Studies on the Selective Reduction of the Amide Link of Acyclic and Macrocyclic Amidoketals: Unexpected Cleavage and trans‐Acetalization with Red‐Al®
摘要:
Selective reduction of the amide moiety of acyclic and macrocyclic amido-ketals was studied in presence of various reagents (BH3 center dot Me2S, iBuAIH(2), Red-Al(R), LiAIH(4)). The best results were obtained with lithium aluminium hydride in the presence of triethylamine traces, whereas borane dimethyl sulfide gave rise to a partial ketal reduction of the acyclic compound and Red-Ale to a cleavage of the macrocyclic molecule accompanied by an unexpected traps-acetalization.
Macrocyclic dioxadilactam 2 was obtained in two steps from R-(-)-phenylglycinol via the chiral amido-ketal 1. Cyclization of 1 in acidic conditions (PTS A) and concentrated solution (c = 0.15 M) afforded 2 as three diasteromers 2a+2b+2c, two of which show a C-2 symmetry. (C) 1997 Published by Elsevier Science Ltd.