Starting from enantiopure 3,6-dihydro-2H-1,2-oxazines syn-1 we introduced an additional hydroxy group in a stereoselective fashion by a standard hydroboration/oxidation protocol. Under "regular" conditions substrate control was sufficient to achieve a very high degree of stereoselectivity. However, a diastereomeric product was isolated when a partially "degraded" borane reagent was used. We could synthesise this new diastereomer on purpose by addition of alcohols to the "fresh" hydroboration reagent. The level of stereoinduction increased with the steric bulk of the added alcohol: MeOH Ͻ nBuOH Ͻ iPrOH Ͻ tBuOH. After a twostep oxidation/reduction sequence, another 5-hydroxy-1,2-oxazine epimer was accessible. The obtained 5-hydroxy-1,2-oxazine diastereomers 2 were used as versatile intermediates