Considerable attention has been focused on branched-chain sugar nucleosides because of their biological importance. They have shown broad spectrum of biological activities such as antitumor, antiviral or antibiotic. The most representative examples of this group are AZT,¹ ECyd, EUrd,² CNDAC,³ TSAO and its derivates.⁴ In our previous communications^5,6 we published the stereoselective synthesis of the branched-chain sugar 3(S)-isothiocyanato-3-deoxy-3-C-vinyl glucose as a suitable precursor for the synthesis of natural compounds and where 1,2-O-isopropylidene group is a decisive factor for the stereocontrol in the aza-Claisen rearrangement of allylic thiocyanates.

We report here the study of stereoselectivity of [3,3]-sigmatropic rearrangement of protected uridine allylic thiocyanates 1 and 3. The key step in the synthesis of branched-chain nucleosides is the stereocontroled formation of a new C-N. The thermal rearrangement of 1 and 3 was carried out at 70-90^oC in various solvents under N₂ for 24 h to give the isothiocyanates 2a-b and 4a-b with a relatively low selectivity (Scheme).

The microwave induced rearrangements of 1 and 3 afforded isothiocyanates 2a-b and 4a-b in good yields within 20-60 min. The reactions were performed in closed vessels in a focused microwave reactor (CEM Discover), with control of power and temperature by an infrared sensor. We have found that the [3,3]-sigmatropic rearrangement of the uridine thiocyanates is strongly accelerated in microwaves conditions with enhanced stereoselectivities of the reactions Wery interesting was result of rearrangement of 1 under microwave condition without solvent. Reaction take place with high conversion and stereoselectivity to afford isothiocyanate 2b.

References

1. Mitsuya H.,Weinhol K., Fumar, P. A. Proc. Natl. Acad. Sci. 1985, 82,7096. 2. (a) Matsuda, A., Hattori, H.,Tanaka, M. Bioorg. Med. Chem. Lett. 1996, 6, 1887. (b) Hattori, H.,Tanaka, M., Matsuda, A. J. Med. Chem. 1966, 39, 5005. 3. Matsuda, A., Nakajima,Y., Azuma, A. J. Med. Chem. 1991, 34, 2917. 4. Camarasa, M. J., Pérez-Pérez, M. J., San-Félix, A. J. Med. Chem. 1992, 35, 2721. 5. Martinková, M., Gonda, J., Šoteková, M., Buděšínský, M., Císařová, I. Chem. Pap. 2005, 59, 113. 6. Gonda, J., Bednáriková, M., Tetrahedron Lett. 1997, 38, 5569.

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