Glycals acetoxy

An entirely different approach to pyranoid 4-enes depends on eliminations from 5-bromo compounds obtained from pyranoid hexuronic acid derivatives by photobromination. Treated with zinc-acetic acid, the bromide 249 gives the glycal -like 250 (62%), while the 4-acetoxy compound 251 is formed when DBU is used to promote elimination. Similarly, base treatment of penta-0-acetyl-5-bromo-/J-D-glucose with DBU causes the analogous loss of hydrogen bromide and formation of the 4-acetoxy-4-ene, but use of zinc-acetic acid affords mainly the 5-exo-methylene alkene by the alternative available elimination process.237... 

Dialkylzincs undergo formal substitution with 4-acetoxy-6-alkyl-l,3-dioxanes in the presence of trimethylsilyl triflate (TMSOTf) with excellent diastereoselectivity (Equations (61) and (62)).123,123a 123c The addition of TMSOTf triggers also the allylic substitution of glycal derivatives, providing the substitution product with excellent regio- and diastereoselectivity (Equation (63)). 

Because two epimers in about equal amounts are produced in the 0X0 reaction on the glycals, it was surprising that the epimer of (50) was not produced in the oxo reaction of 2-acetoxy-tri-O-acetyl-D-glucal. In order to determine whether a different catalyst might influence the composition of the mixture of products, compound (49) was allowed to react with carbon monoxide and hydrogen in the presence of a rhodium, instead of a cobalt, catalyst. Surprisingly, this led to deacetylation, but hydroformylation of the substrate did not occur. 

In one additional example of the formation of C-arylglycosides from glycals, Ichikawa, et al.,65 illustrated the applicability of this chemistry to heterocyclic aryl species. Specifically, as shown in Scheme 2.4.15, tri-O-acetoxy glucal was treated with furan and borontrifluoride etherate. The result was a 54% yield of the desired C-furanoglycoside as a 1 1 anomeric mixture. 

Ireland recognized early on the potential for the application of the Claisen rearrangement to the synthesis of C-glycosides and related oxacycles. In 1979 he reported that the silyl ketene acetals of acetoxy glycals gave the rearranged glycoside in CO. 50-80% yields (Scheme 4.86) [23]. 

For this study, the commercially available triacetyl D-glucal (17) (Table II) was the first substrate subjected to the cycloaddition conditions. Unfortunately, under the conditions used for furanoid glycals, the cycloaddition did not take place. However, when the reaction was carried out at 90 C a 3/2 diastereomeric mixture of adducts 27 was obtained in low yield. Since dihydropyran was found to be a good substrate in the cycloaddition reaction under irradiation, this suggested that the low reactivity of triacetyl D-glucal was due to the acetoxy groups on the ring. 

The endIcT-selective hetero Dids-Alder reaction of chiral oxazolidones 12 with (Z)-l-acetoxy-2-ethoxyethene yields the 1-substituted glycals 13 as the major products when catalysed with dimethylaluminium chloride, and 14 as the major products when catalysed with trimeth silyl triflate. Compounds 13 (R = Et)and 14 (R = Et) were subsequently converted into ethyl P-D-mannopyranoside and ethyl 3-L-mannopyrannoside, respectively. ... 

The methoxymercuration of 4,6-0-benzylidene-D-allal yielded isomeric acetoxy-mercurial adducts having the -D-allo (439) and a-D-altro (440) configurations. Demercuration of (439) with sodium borohydride gave methyl 4,6-0-benzylidene-2-deoxy-P-D-ri6o-hexopyranoside (72%), the ot,p-unsaturated aldehyde (441) (8.3 %), and a trace of the original glycal, while demercuration of (440) gave methyl 4,6-0-benzylidene-2-deoxy-a-D-r/6o-hexopyranoside (40%), (442) (23.5%), and traces of other products. 

A standard displacement with azide ion, etc., has been used in the conversion of 3,4,7-tri-0-acetyl-2,6-anhydro-5-0-methanesulphonyl-D- /ycero-L-ma/jno-heptonamide (113) into 5-amino-2,6-anhydro-5-deoxy-D- /> ccro-D- w/<7-heptonic acid (114), which, after formation of the methyl ester hydrochloride, underwent polycondensation to give oligomeric or polymeric amides on treatment with methanolic sodium methoxide. Both 3-acetamido-2,3,6-trideoxy-L-hexosides (116) and (117) have been synthesized by introduction of an amino-group via azide-displacement of the allylic acetoxy-group in the glycal derivative (115) (116) and (117) are derivatives of L-acosamine and L-ristosamine, respectively. ... 

Ferrier rearrangement on C-2 substituted glycal sterns In the last two years, several reports on the Ferrier rearrangement of 2-halo, 2-nitro, 2-acetoxy and 2-acetoxymethyl glycals have appeared in the literature. 

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