Abstract: The present invention provides synthetic methods and preparation of cholecalciferol (vitamin D3) from cholesterol, which is a by-product of the Omega-3 oil production process. According to the invention, dehydro-cholesterol is prepared and subjected to UV radiation (254 nm). Accordingly, two new synthetic alternatives are provided; wherein the first consists of processes of oxidation-reduction of cholesterol to form dehydro-cholesterol and subsequent UV radiation; which route involves six steps (see scheme 1). The second transformation involves the generation of vitamin D3 by means of a bromination method and a photocatalytic reaction using UV radiation; which alternative requires four steps (see scheme 2).