Synthetic flavours are usually copies of natural components and include substances such as acids, aldehydes, esters and ketones. Many of the simpler artificial fruit flavours are based on esters.
Esters are compounds formed when carboxylic acids react with alcohols.
For example, propionic acid reacts with amyl alcohol to form amyl propionate that gives the fruity smell of pineapple. Natural flavours are usually mixtures of many different chemical compounds and so a large number of compounds need to be blended together to obtain a good artificial flavour.