Zinc-silver graphite

A number of enantiomerically pure chiral building-blocks, such as 292-294, have been prepared (270,271) by zinc-copper cleavage of 5-bromo-5-deoxy-2,3-0-isopropylidene-D-ribono-1,4-lactone, followed by reduction. Similarly, from the 5-iodo lactone analogue the enoic acid 295 was obtained by reaction with zinc/silver-graphite (272). 

Etiolates Type Anions. The well-known Reformatsky reaction allows the introduction of a functionalized two-carbon chain. Modified experimental procedures have now been proposed, such as the reaction of ethylbromoacetate with ketosugar 18 (Scheme 10) in the presence of zinc/silver graphite prepared from CgK, giving 19 in excellent yield [32]. 

Zinc/silver-graphite. A laminated Zn/Ag-C8 can be prepared by reaction of C8K with ZnCl2/AgOAc in THF. 

Silver zeolites, 296 Zinc-Silver-Graphite, 348 Thallium Compounds... 

Zinc iodide, 88, 112, 280, 349, 350 Zinc-Silver-Graphite, 348 Zirconium Compounds Chlorobis(cyclopentadienyl)hydrido-zirconium(IV)-Magnesium, 351 Chlorocrotylbis(cyclopentadienyl)-zirconium, 46... 

In parallel fashion, compound 267, made from 2,3 5,6-di-<9-isopropyl-idene-D-mannono-1,4-lactone by Reformatsky addition using ethyl bromo-acetate and zinc-silver graphite, undergoes ready elimination when converted to the methanesulfonic ester to give a separable mixture of the E- and Z-alkenes (268).251 Quite a different approach involves treatment of the 1-C-nitromannofuranosyl chloride, derived from the oxime of 2,3 5,6-di-O-isopropylidene-D-mannose, with the anion of diethyl malonate to give the disubstituted alkene 269.252... 

The favored formation of acetylated l-a-deuterio-l,5-anhydro-D-glucitol 78 in labeling experiments has been put forward to support the occurrence of carbohydrate radicals as intermediates in the photolytic decomposition of glucopyranosyl phenyl sulfone acetates104 as well as the Cp2TiBD4193 and zinc-silver/graphite-197 mediated reduction of glycosyl halides. Cationic intermediates would have favored the opposite stereochemistry 26... 

Reformatsky reactions can be carried out using a zinc/silver-graphite, prepared from CgK, ZnC and AgOAc, at temperatures as low... 

Other means of forming biaryl derivatives include the use of zinc/silver-graphite reaction with aryl and heteroaryl (including thiophenyl) iodides... 

Zinc-Silver-Graphite reacts with ethyl dihaloacetates at low temperatures in a very efficient Reformatsky-type reaction to form a-halo-jS-hydroxy alkanoa-tes [117] ... 

Reaction of methyl 4,6-0-benzylidene-2-deoxy-a-D-e /Aro-hexos-3-ulopyranoside with (Z)-ethyl-2-bromomethyI-2-alkenoates 14, in the presence of zinc-silver graphite, produce spiro-compounds 15. 

A paper describing the methylenation of aldonolactones using dicyclopentadienyldimethyl titanium as a more stable alternative to the Tebbe reagent has been presented (Scheme 9). Also described is the use of the zinc-silver/graphite reagent for effecting a Reformatsky-like reaction on aldonolactones with ethyl bromoacetate... 

The reductive dealkoxyhalogenation of some 6-iodopyranoses and 5-iodofuranoses using zinc-silver/graphite complex has been described (Scheme 19). In the latter series some alternative reactions are observed (see also Vol. 24, p.l60). 

Reactions of a-sUylalkyl halides with carbonyl compounds using activated zinc have been reported. Reactions of ethyl bromo(trimethylsilyl)acetate with aldehydes and ketones in the presence of zinc/silver-graphite rapidly afford the corresponding a,j8-unsaturated esters (Scheme 2.32) [87]. In this case, the Reformatsky-Peterson reaction sequence proceeds under the mild conditions. 

The original procedure using acid-washed zinc powder in aqueous alcohols at reflux is still the most used protocol, but some other metallic sources were also employed in the past few years [19,20], For example, the use of more reactive zinc sources, as zinc-Rieke [21] or zinc-silver graphite [22,23], allowed the fragmentation to be carried out in tetrahydrofuran (THF) at room temperature. On the contrary, the fragmentation can also be effected by treatment of the halosugar with n-BuLi in THF at -80°C, as depicted in Scheme 3.3 for iodoftiranoside 3 [24]. 

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