Lithium triethoxyaluminum hydride, reduction

This reduction has been achieved previously with lithium triethoxyaluminum hydride (1,625). 

REDUCTION TO THE CORRESPONDING ALDEHYDE Lithium triethoxyaluminum hydride [LiAlH(OEt)j] was found to readily convert pseudoephedrine carboxamides to the corresponding aldehyde with only limited erosion of the ees (Table 2.5). To do so, a solution of the pseudoephedrine carboxamide in THF is added at -78°C to a suspension of LiAlH(OEt)j (2.3 equiv) in hexanes. The reaction mixture is then warmed to 0°C before a dilute acidic aqueous solution (typically HCl 0.5 M) is added, resulting in the formation of the desired aldehyde along with the pseudoephedrine aminal 27. To avoid the formation of the byproduct, the quenching can be performed using a 1M aqueous solution of hydrochloric acid and 10 equivalents of trifluoroacetic add. 

Several reduction methods are available to convert nitriles (Stephen reaction) or acyl chlorides (Rosenmund reduction) or amides to aldehydes. The use of lithium aluminum hydride at low temperatures or lithium triethoxyaluminum hydride appears to be a more effective means of reduction than the stannous chloride-HCl used in the Stephen method or palladium and hydrogen used in the Rosenmund reduction. 

Reduction of o /i-unsatin-ated lactams, S,6-dihydro-2-pyridones, with lithium aluminum hydride, lithium alkoxyaluminum hydrides and alane gave the corresponding piperidines. 5-Methyl-5,6-dihydro-2-pyridone (with no substituent on nitrogen) gave on reduction with lithium aluminum hydride in tetrahydrofuran only 9% yield of 2-methylpiperidine, but l,6-dimethyl-5,6-dihydro-2-pyridone and 6-methyl-l-phenyl-5,6-dihydro-2-pyridone afforded 1,2-dimethylpiperidine and 2-methyl-1-phenylpiperidine in respective yields of 47% and 65% with an excess of lithium aluminum hydride, and 91% and 92% with alane generated from lithium aluminum hydride and aluminum chloride in ether. Lithium mono-, di- and triethoxyaluminum hydrides also gave satisfactory yields (45-84%) [7752]. 

---