Involved in the biosynthesis of the alpha-glucosidase inhibitor acarbose. Catalyzes the 2-epimerisation of 2-epi-5-epivaliolone 7-phosphate to yield 5-epi-valiolone 7-phosphate. 2-epi-5-epi-valiolone 7-phosphate = 5-epi-valiolone 7-phosphate Belongs to the hyi family.