Ring opening reactions of sucrose

R. Khan, M. R. Jenner, H. Lindseth, K. S. Mufti, and G. Patel, Ring-opening reactions of sucrose epoxides Synthesis of 4 -derivatives of sucrose, Carbohydr. Res., 162 (1987) 199-207. 

M. K. Gurjar, L. Hough, A. C. Richardson, and L. V. Sincharoenkul, Preparation and ring-opening reactions of a 2,3-anhydride derived from sucrose, Carbohydr. Res., 150 (1986) 53-61. 

The first one of these converts sucrose into valuable noncarbohydrate compounds by degradation of the sucrose skeleton itself. Thereby, hydrolysis of sucrose (the usual first step) is subsequently followed by further degradation under elevated temperature and pressure conditions in the presence of certain catalysts. Chemical reactions or reaction cascades, e.g., dehydration/rehydration, ring opening/closure, bond scission/formation, lead to noncarbohydrate structures which are of known value from petrochemically-derived compounds. One of the most prominent representatives of these sucrose degradation products is 5-hydroxymethylfurfural (HMF). 

Ans. The previous discussion was an oversimplification. Hemiketal formation in o-fructose and other ketohexoses results in reaction at both C-5 and C-6 to give a mixture of 5- and 6-membered rings. Pure fructose in solution consists of a mixture of the a- and /3-anomers of both furanose and pyranose structures, as well as a very small amount (< 0.2 percent) of the open-chain form. However, all combined forms of o-fructose, e.g., fructose combined with glucose in sucrose, involve only the 5-membered ring. For this reason, most texts simplify matters for the student by ignoring the pyranose structures. The exclusive use of the 5-membered ring in combined forms of o-fructose is not understood. 

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