Oxygen benzylic, removal

Hydrogenolysis (s. a. Hydrocarbons, Oxygen, benzylic, removal. Replacement by hydrogen)... 

The large sulfur atom is a preferred reaction site in synthetic intermediates to introduce chirality into a carbon compound. Thermal equilibrations of chiral sulfoxides are slow, and parbanions with lithium or sodium as counterions on a chiral carbon atom adjacent to a sulfoxide group maintain their chirality. The benzylic proton of chiral sulfoxides is removed stereoselectively by strong bases. The largest groups prefer the anti conformation, e.g. phenyl and oxygen in the first example, phenyl and rert-butyl in the second. Deprotonation occurs at the methylene group on the least hindered site adjacent to the unshared electron pair of the sulfur atom (R.R. Fraser, 1972 F. Montanari, 1975). 

Again, as with pyridopyrimidines, the main reaction is oxidation of di- or poly-hydro derivatives to fully aromatic structures, often merely by air or oxygen. In some cases the reagent of choice is mercury(II) oxide, whilst other reagents used include sulfur, bromine, chloranil, chromium trioxide-acetic acid, hydrogen peroxide, and potassium ferricyanide, which also caused oxidative removal of a benzyl group in the transformation (306) (307)... 

You will note that the oxygen atoms attached to carbons 5 and 12 in 43 reside in proximity to the C-9 ketone carbonyl. Under sufficiently acidic conditions, it is conceivable that removal of the triethylsilyl protecting groups would be attended by a thermodynamically controlled spiroketalization reaction.30 Indeed, after hydro-genolysis of the C-26 benzyl ether in 43, subjection of the organic residue to the action of para-toluenesulfonic acid in a mixture of methylene chloride, ether, and water accomplishes the desired processes outlined above and provides monensin methyl ester. Finally, saponification of the methyl ester with aqueous sodium hydroxide in methanol furnishes the sodium salt of (+)-monensin [(+)-1], Still s elegant synthesis of monensin is now complete.13... 

Benzylic or allylic oxygen functions react with Lewis acids such as trifluoroacetic acid to generate allyl or benzylic cations which abstract a hydride from silanes such as triethylsilane 84 b to result in the removal of the oxygen function in a process which has been called ionic hydrogenation and which has been reviewed [34-38]. 

The removal of a carbobenzyloxy group can be separated into two steps (Figure 1). The first step comprises the hydrogenolysis of the benzyl oxygen bond of the Cbz-protected amino acid 1 to form a carbamic acid intermediate 2 and toluene 3. The carbamic acid intermediate decaiboxylates to give the deprotected amino acid 4 and one equivalent of carbon dioxide 5. 

The benzyl—nitrogen bond is not so easily cleaved as the benzyl—oxygen bond, unless the O-benzyl group is sterically hindered. This difference in activity allows the selective removal of the O-benzyl function in a molecule containing both N-benzyl and O-benzyl protecting groups. The selectivity can be reversed if the amine is protected by the Cbz group. If a small amount of amine (e g., butylamine) is added, then the selective removal of N-benzyl amines can be achieved.292... 

Quantitative determination of the products from Haworth methyla-tion of benzyl 4-0-methyl-/3-D-xylopyranoside gave277 the ratio of rate constants k2 k3 as 3.2 1. Satisfactory agreement between predicted and observed product-ratios was found if it was assumed that, after methylation of HO-2, the reactivity of HO-3 increases by a factor of 3, but that methylation of HO-3 does not alter the reactivity of HO-2. The greater reactivity at HO-2 is, presumably, a result of its greater acidity, resulting from the inductive effect of two acetal oxygen atoms on C-l. When this group is ionized, the acidity of HO-3 should be decreased, but methylation at HO-2 removes the effect. Methylation at HO-3 should not, however, similarly affect HO-2. 

FIGURE 8.7 Synthesis of a protected tripeptide containing a 2-hydroxy-4-methoxybenzyl-protected peptide bond.38 (A) Acylation of the carboxy-terminal residue, (B) removal of both protecting groups, (C) O-acylation of the benzyl-protector by the symmetrical anhydride of the amino-terminal residue, and (D) migration of the protected amino-terminal residue from the oxygen atom to the amino group of the dipeptide ester. 

Figure SJ Activity of the various states of the [NiFe] hydrogenase from A. vinosum as determined with a Pt electrode at 30°C.The reaction was performed in SOmM Tris/HCI (pH 8.0) in a volume of 2 ml. Oxygen was scavenged by adding glucose (90 mM) and glucose oxidase (2.5 mg/ml). Hydrogen peroxide was removed by catalase. When the system was anaerobic, an aliquot of H2-saturated water was added, and a little later enzyme (S-IOnM) was injected. Benzyl viologen (4.2mM) was used as electron acceptor.

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