Concise and highly stereoselective syntheses of d-fagomine and 2-epi-fagomine

Recent advances in the total synthesis of polyhydroxylated alkaloids via chiral oxazines

This review summarizes recently established methodologies developed for the enantioselective and diastereoselective synthesis of chiral 1,3-oxazines. These compounds are of interest as advanced synthetic intermediates in the total synthesis of structurally complex and biologically active polyhydroxylated alkaloids such as (+)-1-deoxynojirimycin, (-)-anisomycin, (+)-castanospermine, (+)-casuarine, (-)-conduramine F-1, (-)-sphingofungin B, Neu5Ac methyl ester, and other natural products. The devised synthetic approach aims to offer a direct, efficient, and adaptable method for obtaining both pure enantiomers and pure diastereomers. It revolves around utilizing chiral building blocks like syn,syn-, syn,syn,anti-, syn,anti-, syn,anti,syn-, anti,syn-, anti,syn,syn-, and anti,syn,anti-oxazines. By integrating oxazine chemistry with established and innovative transformations, this approach enabled the synthesis of 30 polyhydroxylated amines across various studies conducted between 2007 and 2022.

Glycal mediated synthesis of piperidine alkaloids: fagomine, 4-epi-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-epi-fagomine. In one of the strategies, reduction of 2-deoxyglycolactam-N-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 C1-C2 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.

Versatility of glycals in synthetic organic chemistry: coupling reactions, diversity oriented synthesis and natural product synthesis

Glycals, 1,2-unsaturated sugar derivatives, are versatile starting materials for the synthesis of natural products and the generation of novel structural features in Diversity Oriented Synthesis (DOS). The versatility of glycals in synthesis emanates, among others, from the presence of the ring oxygen and the enol-ether type unsaturation, the different types of stable conformations they can adopt depending on the nature of the protecting groups present and the ease with which the protecting groups of the three hydroxy groups could be tailored to suite for a desired manipulation. This review summarizes the literature on the different transformations of the endo glycals into biologically relevant compounds such as chromans, thiochromans, chromenes, thiochromenes, peptidomimetics, bridged benzopyrans etc., as well as on the use of glycals as chiral building blocks for the synthesis of various natural products such as aspicilin, reblastatin, diospongins, decytospolides, osmundalactones, paclitaxel, isatisine, d-fagomine, and spliceostatin, reported post 2014.