Alkyl benzyl carbamates, reduction

From intermediate 28, the construction of aldehyde 8 only requires a few straightforward steps. Thus, alkylation of the newly introduced C-3 secondary hydroxyl with methyl iodide, followed by hydrogenolysis of the C-5 benzyl ether, furnishes primary alcohol ( )-29. With a free primary hydroxyl group, compound ( )-29 provides a convenient opportunity for optical resolution at this stage. Indeed, separation of the equimolar mixture of diastereo-meric urethanes (carbamates) resulting from the action of (S)-(-)-a-methylbenzylisocyanate on ( )-29, followed by lithium aluminum hydride reduction of the separated urethanes, provides both enantiomers of 29 in optically active form. Oxidation of the levorotatory alcohol (-)-29 with PCC furnishes enantiomerically pure aldehyde 8 (88 % yield). 

Benzyl ethers can be prepared from THP ethers by the Me3SiOTf-catalyzed reaction with PhCHO and EtiSiH." Analogously, amides, carbamates, and ureas are A -alkylated via condensation with aldehydes followed by in situ reduction with Et SiH. The aldehyde may be replaced by a thioester that is subject to reduction in situ. Thus, a mixture of sodium triacetoxyborohydride, triethylsilane, and Pd-C catalyst is employed. ... 

---