Do SN2 substition twice! Or start with R to give S(desired).

I agree with Xiang.

First CH(R/S)-OTs/Ms/Tf with NaI/Br => CH(S/R)-I/Br with NaN3 => CH(R/S)-N3

Alternative method:

Dry high-energy ball milling of the CH-O-leaving group and NaN3 often leads to configuration retention, which is molecular structure-dependent. In the solid state, the Nu reaction proceeds via a four-centered mechanism. If the molecule has a more hindered reaction center to the "back" attack than the "front" attack, the reaction will dominantly go by retaining the configuration. In this case, bulkier reagents may also be more advantageous.

Good Luck,

Laszlo

Substitution of OH to Cl with thionyl chloride proceeds with SNi mechanism (substitution nucleophilic internal), resulting in the retention of the configuration if the carbon atom connected to OH is chiral. The degree of retention may vary depending on the structure of the alcohol.

Hope this helps,

Vladimir

When you are looking at experimental picture mitsunobu reaction is not that effective in terms of yield and you need to go through tedious purification process and the in second method where you have talked about oxidation followed by reduction here you need to deal with separation of enantiomers which is again a tough job so as somebody you can go for consecutive SN2 reactions. Convert OH to Br using PBr3 then treat that with sodium azide

Mitsunobu reaction