Dibenzyl acetals

Dibenzyl Acetals and Ketals R2C(OCH2Ph)2 Fonnation/Cleayage ... 

The use of benzyl alcohol as solvent produces the dibenzyl acetal at C-21, and this can be reductively removed to produce the 21-hydroxy compound... 

S,S -Dipropyl Acetals and Ketals RR C(SC3H7)2 S,S -Dibutyl Acetals and Ketals RR C(SC4H9)2 S,S -Dipentyl Acetals and Ketals RR C(SC5H i, )2 S,S -Diphenyl Acetals and Ketals RR C(SC6H )2 S,S -Dibenzyl Acetals and Ketals RR C(SCH2C6H5)2... 

Oxabicyclo[2.2.1]hept-5-cn-2-one derivatives (ketals, cyanohydrins) were prepared in an optically pure form, hence optically pure -functionalized amines can be prepared via the diastereoselective cycloaddition of organic azides. Such transformations have been described for ethoxycarbonyl azide and /ert-butoxycarbonyl azide54, 5. For example 3-amino-3-deoxy-a-altropyranoside hydrochloride 20 was prepared from the dibenzyl acetal 19ss. 

Bemytation of nucleosides Uridine (1) is converted into N -benzyluridine (2) in quantitative yield when heated with DMF dibenzyl acetal in DMF for 3 hrs. at 80°. This derivative is obtained in only low yield using benzyl bromide and sodium hydride. The blocking group of (2) is removed by sodium naph-thalenide (THF, 3 hr., 84% yield). Guanosine (3) has also been blocked in the same way. 

Stereoselective reactions with acetals. Noyori et al. (10,438) have used this Lewis acid to promote an aldol-type reaction between enol silyl ethers and acetals and have noted high. syn-selectivity in this process. Molander and Haar report that reaction of acetals with cyanotrimethylsilane promoted by TMSOTf results in a-alkoxy cyanides and that this reaction can be diastereoselective when the acetal is substituted at the 4-position by an alkoxy group. The diastereoselectivity depends on the nature of the acetal and the 4-alkoxy group. Dimethoxy acetals show slight diastereoselectivity, but diisopropoxy and dibenzyl acetals can show diastereoselectivity of 5-10 1. The diastereoselectivity also depends on the type of 4-substituent. Acetoxy and t-butyldimethylsilyloxy groups have no effect on the diastereoselectivity, but methoxy, benzyloxy, and allyloxy groups promote anri-selectivity. Since a metal template is not involved, the diastereoselectivity... 

Treatment of methyl and -D-glucopyranoside with two moles of 2-methoxypropene in DMF gave the expected 4 6-0-isopropylidene derivatives on reaction with a further 2.5 moles of the reagent, a 73% yield of the 2,3 4 6-di-0-isopropylidene-glucoside was obtained. Tha yields of the diacetal were lower when the glycoside was treated directly with excess 2-methoxypropene. A mixture of the dibenzyl acetals of 2,3 5,6- and 3 4s5,6-di-0-isopropylidene-D-glucose was... 

Tri-O-benzyl-D-arabinofuranose stirred at 0° with a mixture of ethanethiol and anhydrous Mg-sulfate satd. with HGl, and the product treated with benzoyl chloride in pyridine 4-0-benzoyl-2,3,5-tri-0-benzyl-D-arabinose diethyl di-thioacetal (Y 94%) added with Drierite to henzyl alcohol, stirred 1 hr. after addition of Cd-carbonate and HgClg the product isolated after 2 hrs. -> 4-0-benzoyl-2,3,5-tri-0-henzyl-D-arabinose dibenzyl acetal (Y 80%) stirred 16 hrs. under reflux at 56-58° with BaO, Drierite, methyl iodide, and dimethylformamide, the resulting crude 2,3,5-tri-0-henzyl-4-0-methyl-D-arabinose dibenzyl acetal hydrogenated ca. 2.5 hrs. with palladous chloride in methanol, and the product refluxed 1.5 hrs. in 0.5 N HGl to hydrolyze the methyl glycoside present 4-0-methyl-y -D-arabinopyranose (Y 64%). — This pathway is not dependent on the stereochemistry of the aldose. H. G. Fletcher, Jr., and H. W. Diehl, J. Org. Ghem. 30, 2321 (1965). 

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