Benzylic compounds reductions, lithium aluminum hydride

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). 

Brimacombe and How reduced methyl 3,4-0-isopropylidene-2,6-di-O-p-tolylsulfonyl-a-D-galactopyranoside with lithium aluminum hydride in tetrahydrofuran, to provide methyl 3,4-O-isopropylidene-2-0-p-tolylsulfonyl-a-D-fucopyranoside in 54% yield. In later reductions of closely related compounds, the solvent chosen was ether-benzene methyl 3-0-benzyl-2-deoxy-6-0-p-tolylsulfonyl-a-D-Ii/xo-hexopyranoside afforded the corresponding 6-deoxy compound which, on methylation, debenzylation, and acid hydrolysis, gave 2,6-dideoxy-4-O-methyl-D-Zi/Jco-hexose (chromose A). An improved synthesis of this sugar involved the reduction, in ether—benzene (1 1), ofmethyl 2-deoxy-4-0-methyl-3,6-di-0-p-tolylsulfonyl-a-D-/i/xo-hexopyranoside to methyl 2,6-dideoxy-4-0-methyl-a-D-Z /xo-hexopy-ranoside in 25% yield a synthesis of 3-0-acetyl-2,6-dideoxy-D-Z /xo-hexose (chromose D) was effected by way of reduction in ether-benzene (2 1) of methyl 2-deoxy-6-0-p-tolylsulfonyl-a-D-Zyxo-hexo-pyranoside to the corresponding 6-deoxy compound in 27% yield, followed by partial acetylation and acid hydrolysis. 

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