A protocol for stereoselective C-radical addition to a chiral glyoxylate-derived N-sulfinyl imine was developed through visible light-promoted photoredox catalysis, providing a convenient method for the synthesis of unnatural α-amino acids....
Highly regioselective acetylation of primary hydroxy groups in thioglycoside derivatives was achieved by treatment with aqueous or anhydrous acetic acid (60–100%) at elevated temperatures (80–118 °C), avoiding manipulations with protective groups.
In the present work, the synthesis
of acetylated salicins, which
occur naturally in many Salicaceae species,
is reported. The preparation of 2-O-acetylsalicin,
2-O-acetylchlorosalicin, and 2-O-acetylethylsalicin from peracetylated bromosalicin with selective
acid-catalyzed deacetylation and one-pot nucleophilic substitution
of bromine as the key steps is described. The base-catalyzed O-2 →
O-6 acetyl migration afforded 6-O-acetylsalicin derivatives
in good yields. Thus, the first synthesis of 6-O-acetylsalicin
(fragilin) using acetyl group migration is reported as well as the
synthesis of 6-O-acetylchlorosalicin and 6-O-acetylethylsalicin. The NaOMe-catalyzed deacetylation
of acetylated glycosides gave salicin, chlorosalicin, and ethylsalicin
recently reported from Alangium chinense.
Phase transfer catalysis (PTC) is widely used for the preparation of anomerically pure aryl glycosides by glycosylation of phenols with glycosyl halides. Here, we attempted to synthesize 4‐(3‐chloropropoxy)phenyl (CPP) sialoside – a useful Janus glycoside for neoglycoconjugate synthesis. A batch glycosylation reaction under typical PTC conditions (N‐acetylsialyl chloride, 4‐(3‐chloropropoxy)phenol, Bu4NHSO4, AcOEt, 10 % aq Na2CO3) resulted in formation of CPP sialoside as a single anomer, albeit in low yield. Conversely, the PTC glycosylation in flow using Comet X‐01 micromixer doubled the yield of CPP sialoside, which was obtained as a mixture of anomers suggesting the change in the reaction mechanism apparently induced by the change in mixing mode. An increase in flow rate led to further loss of stereoselectivity while the use of T‐shaped mixer increased stereoselectivity. Similar results were obtained for N,N‐diacetylsialyl chloride and two other para‐alkoxy‐substituted phenols suggesting generality of the discovered phenomenon.
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