Highly Efficient Asymmetric Synthesis of Enantiopure Dihydro-1,2-oxazines: Dual-Organocatalyst-Promoted Asymmetric Cascade Reaction

A one-pot dual-organocatalyst-promoted asymmetric α-aminoxylation/<i>aza</i>-Michael/aldol consendation cascade reaction is presented. The targeted optically active 1,2-oxazine derivatives are synthesized in moderate yields (up to 70%), excellent enantioselectivities (ee >99% in all cases), and excellent diastereoselectivities (dr up to >99:1) under mild conditions. To further elucidate the synthetic utility of the cascade products, cleavage of the N–O bond is demonstrated and an enantiopure <i>syn</i>-1,4-amino alcohol derivative is achieved in excellent yield

Modulating Viscoelasticity, Stiffness, and Degradation of Synthetic Cellular Niches via Stoichiometric Tuning of Covalent versus Dynamic Noncovalent Cross-Linking

Viscoelasticity, stiffness, and degradation of tissue matrices regulate cell behavior, yet predictive synergistic tuning of these properties in synthetic cellular niches remains elusive. We hypothesize that reversible physical cross-linking can be quantitatively introduced to synthetic hydrogels to accelerate stress relaxation and enhance network stiffness, while strategic placement of isolated labile linkages near cross-linking sites can predict hydrogel degradation, both of which are essential for creating adaptive cellular niches. To test these hypotheses, chondrocytes were encapsulated in hydrogels formed by biorthogonal covalent and noncovalent physical cross-linking of a pair of hydrophilic building blocks. The stiffer and more viscoelastic hydrogels with DBCO–DBCO physical cross-links facilitated proliferation and chondrogenic ECM deposition of encapsulated cells by dissipating stress imposed by expanding cell mass/ECM via dynamic disruption/reformation of physical cross-links. Degradation of labile linkages near covalent cross-linkers further facilitated cell proliferation and timed cell release while maintaining chondrogenic phenotype. This work presents new chemical tools for engineering permissive synthetic niches for cell encapsulation, 3D expansion, and release

Anionic and Zwitterionic Residues Modulate Stiffness of Photo-Cross-Linked Hydrogels and Cellular Behavior of Encapsulated Chondrocytes

Photo-cross-linked poly­(ethylene glycol) dimethacrylate (PEGDMA) hydrogels have been widely utilized for cartilage tissue engineering. However, strategies for improving their stiffness have been predominantly limited to increasing the degree of photo-cross-linking or weight fraction of the polymer. In this study, we tested the hypothesis that covalent incorporation of anionic sulfonate or zwitterionic sulfobetaine residues into photo-cross-linked PEGDMA hydrogels could enhance their mechanical properties without altering overall polymer content or swelling behavior. In addition, we investigated whether and how covalent incorporation of these chemical residues would affect cartilage extracellular matrix secretion by encapsulated chondrocytes. With the incorporation of 5–10% anionic or zwitterionic residues, the compressive moduli of PEGDMA hydrogels increased and the stress relaxation expedited while the swelling behavior and overall polymer fraction were kept the same. The incorporation of anionic residues exerted a more profound incorporation content-dependent impact on compressive moduli than zwitterionic residues. Higher-content incorporation of the anionic residue (10% vs 5%) also reduced the metabolic activity and type II collagen secretion by encapsulated murine chondrocytes and limited the pericellular diffusion of secreted proteoglycans within the 3D hydrogel. Although encapsulated human chondrocytes exhibited different sensitivity to serum level in culture than murine chondrocytes, the general trend of the impact of covalent incorporation of the chemical residues on their ECM secretion was the same. Overall, covalent incorporation of anionic and zwitterionic residues at an appropriate content presents a viable alternative to increasing the degree of photo-cross-linking for modulating the stiffness of PEGDMA hydrogels and the metabolism and phenotypical matrix secretion by encapsulated chondrocytes. It underscores the significance of noncovalent interactions imposed by charged residues in modulating biomechanical and cellular properties in tissue engineering scaffold designs

<i>F</i> values of a two-way ANOVA with Tukey’s HSD testing for the concentrations of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) as well as the DOC:TDN ratio (DOC/TDN) for the effects of the litter species, soil fauna and their interactions at each stage.

<p>* <i>P</i><0.05</p><p>** <i>P</i><0.01</p><p>*** <i>P</i><0.001.</p><p><i>F</i> values of a two-way ANOVA with Tukey’s HSD testing for the concentrations of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) as well as the DOC:TDN ratio (DOC/TDN) for the effects of the litter species, soil fauna and their interactions at each stage.</p

The concentration of dissolved organic carbon in (a) cypress, (b) willow, (c) wormwood and (d) sedge foliar litter in the soil fauna (SF) and no soil fauna (NSF) treatments at different stages in the alpine forest and the alpine meadow.

<p>* <i>P</i><0.05 and ** <i>P</i><0.01 tested by pairwise t-test.</p

The ratios of dissolved organic carbon and total dissolved nitrogen in (a) cypress, (b) willow, (c) wormwood and (d) sedge foliar litter in the soil fauna (SF) and no soil fauna (NSF) treatments at different stages in the alpine forest and the alpine meadow.

<p>* <i>P</i><0.05 tested by pairwise t-test.</p

Results of repeated measures ANOVA of time, litter species and soil fauna on the concentrations of dissolved organic carbon (DOC) and total dissolved nitrogen (TDN) as well as the DOC:TDN ratio (DOC/TDN) over time.

<p>* <i>P</i><0.05</p><p>** <i>P</i><0.01</p><p>*** <i>P</i><0.001.</p><p>The interaction terms are nested in parentheses.</p

Initial concentration of total carbon (TC), total nitrogen (TN), total phosphorus (TP), cellulose and lignin of four litter types (mean ± SE, <i>n</i> = 3).

<p>Different lowercase letters indicate significant differences among species (<i>p</i><0.05).</p><p>Initial concentration of total carbon (TC), total nitrogen (TN), total phosphorus (TP), cellulose and lignin of four litter types (mean ± SE, <i>n</i> = 3).</p

The mean temperature (MT), positive (PAT) and negative accumulated temperature (NAT), and number of freeze-thaw cycles (NFTC) in the air and litterbags during the various stages in the alpine forest and the alpine meadow.

<p>The mean temperature (MT), positive (PAT) and negative accumulated temperature (NAT), and number of freeze-thaw cycles (NFTC) in the air and litterbags during the various stages in the alpine forest and the alpine meadow.</p

The concentration of total dissolved nitrogen in (a) cypress, (b) willow, (c) wormwood and (d) sedge foliar litter in the soil fauna (SF) and no soil fauna (NSF) treatments at different stages in the alpine forest and the alpine meadow.

<p>* <i>P</i><0.05 and ** <i>P</i><0.01 tested by pairwise t-test.</p