Aluminoxy Acetals from α-Amino Esters: Chirality Transfer via Sequential Addition of Hydride and C-Nucleophiles. 2-Amino Alcohols and Sphingosines

The reaction of α-imino esters (O'Donnell's Schiff bases) with aluminum hydrides to produce acetal-like intermediates and subsequent reaction with carbon nucleophiles has been studied. Treatment of optically pure imine-protected amino esters with iBu₂AlH or iBu₂AlH·Bu₃Al, followed by RMgX or RLi provided fhreo-2-amino alcohols in high yield (73-85%) and excellent “syn” stereoselectivity (8:1 to >20:1, threo or like product preferred). Use of nonpolar solvents (CH₂Cl₂-hexane) provided the highest stereoselectivities. Use of the less-reactive iBu₂AlH·iBu₃Al complex lowered the amount of undesired primary alcohol products observed. Thermally labile aluminoxy acetal intermediates were observed by NMR and were trapped with N-(trimethylsilyl)imidazole to produce relatively stable monosilyl acetals (mixed acetals). Alanine-derived Schiff bases 2a-e showed a correlation between the steric bulk of the ester and threo selectivity. The presence of THF reduced this correlation, suggesting the C-nucleophile addition involves a Lewis acid-assisted S_N2-like displacement of the aluminoxy acetal or displacement of a tight-ion pair. In addition to the synthesis of optically pure arylethanolamines 6a-d from representative amino acids, threo-sphingosines 8a-d were synthesized from L-serine-derived Schiff base 4b, and 1-deoxy-threo-sphingosines 9a-d were synthesized from l-alanine in a similar fashion. Experimental details are provided.