Pyranoses derivatives

The q cloaddition reaction catalyzed by complex 9d proceeds well for several aromatic and a,/ -unsaturated aldehydes and has been used in an enantioselective route to the carbon-branched pyranose derivative cis-3c (Scheme 4.10) [17]. 

The study of Lemieux and co-workers (38, 39) which first delineated the stereospecific dependences of the P.M.R. parameters of pyranose derivatives has since been confirmed by many workers (24, 40, 44). In the following section we shall discuss some of the more recently recognized stereospecific dependences, dealing first with the dependences of coupling constants and then with those of chemical shifts. Reference will be made to the P.M.R. parameters of the hexopyranose derivatives which were obtained in experiments outlined in the previous section, together with the parameters of the a-(4) and, / -(5) anomers of 1, 3, 4, 6-tetra-0-acetyl-2-deoxy-D-arafoino-hexopyranose. 

The widespread occurrence of long-range couplings in both furanose and pyranose derivatives explains why so many of the P.M.R. spectra of carbohydrate derivatives are apparently poorly resolved, even when the resolution of the spectrometer is above reproach. For example, the Hi resonance of the 1,6-anhydro-D-glucose derivative (12) is coupled to all of the other six ring protons. A further example of the line-broadening effect follows a consideration of the spectrum of 5,6-dideoxy-5,6-epithio-l,2-0-isopropylidene-/ -L-idofuranose for which the half-height... 

It is evident from the above discussion that more-detailed studies of the P.M.R. spectra of carbohydrate derivatives will reveal an ever-increasing number of long-range couplings. Studies in this laboratory are directed towards the investigation of the occurrence of these couplings in the spectra of both furanose and pyranose derivatives. 

One of the most interesting stereospecific dependences which followed from the original study (38,39) of O-acetylated hexopyranose and pento-pyranose derivatives was that of the anomeric proton shifts. In general it was found that the axially oriented anomeric protons gave resonances to higher field than the equatorially oriented protons. Many subsequent studies of pyranose derivatives (24,40) and of inositol derivatives (43)... 

Lopez J. C., Lukacs G. Pyranose-Derived Dienes and Conjugated Enals. Preparation and Diels-Alder Cycloaddition Reactions ACS Symp. Ser. 1992 494 33-49 Keywords carbohydrate, befera-Diels-Alder reactions, stereoselectivity... 

As conformational anomeric effects represent only a fraction (—1 to —3 kcal/mol) of the global enthalpic anomeric effect or gem-dioxy stabilizing effect (—6 to —17 kcal/mol), additional factors have to be taken into account. Depending on substitution, steric factors can affect the relative stability of acetal conformers. Dubois et al.2S have demonstrated that in furanose pyranose derivatives a bulky substituent at the furanose... 

In addition to the major strategies previously outlined, other methods have also been utilized to synthesize septanosides. Among these, a bis-aldol addition of nitroalkanes onto pyranose-derived dialdehydes has been utilized to synthesize 3-nitro- and 4-nitro-septanosides.81-84 Such septanose derivatives are useful as they... 

Using the same experimental conditions, the pentoses o-xylose (7), o-ribose (9), and D-arabinose (8) afforded the pyranoses derivatives 36f-h or 37f-h, respectively (Fig. 3). 

The reactions of fused and unfused a-pyrones with magnesium alkoxides, sodium alkoxides, and water as the nucleophile have been studied in depth, particularly with respect to the effects of chelation <1996J(P1)2715>. The annelation reactions of pyranose derivatives, with acetylacetone and ethyl acetoacetate, to give a variety of annelated fused pyrans have been reported, with the products obtained as single diastereoisomers (Scheme 13) <1997LA1019>. 

A comparison of the three binary copolymerizations of 1,6-anhydro-/S-D-gluco-, -galacto-, and -manno-pyranose derivatives gives some insight into the mechanism of copolymerization, if it is assumed on this evidence that the per-p-xylyl and perbenzyl derivatives can be used interchangeably.107... 

