In Situ Switching of Site-Selectivity with Light in the Acetylation of Sugars with Azopeptide Catalysts

Abstract

We present a novel concept for the in situ control of site-selectivity of catalytic acetylations of partially protected sugars using light as external stimulus and oligopeptide catalysts equipped with an azobenzene moiety. The isomerizable azobenzene-peptide backbone defines the size and shape of the catalytic pocket, while the π-methyl-l-histidine (Pmh) moiety transfers the electrophile. Photoisomerization of the E- to the Z-azobenzene catalyst (monitored via NMR) with an LED (λ = 365 nm) drastically changes the chemical environment around the catalytically active Pmh moiety, so that the light-induced change in the catalyst shape alters site-selectivity. As a proof of principle, we employed (4,6-O-benzylidene)­methyl-α-d-pyranosides, which provide a change in regioselectivity from 2:1 (E) to 1:5 (Z) for the monoacetylated products at room temperature. The validity of this new catalyst-design concept is further demonstrated with the regioselective acetylation of the natural product quercetin. In situ irradiation NMR spectroscopy was used to quantify photostationary states under continuous irradiation with UV light.

Light-Controlled Lyotropic Liquid Crystallinity of Polyaspartates

“…Sample homogeneity was established by centrifuging the sample back and forth while heating the sample in between each centrifugation step. Prior to the first NMR measurements, the samples were irradiated by two violet LEDs (from opposite sides) inside an aluminium box 44 resulting in pss-E. To change the E/Z ratio, the samples were irradiated with a green LED or two violet LEDs either homogeneously or with spatial control (detailed sample history and irradiation times are given in the supporting information). Homogeneously irradiated samples were further centrifuged back and forth again after irradiation to enhance sample homogeneity.…”

Azo-dimethylaminopyridine-functionalized Ni(II)-porphyrin as a photoswitchable nucleophilic catalyst

“…Calculated rate constants are based on the assumption of complete isomerization and no intermolecular coordination. The concentration of the catalyst is given in mol % (in parentheses) with respect to the initial nitrobenzaldehyde (6) because of the electron-withdrawing effect of the azo group. Surprisingly, however, the catalytic effect drops by a factor of 54 (4 mM pure trans-1 vs trans-4) or by a factor of 18 (40 mM pure trans-1 vs trans-4), if the azopyridine unit of trans-4 is attached to the porphyrin core as realized in trans-1.…”

“…Diarylethenes were reported in the context of photoswitchable catalysis as inhibitors of the Karstedt´s catalyst [ 3 ] and for p K a modulation in acid–base-controlled processes [ 4 ]. Molecular motors for stereodivergent anion binding catalysis [ 5 ], azopeptides for the acetylation of sugars [ 6 ], enzyme mimics [ 7 ], and the utilization of intermolecular cooperative effects [ 8 ] are further applications of photoswitchable catalysis.…”

“…Conversion of 4-nitrobenzaldehyde(6) in the Henry reaction with nitroethane (5) as a function of time, using (a, left) 4 mM concentration of catalyst or reference compound (1 mol % compared to 4-nitrobenzaldehyde) or 40 mM catalyst (10 mol % compared to 4-nitrobenzaldehyde) (b, right). Rate constants (k tot ) are given for DMAP (2) (black dots), azopyridine trans-4 (orange), photoswitchable porphyrin 1 at PSS 435nm (predominantly trans-1, blue), and 1 at PSS 530nm (predominantly cis-1, green).…”

Azo-dimethylaminopyridine functionalized Ni(II)-Porphyrin as a Photoswitchable Nucleophilic Catalyst

“…Diarylethenes were reported in the context of photoswitchable catalysis as inhibitors of the Karstedt´s catalyst [3] and for pKa modulation in acid-base controlled processes [4]. Molecular motors for stereodivergent anion binding catalysis [5], azopeptides for the acetylation of sugars [6], enzyme mimics [7] and the utilization of intermolecular cooperative effects [8] are further applications of photoswitchable catalysis.…”