Stereoselective hydroxylation of glycals

Stereoselective Tandem Epoxidation–Alcoholysis/Hydrolysis of Glycals with Molybdenum Catalysts

“…When the reaction was carried out in water with readily available MoO 3 as a catalyst precursor and H 2 O 2 as stoichiometric oxidant, mannose (2b) was almost exclusively obtained, in agreement with that reported by Bilik [23] et al (Table 1, entry 1). Note that in no case was the epoxide obtained, as a consequence of the high reactivity of the anhydro sugar in the presence of nucleophilic solvents.…”

“…[27] The theoretically predicted energies of the transition states and the calculated intrinsic reaction coordinates show that the oxygen transfer takes place in a single-step reaction via a direct attack of the alkene to the peroxo group as proposed by Sharpless, [28] thus ruling out Mimouns metalla-2,3-dioxolane model. [26] We initially evaluated the epoxidation of unprotected glucal (1) under similar conditions to those previously explored, [23] using two molybdenum catalysts in polar solvents such as water and methanol and using different oxidizing reagents. The results are collected in Table 1.…”

“…Interestingly, the dihydroxylation (epoxidation/hydrolysis) of unprotected glucal using MoO 3 as a catalyst in water as a solvent affords mainly mannose, in a process that involves directed b-epoxidation, probably via a hydrogen bond between the neighbouring allylic hydroxy group and the catalyst. [23] Only Venturellos peroxotungstate (PW 4 O 24 3À ) [19] provided a similar selectivity in unprotected glycals.…”

Tandem Epoxidation‐Alcoholysis or Epoxidation‐Hydrolysis of Glycals Catalyzed by Titanium(IV) Isopropoxide or Venturello’s Phosphotungstate Complex

“…Although these diols are synthetically very useful compounds (vide supra, Introduction), only a few methods for their selective preparation have been reported [6b,d,12-14] . The existing methods for direct dihydroxylation of glycals all have drawbacks such as the removal of Os salts [12][13][14] problems in scaleup in the case of DMDO, [13] formation of C-2 epimers or use of high mol% of metal. [14] In a first approach, the same reaction conditions as for the incorporation of long-chain alcohols were used, but the nucleophile was replaced by an excess of water.…”

“…In one of the earliest reports on the matter, Bilik et al [12] used a range of metal oxides among which MoO 3 gave the best results. Other methods include the use of oxone-acetone, [13] OsO 4 -NMO [14] and the bimetallic system RuCl 3 /CeCl 3 /NaIO 4 .…”