The first and asymmetric total synthesis of phomarol, an uncommon C25 steroid, is described. The synthetically challenging benzocycloheptane motif, found in phomarol and some other naturally occurring molecules, was synthesized efficiently using a very mild acid-promoted type I [5 + 2] cycloaddition, followed by regio-and chemoselective cleavage of the C-O bond and aromatization cascade. This work is the first example of using the hydrogen bonding between the hydroxy group and oxidopyrylium ylide to control the stereoselectivity of cycloadditions. The highly functionalized tetrahydropyran ring of phomarol was produced efficiently based on our suggested biomimetic pathway.