The thio-Mitsunobu reaction: a useful tool for the preparation of 2,5-anhydro-2-thio- and 3,5-anhydro-3-thiopentofuranosides.
The unprotected methyl L-arabinofuranosides, D-ribofuranosides and D-xylofuranosides are transformed into the corresponding S-acetyl-5-thio derivatives by the thio-Mitsunobu reaction. Mesylation and subsequent reaction with sodium hydrogen carbonate led, depending on the configuration of the intermediate, to 2,5-anhydro-2-thio- or 3,5-anhydro-3-thiopentofuranosides. Due to inversion at C-3 or C-2 during the intramolecular nucleophilic displacement the products exhibit L-lyxo-, D-arabino- or D-lyxo-configuration. Analogously, the methyl 2,3-anhydro-D-ribofuranosides yielded 5-thio-S-acetates with intact 2,3-oxirane groups, which were cyclised with sodium hydrogen carbonate by epoxide ring opening and concomitant ring closure to form exclusively 3,5-anhydro-3-thio-D-xylofuranosides. A related 3,5-anhydro-3-seleno-D-lyxofuranoside was obtained by reaction of a 3,5-di-O-mesyl-D-arabinofuranoside with sodium hydrogen selenide. Several X-ray diffraction analyses proved the structures of the products.
Four-membered Rings with One Selenium or Tellurium Atom
This chapter is intended to update the previous version and to highlight major new preparations, reactions, and concepts. We have provided at the beginning of each main section a short paragraph explaining the major advances since the publication of the earlier chapters and some deficiencies in CHEC-II(1996) that we have now attempted to address. Four-membered cyclic compounds with one selenium or tellurium atom are named selenetane, telluretane, selenete, and tellurete. In this chapter, we describe crystal structures, NMR, syntheses of the selenetanes, and selenetes which have been mentioned. No discussions on telluretane and tellurete were found in the literature in the period 1995–2005. For the first time in 1995, selenetane derivatives have been characterized by X-ray diffraction and since then six selenetane derivatives have been determined by X-ray diffraction. Specific 1H, 13C, and 77Se NMR spectra of selenetane derivatives have been summarized. The selenetanes that have been described in the literature have been constructed via three types of reactions (1) via a [2+2] two-component synthesis; (2) via the [1+3] two-component synthesis through the reaction of a selenium nucleophile with a three-carbon unit, and (3) by a one-component synthesis via rearrangement-cyclization.
Facile cleavage of carbohydrate benzyl ethers and benzylidene acetals using the NaBrO3/Na2S2O4 reagent under two-phase conditions
Benzyl ether and benzylidene acetal carbohydrate protecting groups can be selectively cleaved by reaction with sodium bromate/sodium dithionite in ethyl acetate/water. Under the mild (room temperature, short reaction time) conditions needed, a variety of other protecting functionalities such as acetyl, chloroacetyl, benzoyl, pivaloyl, tosyl, t-butyldimethylsilyl, trityl, and isopropylidene groups remain unaffected.