Glucal 3-0-methyl

Dibutylstannylene acetals have yielded propenylidene acetals on reaction with tetrakis(triphenylphosphinepalladium and acrolein diacetate, in some cases with excellent regioselectivity.72 For instance,D-glucal, methyl a-D-glucopyranoside, methyl a-D-mannopyranoside, and methyl /J-d-glucopyranoside yield the 4,6-O-propenylidene acetals in 89, 85, 83, and 80% yields, respectively, as mixtures of diastereomers at the acetal center. Galactose derivatives give mixtures of 3,4- and 4,6-O-propenylidene acetals.72... 

Neher and Lewis177 obtained 2-furaldehyde from 2,3,4-tri-O-methyl-L-arabinose by heating with dilute acid after preliminary enolization with alkali. Isbell83 proposed a mechanism for this conversion similar to that for the conversion of tetra-0-methyl-(2-hydroxy-D-glucal) into 5-(hydroxymethyl)-2-furaldehyde XLIV was suggested as an intermediate. In Hurd and Isenhour s178 scheme for the formation of 2-furaldehyde from free pentose, the enol (XLV) of a 3-deoxypentosone was regarded as an inter-... 

Similarly, tri-O-benzyl-glucal affords the corresponding manno-configurated methoxycarbonyl cyclopropanes upon treatment with methyl diazoacetate under... 

Hirst and Woolvin169 preferentially methylated positions 3, 4 and 6 by the use of glucal, an unsaturated sugar derivative having a 1,2-olefinic bond, introduced by the reduction of triacetyl-a-D-glucopyranosyl bromide with zinc dust and acetic acid. By the action of an ethereal solution of perbenzoic acid on an aqueous solution of 3,4,6-trimethyl-glucal a sirupy product was obtained. This was shown to contain... 

Unsaturated branched-chain sugars were synthetized with 72-84 % yield from both protected and unprotected 2-bromo-D-glucal with methyl acrylate in CH3CN/H2O 5/1 or in DMF/H2O 5/1 with a catalyst prepared from [Pd(DBA)2] and P(o-tolyl)3. Et3N or K2CO3 + /1-BU4NHSO4 could be used as base with similar results. 

On their side, Yin and Linker [216] made use of a 2-C-branched hexopyranoside, the synthesis of which was achieved by addition of dimethyl malonate to tri-O-benzyl-D-glucal (TUPAC name 3,4,6-tri-0-benzyl-l,5-anhydro-2-deoxy-D-araZtino-hex-l-enitol, Scheme 45) [217], Thus, saponification of the 2-C-[bis(meth-oxycarbonyl)]methyl derivative 184 to the corresponding malonic acid 185 was followed by heating in refluxing toluene. This led to decarboxylation and lactoniza-tion giving 186. The method was optimized and applied to the synthesis of pentoses and disaccharides. 

If the glycal is treated with a 2-3 percent solution of hydrogen chloride in methanol instead of with dilute aqueous acid, then the methyl glycoside of the 2-desoxysugar is obtained. In this way L-arabinal was converted into methyl 2-desoxy-/3-L-ribopyranoside,141 D-glucal into methyl... 

The reactivities of the hydroxyl groups of D-glucal and D-galactal are in the order 0-4 > 0-3 > 0-6 for methylation and benzylation conducted in Ar,Ar-dimethy 1 form am ide via the sodium or lithium alkoxides or electrochemically.86 These observations are corroborated by semiempirical calculations87 and are in interesting contrast to the selectivities just given. A possible explanation is that while the last-reported data pertain to reactions of oxyions, the others may refer to those of the undissociated alcohols. 

Simple alkylation can be accomplished by use of trialkylaluminums which, with Lewis acids, afford 1-C-alkyl products such as 90, available in 72% yield by reaction between di-O-acetyl-6-deoxy-D-galactal and trimethylaluminum in the presence of titanium tetrachloride in dichloro-methane at —78 °C.109 Importantly, the reaction can also be applied to 1-alkylglycals, also with high stereoselectivity, so that the doubly substituted compound 91 can, for example, be made by allylation of tri-O-acetyl-l-C-methyl-D-glucal. Its C-l epimer is available by C-methylation of the 1-C-allylglucal.110... 

Ozonolysis of tri-O-acetyl-D-glucal (22) followed by work-up with Et3N-Ac20, led to the O-formylated derivative 23 of methyl-D-arabinonate in 77% yield.48... 

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