The sweetness in coffee comes from saccharides, better known as carbohydrates. Saccharides relevant to coffee are mono and disaccharides. The monosaccharides are composed of glucose and fructose, with the disaccharides made up of sucrose. The roasting process brings about a significant degradation of sucrose content in coffee beans with scientific literature reporting numbers ranging from up to 97-99% degradation of sugars. (Refer to page 188.8.131.52) One suspicion we have of existing scientific literature is that a lot of research on this subject is carried out using low scoring coffees which could suggest some of these figures are understated.
James Hoffmann walks you through the processes which contribute to a reduction in sucrose content in his very succinct post on Maillard Reaction,