Glycals and 4-deoxypentenosides (4-DPs), unsaturated pyranosides with similar structures and reactivity profiles, can exhibit a high degree of stereoselectivity upon epoxidation with dimethyldioxirane (DMDO). In most cases, the glycals and their corresponding 4-DP isosteres share the same facioselectivity, implying that the pyran substituents are largely responsible for the stereodirecting effect. Fully substituted dihydropyrans are subject to a “majority rule,” in which the epoxidation is directed toward the face opposite to two of the three groups. Removing one of the substituents has a variable effect on the epoxidation outcome, depending on its position and also on the relative stereochemistry of the remaining two groups. Overall, we observe that the greatest loss in facioselectivity for glycals and 4-DPs is caused by removal of the C3 oxygen, followed by the C5/anomeric substituent, and least of all by the C4/C2 oxygen. DFT calculations based on polarized-π frontier molecular orbital (PPFMO) theory support a stereoelectronic role for the oxygen substituents in 4-DP facioselectivity, but less clearly so in the case of glycals. We conclude that the anomeric oxygen in 4-DPs contributes toward a stereoelectronic bias in facioselectivity whereas the C5 alkoxymethyl in glycals imparts a steric bias, which at times can compete with the stereodirecting effects from the other oxygen substituents.

Glycal assembly offers an expedient entry into β-linked oligosaccharides, but epoxyglycal donors can be capricious in their reactivities. Treatment with Et2NH and CS2 enables their in situ conversion into glycosyl dithiocarbamates, which can be activated by copper triflate for coupling with complex or sterically congested acceptors. The coupling efficiency can be further enhanced by in situ benzoylation, as illustrated in an 11-step synthesis of a branched hexasaccharide from glucals in 28% isolated yield and just 4 chromatographic purifications.

The Pictet−Spengler reaction of tryptophan allyl ester with aryl aldehydes generates cis-tetrahydro-β-carbolines with complete stereo-control and with complete retention of optical integrity, when the reaction is carried out under kinetically controlled conditions.

In this article, we evaluate glycosyl dithiocarbamates (DTCs) with unprotected C2 hydroxyls as donors in β-linked oligosaccharide synthesis. We report a mild, one-pot conversion of glycals into β-glycosyl DTCs via DMDO oxidation with subsequent ring opening by DTC salts, which can be generated in situ from secondary amines and CS2. Glycosyl DTCs are readily activated with Cu(I) or Cu(II) triflate at low temperatures and are amenable to reiterative synthesis strategies, as demonstrated by the efficient construction of a tri-β-1,6-linked tetrasaccharide. Glycosyl DTC couplings are highly β-selective despite the absence of a preexisting C2 auxiliary group. We provide evidence that the directing effect is mediated by the C2 hydroxyl itself via the putative formation of a cis-fused bicyclic intermediate.

The nucleation and growth of crystalline cobalt nanoparticles (Co NPs) under solvothermal conditions can be separated into distinct stages by using (i) polynuclear clusters with multivalent capping ligands to initiate nucleation, and (ii) thermolabile organometallic complexes with low autonucleation potential to promote crystalline growth. Both nucleation and growth take place within an amorphous accretion, formed in the presence of polyvalent surfactants. At the pre-nucleation stage, a calixarene complex with multiple Co2–alkyne ligands (Co16–calixarene 1) undergoes thermal decomposition above 130 °C to form “capped cluster” intermediates that coalesce into well-defined Co nanoclusters, but are resistant to further aggregation. At the post-nucleation stage, a monomer (pentyne–Co4(CO)10, or PTC) with a low thermal activation threshold but a high barrier to autonucleation is introduced, yielding ε-Co NPs with a linear relationship between particle volume and the Co mole ratio ([Cofinal]/[Coseed]). Co nanocrystals can be produced up to 40 nm with a 10–12% size dispersity within the accretion, but their growth rate depends on the activity of the supporting surfactant, with an octapropargyl calixarene derivative (OP-C11R) providing the most efficient transport of reactive Co species through the amorphous matrix. Post-growth digestion with oleic acid releases the Co NPs from the residual accretion, which can then self-assemble by magnetic dipolar interactions into flux-closure rings when stabilized by calixarene-based surfactants. These studies demonstrate that organometallic complexes can be designed to establish rational control over the nucleation and growth of crystalline NPs within an intermediate accretion phase.