanine 1-methyl ester derivatives, which are useful as sweeteners, may be conveniently produced by subjecting a 2-hydroxychroman derivative having various substituents on the benzene ring and aspartame to a reductive alkylation reaction. These compounds may be purified by subjecting such a derivative containing one or more impurities to a crystallization step to crystallize the same. Further, there are provided a process for producing dialkoxychroman derivatives useful as intermediates for the production of such aspartame derivatives in the above process, and novel dialkoxychroman derivatives thus produced, which are excellent as the production intermediate.