Abstract: The gibberellins (GAs) are a group of naturally-occurring tetracyclic diterpenoid plant growth hormones which are essential to the normal growth and development of plants. Over one hundred different GAs have been identified to date and most of these compounds are present in Nature only in minute quantities. Among the biosynthetic pathways of the gibberellins, the oxidation and subsequent removal of carbon-20 in C₂₀-GAS (e.g. 1 and 2) to form C₁₉-GAs (e.g. 3 and 4) is one of the most important process in which GA 20-oxidases are involved¹ (Scheme 1). During the course of our investigation into this process, we needed to have access to C₂₀-GAS such as (2) from a more abundant C₁₉-GA such as GA₃ (5). A successful route for such transformation may then be adapted for the synthesis of isotopically-labelled C₂₀-GAS for biological studies.