Diastereodivergent hydroxyfluorination of cyclic and acyclic allylic amines: synthesis of 4-deoxy-4-fluorophytosphingosines.
Cresswell AJ., Davies SG., Lee JA., Morris MJ., Roberts PM., Thomson JE.
A diastereodivergent hydroxyfluorination protocol enabling the direct conversion of some conformationally biased allylic amines to the corresponding diastereoisomeric amino fluorohydrins has been developed. Sequential treatment of a conformationally biased allylic amine with 2 equiv of HBF(4)·OEt(2) followed by m-CPBA promotes epoxidation of the olefin on the face proximal to the amino group under hydrogen-bonded direction from the in situ formed ammonium ion. Regioselective and stereospecific epoxide ring-opening by transfer of fluoride from a BF(4)(-) ion (an S(N)2-type process at the carbon atom distal to the ammonium moiety) then occurs in situ to give the corresponding amino fluorohydrin. Alternatively, an analogous reaction using 20 equiv of HBF(4)·OEt(2) results in preferential epoxidation of the opposite face of the olefin, which is followed by regioselective and stereospecific epoxide ring-opening by transfer of fluoride from a BF(4)(-) ion (an S(N)2-type process at the carbon atom distal to the ammonium moiety). The synthetic utility of this methodology is demonstrated via its application to a synthesis of 4-deoxy-4-fluoro-L-xylo-phytosphingosine and 4-deoxy-4-fluoro-L-lyxo-phytosphingosine, each in five steps from Garner's aldehyde.