The crystal structure of alpha-L-sorbose

Mini-ISES identifies promising carbafructopyranose-based salens for asymmetric catalysis: Tuning ligand shape via the anomeric effect

This study introduces new methods of screening for and tuning chiral space and in so doing identifies a promising set of chiral ligands for asymmetric synthesis. The carbafructopyranosyl-1,2-diamine(s) and salens constructed therefrom are particularly compelling. It is shown that by removing the native anomeric effect in this ligand family, one can tune chiral ligand shape and improve chiral bias. This concept is demonstrated by a combination of (i) x-ray crystallographic structure determination, (ii) assessment of catalytic performance, and (iii) consideration of the anomeric effect and its underlying dipolar basis. The title ligands were identified by a new mini version of the in situ enzymatic screening (ISES) procedure through which catalyst-ligand combinations are screened in parallel, and information on relative rate and enantioselectivity is obtained in real time, without the need to quench reactions or draw aliquots. Mini-ISES brings the technique into the nanomole regime (200 to 350 nmol catalyst/20 μml organic volume) commensurate with emerging trends in reaction development/process chemistry. The best-performing β-d-carbafructopyranosyl-1,2-diamine-derived salen ligand discovered here outperforms the best known organometallic and enzymatic catalysts for the hydrolytic kinetic resolution of 3-phenylpropylene oxide, one of several substrates examined for which the ligand is "matched." This ligand scaffold defines a new swath of chiral space, and anomeric effect tunability defines a new concept in shaping that chiral space. Both this ligand set and the anomeric shape-tuning concept are expected to find broad application, given the value of chiral 1,2-diamines and salens constructed from these in asymmetric catalysis.

Crystal structure of beta-D-psicopyranose

Here, we report the crystal structure of D-psicose, C(6)H(12)O(6), one of the rare sugars. The compound crystallizes as the beta-anomer with rarely observed in pyranose carbohydrate structures trans-gauche orientation of the hydroxymethyl group relative to the pyranosyl ring. The crystal system is orthorhombic, space group P2(1)2(1)2(1), Z=4, with cell dimensions a=7.727(2), b=8.672(2), c=11.123(3)A, V=745.3(3)A(3). The pyranosyl ring adopts chair (2)C(5) conformation. The crystal structure at 100(2)K is stabilized by three-dimensional network of O-H...O and C-H...O intermolecular hydrogen bonds.

ENDOR-Assisted Study of the Stable EPR Spectrum of X-Irradiated α-𝗟-Sorbose Single Crystals: MLCFA and Simulation Decomposition Analyses

After X irradiation of single crystals of alpha-L-sorbose at 295 K, previous electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and ENDOR-induced EPR (EI-EPR) results have indicated the formation of at least 10 different free radicals, and also that conceivably each carbon in the pyranose ring is a possible radical center. The radicals appear to be formed mostly by net H-abstraction reactions followed by standard elimination (e.g. beta-OH elimination) reactions or proton shifts, in turn leading to ring opening and fragmentation. In the present work, EPR spectra were recorded at room temperature with the external magnetic field along each of the three crystallographic axes subsequent to careful annealing at different temperatures using a high-temperature cavity. Each of the three sets of spectra was subjected to a maximum likelihood common factor analysis (MLCFA) that contributed to a better understanding of the spectral decays. Furthermore, the most stable spectra were simulated by optimization of previous ENDOR and EI-EPR results. The optimized EPR parameters resulted in excellent simulations of the experimental stable sorbose spectra and hence provided an improved insight of their spectral compositions.