The foregoing results show that the deaminations of pyranose derivatives containing equatorially attached amino groups at C-2 or C-4 are dominated by participation of the ring-oxygen atom. 

The higher yield of the pyranose derivatives (see Table II) in the ammonolysis of a,/3-L-rhamnopyranose tetraacetate shows the influence of polar effects, because the methyl substituent on C-5 would enhance the nucleophilic character of its hydroxyl group in relation to that of the one on C-4. 

Among the classic methods for the extension of the aldose chain by one carbon atom from the reducing end [9J, the Kiliani-Fischer cyanohydrin synthesis [10] is a milestone in carbohydrate chemistry. However after 110 years from discovery and numerous applications [11], including the preparation of carbon and hydrogen isotopically labeled compounds for mechanistic and structural studies [12], there are still several drawbacks that make the method impractical. These are the low and variable degree of selectivity and the harsh reaction conditions that are required to reveal the aldose from either the aldonic acid or directly from the cyanohydrin. Synthetic applications that have appeared in recent times confirmed these limitations. For instance, a quite low selectivity was registered [13] in the addition of the cyanide ion to the D-ga/acfo-hexodialdo-l,5-pyranose derivative 1... 

Scheme 1) whereas the reaction with the D-man/io-hexodialdo-l,5-pyranose derivative 3 produced, in low yield, the cyanohydrin 4 having an L-glycero-D-manno configuration and the stereoisomeric epimer at C-5, rather than at C-6 [14J. With an atm toward the synthesis of d- and L-glycero-D-manno-heptoses, the reaction of 3 with other reagents (2-inethyl-furan, alkyl magnesium chlorides) was also explored, without any substantial improvement of either stereoselectivity or yield. 

N. K. Khare, R. K. Sood, and G. O. Aspinall, Diastereoselectivity in the synthesis of D-g/ycero-D-aldoheptoses by 2-(trimethylsilyl)thiazole homologation from hexodialdo-1,5-pyranose derivatives, Can. J. Chem. 72 237 (1994). 

Recently the use of allylic halide (see compound 96, Scheme 34) or sulfonate for the introduction of a malonyl residue at C-2 of pyranose derivatives using tetrakis(triphenyl-phosphine)palladium complex has been reported [132]. In this case, the malonyl chain is introduced anti to the leaving group at the 7-position. 

J. C. Lopez and G. Lukacs, Pyranose-derived dienes and conjugated enals—preparation and Diels-Alder cycloaddition reactions, in Cycloaddition Reactions in Carbohydrate Chemistry, 1992, pp. 33-49. 

R. Tsang and B. Fraser-Reid, A route to optically active trichothecene skeleton by bisannulation of a pyranose derivative, J. Org. Chem. 50 4659 (1985). 

Although the characteristic differences discussed above permit ready distinction between furanose and pyranose derivatives, their fragmentation patterns have much in common. The fragments of the A, C, and J series are formed in a similar manner from both of the two types of derivatives, although there is a difference in the contribution of some of the isomeric ions.1 87... 

Fraser-Reid s stereocontrolled synthesis of the Woodward reserpine precursor 195 relied upon a tandem 5-exol6-exo radical cyclization of pyranose-derived dienes [76-77]. As outlined in Scheme 36, a,P-unsaturated ester 188 was prepared by free radical coupling of iodide 187 with a tin acrylate. After hydrolysis of 188 (MeONa, MeOH, 100%) to give primary alcohol 189, the silicon tethered diene 190 was installed by silylation. Treatment of 190 with n-BujSnH led to the desired cage molecule 192 in high yield via a 5-exo-trig cyclization to intermediate 191 followed by a 6-exo cyclization. Tamao oxidation of tricycle 192 led to diol... 

The facile photobromination at C-4 or C-5 position of different furanose and pyranose derivatives has many applications for further modifications of carbohydrates, mainly by elimination and substitution reactions. 

In the pyranose series, the 6-deoxy hex-5-eno-pyranose derivatives obtained from C-5 photobrominated carbohydrates reacted with mercury (II) salts by an intramolecular aldol process to give [3-hydroxy-cyclohexanones according to Scheme 13. 

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