Monoterpene synthase that catalyzes the formation of (+)-menthofuran from (+)-pulegone. (R)-pulegone + O2 + reduced [NADPH--hemoprotein reductase] = (R)-menthofuran + H(+) + 2 H2O + oxidized [NADPH--hemoprotein reductase] Secondary metabolite biosynthesis; terpenoid biosynthesis. Co-suppression of menthofuran synthase leads to a decrease of pulegone level and an increase of piperitone and piperitenone levels due to an increased pulegone reductase activity. Belongs to the cytochrome P450 family.