Stereoselective Syntheses of Aza, Amino and Imino Sugar Derivatives by Hydroboration of 3,6‐Dihydro‐2 H ‐1,2‐oxazines as Key Reaction

Starting from enantiopure 3,6‐dihydro‐2 H ‐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 < n BuOH < i PrOH < t BuOH. After a two‐step 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 in a series of transformations leading to several amino polyol derivatives. Complete deprotection of both diastereomers without cleavage of the N–O bond led to the novel polyhydroxylated tetrahydro‐2 H ‐1,2‐oxazines 3 and epi ‐ 3 . By change of the deprotection conditions the open‐chain amino polyol 4 with D ‐iditol configuration became accessible. In an alternative sequence 5‐hydroxy‐1,2‐oxazines were utilised to synthesise imino sugars (polyhydroxylated pyrrolidines). Samarium diiodide induced cleavage of the 1,2‐oxazine N–O bond furnished 1,4‐amino alcohols, which were cyclised to give the corresponding pyrrolidine derivatives after activation by mesyl chloride. This sequence either led to a 3‐methoxy‐substituted trihydroxylated pyrrolidine derivative or to the related fully deprotected compound.