Glycal mediated synthesis of piperidine alkaloids: fagomine, 4--fagomine, 2-deoxynojirimycin, and an advanced intermediate, iminoglycal.

Glucal and galactal are transformed into 2-deoxyglycolactams, which are important building blocks in the synthesis of biologically active piperidine alkaloids, fagomine and 4--fagomine. In one of the strategies, reduction of 2-deoxyglycolactam--Boc carbonyl by lithium triethylborohydride (Super-Hydride®) has been exploited to generate lactamol whereas reduction followed by dehydration was utilized as the other strategy to functionalize the C-C bond in the iminosugar substrate. The strategies provide the formal synthesis of 2-deoxynojirimycin, nojirimycin and nojirimycin B. DFT studies were carried out to determine the reason for the failure of the formation of the 2-deoxygalactonojirimycin derivative. Further, DFT studies suggest that phenyl moieties of protecting groups and lone pairs of oxygen in carbamate group plays a vital role in deciphering the conformational space of the reaction intermediates and transition-state structures through cation-π or cation-lone pair interactions. The influence of these interactions is more pronounced at low temperature when the entropy factor is small.