Benzylation selective

The primary alcohol below was selectively benzylated using NaH and BnBr at -70°. ... 

XfjT Conversion m-nitro toluene Conversion bromine Selectivity benzyl bromide Selectivity benzal bromide Selectivity side product... 

Bisacid 91 was used toward three different targets. For the first, a palladium-catalyzed decarboxylative Heck reaction followed by perylenequinone formation provided bis-styryl derivative 92 (Scheme 7.22) [52]. For the second, the C5,C5 -benzyl ethers were cleaved, and the more acidic carboxylic acids were then selectively benzylated using BnBr and K2CO3 (Scheme 7.22). This re-esterification... 

Methyl a-D-glucopyranoside may be converted,271 in a remarkably selective reaction, into its 2,4,6-tribenzyl ether (in 62% yield) on treatment with three molar equivalents of sodium hydride in benzyl chloride at 110°, a substitution pattern that might have been predicted in view of the low, relative reactivity of HO-3 towards alkylation in the mechanistically related, Haworth procedure.268 Similar, selective benzylations have also been achieved on partially substituted derivatives of methyl a- and /3-D-galactopyranoside272 and on methyl 6-deoxy-a-L-galactopyranoside62 in all of these, an unexpectedly high relative-reactivity of HO-4 (ax) compared to that of HO-3 (eq) was noted, indicating that steric factors are not the sole influence on reactivity in these cases. Nevertheless, the primary hydroxyl... 

The difference in the acidities of the indolyl NH group and the amidic NH groups allows for selective benzylation at the 1-position of ALacetyltryptamines (Scheme... 

Figure 9.(A) Reaction scheme for the selective benzyl alcohol oxidation without any over-oxidation to benzoic acid. (B) Plot of % of conversion over the reaction time for the benzyl alcohol oxidation reaction catalyzed by NPyc at various recyclic conditions with 30%H2O2. (C) The reaction step up picture.

As the precursor for the B unit, the selectively benzylated -fucal derivative 75- could be obtained crystalline from L-fucal by applying a phase-transfer-catalyzed process T64). By a Ferrier reaction of di-O-acetyl-L-rhamnal and a subsequent retro enol ether formation trough hydride attack at C-3 (71.) the -amicetal 72 was obtained (64) this reaction was concurrently also es-cribed by others (72). 

Sheldon and coworkers have developed chromium-substituted molecular sieves (CrAPO-5) as recyclable solid catalysts for several selective oxidations, among them also the allylic" and benzylic ° " ° " ° oxidations using TBHP or O2 as the terminal oxidants (equation 63), which yielded the corresponding benzylic ketones in moderate yield (conv. 13-70%) and moderate to good selectivity (41%, 65-97%). The benzylic alcohols were formed as side products. Allylic oxidation also proceeded with good conversions, while selectivities were lower and both possible products, the allylic ketone (31-77% selectivity) and the allylic alcohol (0-47% selectivity), were formed. Chromium sUicalite showed activity for selective benzylic oxidation in the presence of TBHP as well as giving mainly the allylic ketone (2-cyclohexen-l-one with 74% selectivity) and the allylic alcohol as minor product (2-cyclohexen-l-ol with 26% selectivity) -. ... 

The 5-methyl ether of V-acetylmannosamine cannot be prepared by this route. The D-glucofuranose 42 was obtained from the 5,6-diol by selective benzylation by way of the dibutyltin derivative, followed by conventional methylation. It was converted into the benzyl glycoside 43 with benzyl alcohol under acidic conditions. Conversion into the amine 44 and amide 45 followed the same path as in the pyranose series. Hydrogenolysis gave20 28 (see Scheme 12). 

A compound prepared and first described as nicotinium dichromate (NDC) by Palomo et al.,379 was later shown by X-ray-crystal analysis380 to be a betainic mixed anhydride of nicotinic and chromic acid (NACAA). Because of its unique structure, it deserves a close scrutiny of its oxidative properties.381 Replacement of the chloride anion in the quaternary ammonium resin, Dowex 1-X8, for the dichromate anion, leads to a polymer supported dichromate, which is able to make selective benzylic oxidations.382 Finally, poly[vinyl(pyridinium dichromate)] (PVPDC), a polymeric analogue of PDC, must be mentioned whose use in the oxidation of alcohols allows for a very easy work-up.383... 

The primary hydroxyl group of various 3-O-protected iV-acetyl-glucosaminides (1) has been selectively benzylated by stirring the carbohydrate with approx. 1 molar equiv. of benzyl bromide, excess barium oxide and barium hydroxide octahydrate in... 

Excess benzyl bromide and triethylamine in the presence of tin(II)chloride catalyst were used [91] for partial benzylation of methyl a-L-rhamnopyranoside and its 4-O-benzyl ether. Complexation of tin(II)chloride with vicinal m-disposed hydroxyl groups permitted selective benzylation at the 3- and 2-positions. Methyl 3-O-benzyl-or 3,4-di-O-benzyl-a-L-rhamnopyranoside, respectively, could be prepared in good yield by this method. No benzylation took place when benzyl chloride was used instead of benzyl bromide, or when other solvents than ethyl acetate or acetonitrile were tested. 

After the nucleophilic attack of the bromide anion on the Sn atoms of the tributylstannyl ether 36 you get oxyanions at C-3 and C-6 as the reactive species. Now selective benzylation can proceed and the alcohol at C-4 is only benzylated as a by-product in low yield. 

G. Uncatalyzed thermal ortho-alkylation of phenols./. Org. Chem. 1958, 23, 1871-1876. Bram, G. Loupy, A. Sansoulet, J. Vaziri-Zand, F. Highly selective benzylations of /j-riaphthoxide anion in heterogeneous media. Tetrahedron Lett. 1984, 25, 5035-5038. 

Accordingly, 14 was converted under standard dihydroxylation conditions to hemiketal 45 in 80% yield. Selective benzylation of 45 provided benzyl ether 46, which was transformed to the diketone 47, over two steps ketone reduction, and subsequent reoxidation of the resultant diol. Treatment of 47 under our standard reductive amination conditions (1.2 equivalents NH4HCO2, NaCNBH3, dry MeOH, 4A MS, 2h) provided the aza-p-C-galactoside 12 in 72% yield. The structure of 12 was confirmed by HCOSY, 13CNMR and MS analysis. No diastereomeric products were observed within the limits of NMR detection (Scheme 7). 

The reaction of HBpin in toluene in the presence of RhCl P(/-Pr)3 2(N2) (1 mol%) at 140 °C resulted in a mixture of (borylmethyl)benzene (69%) and bis(boryl)methyl benzene (7%), along with several products arising from aromatic C-H borylation (ca. 15%).345 Rhodium-bpy complexes catalyzed the borylation at the benzylic C-H bond.351 Pd/C was found to be a unique catalyst for selective benzylic C-H borylation of alkylbenzenes by B2pin2 or HBpin (Equation (70)).360 Toluene, xylenes, and mesitylene were all viable substrates however, the reaction can be strongly retarded by the presence of heteroatom functionalities such as MeO and F. Ethylbenzene resulted in a 3 1 mixture of pinacol 1-phenylethylboron and 2-phenylethylboron derivatives. 

Efficient syntheses of trehazolamine (5) starting from D-mannose (Scheme 15)79 have been readily achieved by deacetylation of phenyl 1-thio-a-D-mannopyranoside (109),80 followed by monoacetonation under kinetic conditions to give 110. Regio-selective benzylation of the equatorial hydroxyl group of 110 gave 111, whose... 

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