Under the alkaline conditions of Tollen's reagent, fructose (which is a ketose) undergoes tautomerization (Lobry de Bruyn-van Ekenstein transformation) to form an enediol intermediate. This intermediate then converts into aldoses (glucose and mannose). Since aldoses contain an aldehyde group, they readily reduce Tollen's reagent. Thus, the reduction is due to the enolisation of fructose followed by its conversion to an aldehyde by the base.