Synthesis of Heterocyclic Alcohols

Brown et a/ [1] have elegantly hydroborated a variety of heterocyclic olefins, which on oxidation yield the corresponding alcohols in excellent yields. In the case of heterocyclic olefin containing a double bond a to the heteroatom, the hydroboration reaction is highly regioselective, placing boron at the (3-carbon atom. The use of 9-BBN for the synthesis of variety of heterocyclic alcohols is given in Chart 6.21. 

The hydroboration of pyridine derivative having a double bond P to the nitrogen affords the corresponding 3-hydroxy and 4-hydroxy derivatives in an 85 15 ratio (Eq. 6.18), whereas the product distribution ratio is 75 25 in the case of dicyclohexylborane and disiamylborane as the hydroborating reagents. 

Brown and coworkers [2] studied the hydroboration of N-substituted 3-pyr-rolines and the intermediate organoboranes on oxidation afford the corresponding 3-hydroxy pyrrolines in good yield (Table 6.22) [2]. 

The relative reactivity of heterocyclic and carbocyclic olefins toward 9-BBN are depicted in Chart 6.22 [3]. 

---