Acetobromoglucose synthesis

D-glucopyranosyl esters of abscisic acid and /3-ionylideneacetic acid (120) have been prepared by treatment of the acid with a-acetobromoglucose in the presence of triethylamine/ Two papers " describe a new synthesis of y-ionone (64) in which the key intermediate (121) is prepared from y-cyclocitral (122) and acetone in the presence of BU2BSO3CH2CF3. 

When acetobromoglucose was dissolved in ether, and treated with an aqueous, alkaline solution of benzenethiol, phenyl 1-thioglucoside was obtained acetobromolactose was also utilized successfully in this type of synthesis. 

The reactions involved in the glycal synthesis have been studied in considerable detail, especially in the conversion of arabinose into 2-des-oxyribose. For this particular conversion the overall yield has been doubled by recently introduced improvements but it is still very low.112 The reduction of the acetobromoaldose to the acetylated glycal by zinc dust and acetic acid was found to proceed in better yield if a few drops of chloroplatinic acid were added at intervals to maintain a vigorous reaction.112 120 127 Moreover, the reaction could then be conducted at lower temperatures (i. e., — 5° to —10°). This was particularly the case in the pentose series, and the simultaneous formation of the pentose tetraacetate by replacement of the bromo group by an acetyl residue, was much reduced.60 112 Hughes148 demonstrated that the maximum yield of triacetylglucal from acetobromoglucose was obtained when the addition of zinc dust and catalytic amounts of chloroplatinic acid was spread over several hours and the reaction mixture was maintained at 0°. 

The other type of radical chemistry of importance in the carbohydrate field is one-electron reductions. A handful of these reactions (such as the metallic Zn reduction of acetobromoglucose to triacetylglucal) have been used in synthesis for decades, but, starting with the Barton-McCombie deoxygenation of sugars in the mid-1970s there has been an explosion of interest, as increasingly sophisticated cascades of elementary radical steps have been devised. Such reactions are driven by the homolysis of weak bonds such as Sn-H or N-O under conditions of photolysis or mild thermolysis. Nature uses a similar basic principle in Type II ribonucleotide reductases, where the weak bond in question is the cobalt-carbon a bond in the corrin cofactor. ... 

The synthesis of C-glycosides may be achieved by B 2-catalyzed C-C bond formation. Thus, 3-(2,3,4,6-tetra-0-acetyl-a-D-glucosyl)propionitrile (13) can be prepared from acetobromoglucose (12) by reduction with Zn in DMF in the presence of acrylonitrile and 3 mol% hydroxocobalamin hydrochloride. Similarly, the ribo-furanosyl derivative 15 can be prepared from the corresponding acetobromofur-anose 14 (Scheme 5). 

TaxicatFn. 3,5-Dimethoxyphenol glucoside. CH-HjqOj mol Wt 316.30. C 53.16%, H 6,37%. O 40.477c. Isojn from leaves of Tax us baccata L Taxaceae Lefebvre, Arch. Pharm. 245, 486 (1907) Merz, Preuss, ibid. 279, 134 (1941). Structure and synthesis from 3,5-dimethoxyphenol and a-acetobromoglucose Merz, Preuss, foe. cit. 

Some carbohydrate acetylenic alcohols have been converted to metal carbene spirocyclic derivatives, e.g. 22 (Scheme 4) Acetobromoglucose has been converted into tri-O-acetyl-D-glucal in 90% yield by way of a glycosyl chromium-(III) complex, and an organomercury intermediate featured in a synthesis of tri-O-benzyl-2-C-methyl-D-glucal covered in Chapter 14. 

An efficient and stereoselective synthesis of 3,4,6-tri-O-acetyl-ot-D-glucopyranose 1,2-exo-alkyl ortho-acetates (67) has been achieved using DMF dialkyl acetals (68) and tetrabutyl ammonium bromide on acetobromoglucose. The DMF acetal from 1,2 3,4-di-0-isopropylidene bC-D-galactopyranose (69) was also prepared and used to synthesize the mixed sugar orthoester (70). 

An improved synthesis of "tempol" 4 involved the use of acetobromoglucose with silver triflate as activator in dichloromethane at -20°C. ... 

---