 If the cycle has four carbon atoms (five in total), the form is called furanose after the compound tetrahydrofuran.The conventional numbering of the carbons in the closed form is the same as in the open-chain form.If the sugar is a aldohexose, with the carbonyl in position 1, the reaction may involve the hydroxyl on carbon 4 or carbon 5, creating a hemiacetal with five- or six-member ring, respectively.