The acetalization of phenyl 1-thio-α-L-arabinopyranoside with 2,3-butanedione in the medium of MeOH-CH(OMe) 3 -CSA proceeded with the prevailing formation of the corresponding 3,4-bisacetal that further was converted in compounds, which were regio-and stereoisomers of disaccharide block of OSW-1. O HO O SPh HO OH HO O SPh HO OAc I II IIIIn the sugar chemistry the necessity to work with awkward polyol systems makes the protective groups extremely important [1]. In the course of the planned synthesis of disaccharide block I for anticancer steroid OSW-1 based on the known components, glycosidedonor II and acceptor III [2, 3] (Scheme 1) we tested 2,3-butanedione in conditions described in [4] for the selective protection of the 3,4-diol moiety of triol II in the form of butane 2,3-bisacetal. The butane 2,3-bisacetal and the other similar protective groups introduced into the synthetic practice by Liu group [5] are known to provide a selective blocking in cycles of trans-vicinal diols, impart the products a crystalline structure, desired conformational rigidity, etc. [6].As seen from the structure of triol II all its hydroxy groups are located in the trans-equatorial reciprocal position, and here the possibility of the regiocontrolled acetalization depends mainly on the steric factors. In the experimental test of triol II acetalization by heating with excess 2,3-butanedione in the medium of MeOH-CH(OMe) 3 -camphorsulfonic acid (CSA) (catalyst) we obtained two expected acetals IV and V in the ratio ~3 : 1 and in the overall yield 80%. The main bisacetal IV is a crystalline substance easily isolated from compound V by chromatography on SiO 2 . Minor bisacetal V is oily substance containing ~10% of unidentifi ed isomeric compound. The assignment of the structure of regioisomeric acetals IV and V was carried out applying spectral data. Their 1 H NMR spectra contain a characteristic signal C 1 H which in the spectrum of 2,3-acetal V is located downfi eld (doublet, δ 4.77 ppm, J 9.6 Hz) with respect to the corresponding signal in the spectrum of compound IV (δ 4.47 ppm, d, J 9.35 Hz) because of the electron-withdrawing inductive effect of the closely Scheme 1.

The intramolecular cyclization of (-)-cis-carveol under iodine treatment afforded (1R,5R,6S)-6-iodomethyl-2,6-dimethyl-7-oxabicyclo[3.2.1]oct-2-ene that was subjected to allyl oxydation with the complex CrO 3 DMP giving a synthetically valuable building block, (1R,5R,6S)-6-iodomethyl-2,6-dimethyl-7-oxabicyclo[3.2.1]oct-2-ene-4-one. In the latter the double bond was cleaved by ozonization to obtain the expected trioxo derivative, and the subsequent ozonolysis of its enol form provided a multiple functionalized tetrahydrofuran derivative.

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