Involved in the biosynthesis of A factor (2-isocapryloyl-3R-hydroxymethyl-gamma-butyrolactone), a gamma-butyrolactone autoregulator that triggers secondary metabolism and morphogenesis in Streptomyces (PubMed:17277085). Catalyzes the reduction of the butenolide phosphate produced by nonenzymatic intramolecular condensation of the 8-methyl-3-oxononanoyl-DHAP ester (PubMed:17277085). a [(3S,4R)-4-alkanoyl-5-oxooxolan-3-yl]methyl phosphate + NADP(+) = a (4-alkanoyl-5-oxo-2,5-dihydrofuran-3-yl)methyl phosphate + H(+) + NADPH [(3S,4R)-4-(6-methylheptanoyl)-5-oxooxolan-3-yl]methyl phosphate + NADP(+) = [4-(6-methylheptanoyl)-5-oxo-2H-furan-3-yl]methyl phosphate + H(+) + NADPH Belongs to the NmrA-type oxidoreductase family.