Model synthesis of 2,3:5,6-di-O -isopropylidene- S -α- D - mannofuranosyl-N , N -diisopropylphosphoramidofluoridothioate and 2,3:5,6-di-O -isopropylidene-Se-α- D -mannofuranosyl- N , N -di- isopropylphosphoramidofluoridoselenoate via P(III)-OAr intermediates and a thiono-thiolo (selenono-selenolo) rearrangement

Phosphitylation of the glycosidic center of sugar derivative 2 by phosphoroamidite 1 leads to the intermediate P(III) ester 3 . Conversion of 3 into fluorophosphoroamidite 4 and subsequently oxidation by elemental sulfur or selenium affords the corresponding thiono-and selenono-esters 5 and 6 . The latter compounds are readily rearranged into their thiolo-or selenolo-isomers in the presence of Bu 4 NI as catalyst (TBAI). The total yield of 7 and 8 exceeds 90%.


Introduction
Thio-and seleno-analogs of glycosyl phosphates are useful intermediates in the synthesis of modified monosaccharides and as glycosyl donors. 1 Michalska and associates have found that this class of compounds can be readily prepared e.g. from glycosyl bromides by condensation with thiophosphate or selenophosphate salts. 2 Glycosyl S-or Se phosphates are major products of these reactions.Another pathway leading to glycosyl thiophosphates is based on the known reaction between P(III) esters and alkyl (aryl) thiocyanates. 3Alkyl and aryl thiocyanates react with trialkyl phosphite and their structural analogues to give alkyl or aryl cyanates and the corresponding thiolo-esters.This reaction is believed to proceed by a mechanism involving displacement of the CN group by nucleophilic attack of P(III) phosphorus on electrophilic sulfur via formation of a phosphonium-type intermediate.The procedure has been recently used to prepare glycosyl thiophosphates as potential glycosyl donors. 4However, this method is limited to glycosyl thiocyanates that do not rearrange readily into isomeric isothiocyanates.Such isomerization may be slowed down by bulky protecting groups at the sugar moiety.In every described case, however, isomeric products are formed in substantial amounts.Because glycosyl thiophosphates and selenophosphates can be synthesized directly from glycosyl halides, as mentioned above, this approach seems to be of limited use. 2 Since the seminal work of Letsinger and Caruthers it became obvious that phosphitylation procedures are of special importance in the synthesis of biophosphates and their structural analogs. 5We have recently become interested in pursuing the use of specially designed phosphitylation reagents in the synthesis of glycosyl thiophosphates and their seleno-analogs as a possible alternative to the methods described above.We anticipated that such an approach could not only be more efficient but also would allow the construction of more complex glycosyl-phosphorus ester structures, such as those derived from thio-and seleno-fluorophosphoric acids.

Results and Discussion
We discuss a model strategy based on O,O-di-(4-nitrophenyl)-N,N-diisopropylphosphoroamidite 1 as a phosphitylation reagent and 2,3,5,6-di-isopropylidene-α-D-mannofuranose 2 as an example sugar.The amidite 1 belongs to a group of ambident reagents containing two different types of leaving group at tricoordinate phosphorus center: 4-nitrophenoxy and diisopropylamino. 6The former can be activated by strong bases like DBU, the latter by tetrazole or -more convenientlyby trimethylchlorosilane. 7It is important to mention that phosphitylation employing an aryloxy leaving group does not interfere with subsequent use of amido group.The opposite is also true.
When α-D-mannofuranose 2 was allowed to react with the phosphitylating reagent 1 in acetonitrile at 20 ºC in the presence of DBU, the α-D-mannofuranosyl-O-(4-nitrophenyl) phosphoroamidite 3 was formed within 10 min in over 95% yield after purification by silica gel column chromatography, and was obtained as a mixture of diastereoisomers in a ratio of 1:1.The compound 3 is potentially useful as a phosphitylating reagent by replacement of the diisopropylamino group and as an intermediate in the synthesis of α-D-mannofuranosyl fluorophosphoroamidite 4 via exchange of the 4-nitrophenoxy group.It has been earlier established that an aryloxy group can readily be replaced by a fluoride anion. 8Indeed, the fluorophosphoroamidite 4 was formed in almost quantitative yield when 3 was kept for 10 min.with tetrabutylammonium fluoride (TBAF) in acetonitrile solution.Both reactions a and b shown in Scheme 1 can be performed as a one-flask procedure at 20 ºC.

Scheme 1
Transformation of 4 into the thionofluorophosphate or selenonofluorophosphate takes place by addition of elemental sulfur or selenium at 20ºC in diisopropylamine.Rates of addition of sulfur and selenium are distinctly different: at 20ºC the sulfurization is complete in 1 h while under the same reaction conditions the selenization reaction requires 10 h.

Scheme 2
Compounds 5 and 6 were formed in almost quantitative yield after purification by silica gel chromatography, and isolated in 95% yield, as a mixture of diastereoisomers in a ratio of 1:1.The thiono-and seleno-esters 5, 6 were readily rearranged into their thermodynamically more stable thiolo-and selenolo-isomers 7, 8.This transformation (Scheme 2, steps c and d), related to the Emmet-Pishchimuka reaction, requires heating to over 100 ºC but the temperature can be effectively reduced to below 60 ºC when tetra-butylammonium iodide (TBAI) is used as the catalyst. 9The rearrangement of both compounds 5 and 6 proceeds in acetonitrile solution in almost quantitative yields.A plausible intermediate in this rearrangement is the iodoglycosyl compound 9 which is formed via nucleophilic displacement at the sugar glycosylic center by iodide anion derived from the catalyst.This intermediate, formed with inversion of configuration at glycosylic carbon, provides β-Diodomannofuranose 10 which undergoes a second nucleophilic displacement by the fluoroamidothiophosphorus acid anion.This second displacement proceeds more likely with inversion of configuration at the glycosylic carbon atom.Two inversions result in the final retention (Scheme 3). 31P NMR spectroscopy failed to show formation of the iodide 9 and salts 10.This can be explained if the reaction b (Scheme 3) is very fast.

Scheme 3
A similar mechanistic picture is likely to be valid in the case of the seleno compound.The α-D-mannofuranosyl structure of compounds 3-8 was rigorously confirmed by 1 H-, 13 C-19 F-and 31 P-NMR spectroscopy.They all have the same α-D-configuration at the glycosylic mannose furanose center.In respect to the center of chirality at the phosphorus atom, they are all isolated as 1:1 mixtures of diastereomers.

Conclusions
It has been demonstrated earlier that the thermal >P(S)(OR) → >P(O)(SR) rearrangement may proceed with either retention or inversion of configuration at C or P centers.Our results do not allow us to draw any definite mechanistic conclusion. 10n summary, we have shown that even complex P-thiolo compounds can be synthesized readily in excellent yield and under mild conditions from a 1-OH sugar via phosphitylating procedures.This methodology is superior to methods employed earlier and in particular to those based on the intermediate formation of glycosylthiocyanate.