Replacement of hydroxyl groups with

The reaction between silica and halogenating reagents permits the direct replacement of hydroxyl groups with halogen atoms, yielding reactive =Si-X surface groups. 

The most popular way to modify alkaloids 1-4 is a derivatization of the alcohol or a replacement of hydroxyl group with retention or inversion of the absolute configuration at C-9 (Scheme 21.1 note quinine is given as a representative example, but most of the reaction could be also carried out with alkaloids 2-4 and the 9-epi series). Alkaloids could be easily transformed into the corresponding 9-0-ethers 16 [122-125], esters 17 [126, 127], and carbamates 18 [128, 129]... 

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