Diastereoselective Reformatsky reaction of methyl 4-bromocrotonate with 1,2:5,6-di-O-isopropylidene-α-d-ribo-hexofuranos-3-ulose: application to novel bicyclic nucleosides

Applications of ultrasound in total synthesis of bioactive natural products: A promising green tool

Total synthesisis frequently compared to climbing as it provides a suitable route to reach a high point from the floor, the complex natural product from simple and commercially available materials. The total synthesis has a privileged position of trust in confirming the hypothetical complex structures of natural products despite sophisticated analytical and spectroscopic instrumentation and techniques that are available presently. Moreover, total synthesis is also useful to prepare rare bioactive natural products in the laboratory as several bioactive secondary metabolites are obtained in small quantities from natural sources. The artistic aspect of the total synthesis of bioactive natural products continues to be praised today as it may provide environmental protection through the concept of green or clean chemistry. The use of ultrasound waves as a non-polluting source of energy is of great interest in the field of sustainable and pharmaceutical chemistry as it differs from conventional energy sources in terms of reaction rates, yields, selectivities, and purity of the products. The present review highlights the application of ultrasound as a green tool in the total synthesis of bioactive natural products as well as this article is also aimed to offer an overview of natural sources, structures, and biological activities of the promising natural products for the first time from 2005 to 2020 elegantly.

A rapid synthetic route to conformationally restricted [5,5]-bicyclic nucleoside-amino acid conjugates

A concise synthetic route to a novel class of conformationally rigid 3',4'-cis-fused bicyclic nucleoside derivatives has been developed. The synthetic strategy and approach involves initial synthesis of a key [5,5]-bicyclic 6-aminofurofuran-2-one scaffold, employing an L-serine derived aminobutenolide as a strategically functionalized chiral template. Subsequent utilization of the carbonyl functionality of the above bicyclic lactone toward nucleobase incorporation, and linking of the resident amine functionality with appropriately protected amino acids completed the syntheses of the target bicyclic nucleoside-amino acid conjugates. Following the above route, and utilizing a combination of easily available nucleobases (4) and amino acids (4) as the two diversity elements, combinatorial synthesis of a 16-member demonstration library of the title amino acid-linked nucleosides has been accomplished.

First example of 5/6-O-linked pseudosaccharides: synthesis of bicyclic nucleosides containing azido or extended carbohydrate moiety

Treatment of the d-glucose-derived substrate 1 with sodium hydride in tetrahydrofuran provided 3,6-anhydro monosaccharide 2, along with the 5,6-ether linked pseudodisaccharide 3, and pseudotrisaccharide 4. However, reaction of 1 with sodium ethoxide in ethanol afforded 2 as the sole product, elaborated to the bicyclic azidonucleosides 9 and 16. Acetylated bicyclic nucleosides 17-19 with extended carbohydrate residues have been synthesized from 3.