Synthesis of 1,7-dioxaspiro[5.5]undecanes and 1-oxa-7-thiaspiro[5.5]undecanes from exo-glycal

Formal Glycosylation of Quinones with exo-Glycals Enabled by Iron-Mediated Oxidative Radical-Polar Crossover

The intermolecular C-O coupling reaction of 1,4-quinones with exo-glycals under iron hydride hydrogen atom transfer (HAT) conditions is described. This method provides a direct and regioselective access to a wide range of phenolic O-ketosides related to biologically relevant natural products in diastereomeric ratios up to >98:2 in the furanose and pyranose series. No trace of the corresponding C-glycosylated products that might have resulted from the radical alkylation of 1,4-quinones was observed. The results of mechanistic experiments suggest that the key C-O bond-forming event proceeds through an oxidative radical-polar crossover process involving a single-electron transfer between the HAT-generated glycosyl radical and the electron-acceptor quinone.

Structures and hydrogen bonding of 1,7-dioxaspiro[5.5]undecane and its hydrates

The conformations of 1,7DSU and its stepwise solvation by up to 5 water molecules were explored using supersonic-jet Fourier transform microwave spectroscopy with the supplement of theoretical calculations. Experimentally, the rotational spectra of the most stable structures of the monomer, monohydrate and dihydrate were observed and assigned. The characteristics of the stability and intermolecular interaction topologies of the 1,7DSU monomer and its hydrated clusters were obtained by CREST conformational sampling followed by B3LYP-D3(BJ)/def2-TZVP geometrical optimizations and MP2/aug-cc-pVTZ single-point energy calculations. The first water molecule links to the 1,7DSU monomer through an O_wH⋯O hydrogen bond. The water molecules tend to aggregate with each other and form cyclic structures for the n = 2-5 clusters. The interactions between water and the 1,7DSU monomer as well as those between water and water were revealed. The analyses of non-covalent interactions and the natural bond orbital suggest that the O_wH⋯O_1,7DSU, O_wH⋯O_w, and CH⋯O_w hydrogen bonds play a prominent role in structural stability.

Pyranoid Spirosugars as Enzyme Inhibitors

Background:

Pyranoid spirofused sugar derivatives represent a class of compounds with a significant impact in the literature. From the structural point of view, the rigidity inferred by the spirofused entity has made these compounds object of interest mainly as enzymatic inhibitors, in particular, carbohydrate processing enzymes. Among them glycogen phosphorylase and sodium glucose co-transporter 2 are important target enzymes for diverse pathological states. Most of the developed compounds present the spirofused entity at the C1 position of the sugar moiety; nevertheless, spirofused entities can also be found at other sugar ring positions. The main spirofused entities encountered are spiroacetals/thioacetals, spiro-hydantoin and derivatives, spiro-isoxazolines, spiro-aminals, spiro-lactams, spiro-oxathiazole and spiro-oxazinanone, but also others are present.

Objectives:

The present review focuses on the most explored synthetic strategies for the preparation of this class of compounds, classified according to the position and structure of the spirofused moiety on the pyranoid scaffold. Moreover, the structures are correlated to their main biological activities or to their role as chiral auxiliaries.

Conclusion:

It is clear from the review that, among the different derivatives, the spirofused structures at position C1 of the pyranoid scaffold are the most represented and possess the most relevant enzymatic inhibitor activities. Nevertheless, great efforts have been devoted to the introduction of the spirofused entity also in the other positions, mainly for the preparation of biologically active compounds but also for the synthesis of chiral auxiliaries useful in asymmetric reactions; examples of such auxiliaries are the spirofused chiral 1,3-oxazolidin-2-ones and 1,3-oxazolidine-2-thiones.

Synthesis of highly functionalized 2-thiaspiro[4.5]deca-6,8-dienes via atom efficient tandem Michael addition/Thorpe–Ziegler cyclization

The reaction of 2,4-bis((Z)-arylidene)dihydrothiophen-3(2H)-ones and 2-(1-arylethylidene)malononitriles proceeded through a tandem sequence of reactions in a single step affording 2-thiaspiro[4.5]deca-6,8-dienes in quantitative yields.

Photoinitiated hydrothiolation of pyranoid exo-glycals: the d-galacto and d-xylo cases

Radical-mediated addition reactions of thiols to O-peracetylated exo-galactal and exo-xylal with 2,2-dimethoxy-2-phenylacetophenone as the photoinitiator resulted in high yielding formation of the corresponding β-d-glycopyranosylmethyl-sulfide derivatives (2,6-anhydro-1-deoxy-1-S-substituted-1-thio-alditols) with exclusive regio- and very high stereoselectivity, including disaccharide mimicks with Gly-CH2-S-Gly scaffolds.

Iron(III) chloride-tandem catalysis for a one-pot regioselective protection of glycopyranosides

Tandem catalysis by using iron(III) chloride hexahydrate leads to carbohydrate building blocks displaying an orthogonal protecting group pattern as illustrated by the regioselective protection of trehalose and maltose disaccharides.