Talopyranose preparation

An anhydro sugar which will probably find extensive synthetic use has been described by Hann and Hudson.85 This is the tricyclic compound, 3,4 1,6-dianhydro-jS-D-talopyranose (LXXIV) which is prepared by the saponification of the 4-tosyl derivative of mannosan (LXXIII). 

Brimacombe and Tucker48 were able to prepare l,4-anhydro-6-deoxy-2,3-0-isopropylidene-/3-L-talopyranose by an uncomplicated ring-closure, with inversion, of 6-deoxy-2,3-0-isopropylidene-4-0-... 

In the case of 1,6 2,3- (136) and l,6 3,4-dianhydro-/I-D-talopyranoses, the diaxial opening of the oxirane ring prevails, but a trend to diequatorial opening496 498 is apparent with l,6 2,3-dianhydro-4-deoxy- and 1,6 3,4-dianhydro-2-deoxy-/l-D-/yxo-hexopyranoses.169,432,493 Using 1,6 2,3- and l,6 3,4-dianhydrohexopyranoses as starting compounds allowed the preparation of a complete series of 12 isomeric 1,6-anhydro-monodeoxy-/Fn-hexopyranoses and 6 corresponding 1,6-anhydro-dideoxyhexoses,499 mainly by catalytic or complex hydride reductions.462,500... 

A quite different approach was demonstrated in the preparation of 2,4-diamino-1,6-anhydro-2,4-dideoxy-/J-D-talopyranose (147) from 146 by the action of phenylhydrazine and iodine, followed by catalytic reduction.433... 

The acyloxonium rearrangement provides a good preparative access to D-talose from D-galactose. Thus, tetra-0-acetyl-/3-D-galactopy-ranosyl chloride (90) is treated with antimony pentachloride to give the mixture of salts 91 and 92, which is hydrolyzed and the mixture of products acetylated. Penta-O-acetyl-a-D-talopyranose, which is present as the pure a-D anomer, crystallizes out from the mixture, whereas the D-galactose pentaacetate does not. The latter exists as a difficultly crystallizable, a,/3 mixture, because hydrolysis of 91 and acetylation of the product leads to an anomeric mixture. 

Methyl 2,3,5,6-tetradeoxy-2,3,5,6-tetrathio- 8-L-galactofuranoside and -0(-D-altropyranoside have been prepared by the routes shown in Schemes 2 and 3. The syntheses of 6-deoxy-4-thio-a-D-talopyranose and - 3-L-allopyranose have been achieved by applying to D-mannose the methods developed in the D-glucose series (see Vol. 12, p. 94). The unsaturated 4-thio-derivatives (6) and (7) were obtained from the 4-ulose (8) by treatment with phosphorus pentasulphide in pyridine followed by acetylation. ... 

Oxirans. - The synthesis of l,2-anhydro-3,4-di-0-benzyl-6-deoxy-a-D-glucopyranose and its conformational analysis have been reported. A range of epoxides have been prepared by base treatment of bromohydrins, which were made by reaction of hydrogen bromide with aldonolactones. A one-pot conversion of vicinal diols into epoxides employs halohydrin ester intermediates generated from cyclic orthoacetates and either acetyl bromide or trimethylsilyl chloride. Levoglucosenone has been transformed into l,6 3,4-dianhydro-p-D-talopyranose by way of a trn/w-iodo-acetoxylation of the alkene moiety... 

The preparation of some glycosylated ribitols as useful building blocks for the synthesis of certain teichoic acids has been described. The 1,6-anhydro-P-D-talopyranose (13), a key intermediate, is readily accessible from l,6-anhydro-23-< -(S)-benzylidene-p-D-mannopyranose. Thus coupling of (13) with S-ethyl-2,3,4-tri-0-acetyl-l-thio-a-L-rhamnopyranoside affords the L-ribitol derivative (14) after further IM Ocessing. Also the 4-0-allyl derivative (IS) is a suitable precursor of glycoside (16). 

Other Anhydrides. - Stereoselective chemical and chemoenzymatic approaches to the preparation of 1,6-anhydrohexopyranoses have been reviewed, and base treatment of pentabromophenyl glycosides has been investigated as an improved procedure for the synthesis of 1,6-anhydro sugars. Treatment of O-benzylated-3-O-acetyl-glycosyl chlorides with base (KO Bu, THF) has allowed syntheses of l,3-anhydro-2,4,6-tri-0-benzyl-P-D-talopyranose, l,3-anhydro-2,4-di-C -benzyl-a-L-arabinopyranose and 1,3-anhydro-2,4-di-0-benzyl-6-deoxy-P-L-talopyranose. The theoretical solution conformation of methyl 3,6-anhydro-a-D-galactopyranoside has been studied using ab initio calculations. The computed conformation was similar to that adopted in the crystal (X-ray analysis). ... 

Bromination of unsaturated lactone 5 (Br2,CH2Cl2) gave only the dibromo compound 6, whereas unsaturated lactone 7 afforded 8 under the same conditions. 2-Acetamido-4-bromo-2,4-dideoxy-talopyranose derivatives have been prepared via 2-acetamido-l,6 3,4-dianhydro-2-deoxy-P-D-talopyranose. The synthesis of deoxyiminoalditols from brominated aldonolactones is covered in Chapter 18. 

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