Silyl protecting groups regioselective silylation

Two consecutive enolate alkylations were utilized to generate the quaternary carbon atom (Scheme 38). Alcohol 238 was transformed into the protected hydroxy enone 244. Regioselective deprotonation at the a-position of the ketone 244 led to a cross-conjugated enolate that was alkylated with the allylic iodide 245. The vinyl silyl moiety in 245 represents a masked keto group [127]. The choice of the TBS protecting group for the hydroxyl group at of 244 was crucial in order to prevent the deprotonation at the y-posi-... 

Removal of the silyl protecting group using TBAF, and subsequent acetylation, led to the diacetate 818. Regioselective nitration and catalytic hydrogenation provided the (R)-arylamine 814 (613) (Scheme 5.95). 

Different protecting groups such as acetonides, /-butyl dimethyl silyl derivatives proved to be stable under these experimental conditions, but the benzylidene group is completely cleaved.. The rate of photodeoxygenation is greater at the secondary positions [82], as indicated in Scheme 43, with the regioselective photolysis of dipivaloate to give a mixture of monodeoxy and dideoxy derivatives (81) in low yields. 

Bis-protection of the terminal OH-functions of 25 furnished 26, to which Lipshutz cuprate is added. The bulky silyl group directs the nucleophilic attack to the 3-position to form 27, whereas smaller protective groups such as MOM lead to mixtures of 2,3-regioisomers. Monoepoxide 27 is not isolated but adds excess cuprate in-situ to furnish 28 regioselectively. Once introduced the 3-methyl branching in 27 directs the cuprate attack towards the less hindered position at C-5. 

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