NUCLEOSIDES .59. THE 2-(4-NITROPHENYL)ETHYLSULFONYL (NPES) GROUP - A NEW-TYPE OF PROTECTION IN NUCLEOSIDE CHEMISTRY

The 2-(4-nitrophenyl)ethylsulfonyl (npes) group is developed as a new sugar OH-blocking group in the ribonucleoside series. Its cleavage can be performed in a beta-eliminating process under aprotic conditions u sing 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the most effective ba se. Since sulfonates do not show acyl migration, partial protection of 1,2-cis-diol moieties is possible leading to new types of oligonucleo tide building blocks. A series of Markiewicz-protected ribonucleosides 1-10 is converted into their 2'-0-[2-(4-nitrophenyl)ethylsulfonyl] de rivatives 29-38 in which the 5'-O-Si bond can be cleaved by acid hydro lysis forming 39-45. Subsequent monomethoxytritylation leads to 46-50, and desilylation affords the -O-[2-(4-nitrophenyl)ethylsulfonyl]ribon ucleosides 51-55. Acid treatment to remove trityl groups do also not h arm the npes group (--> 56-58). Unambiguous syntheses of fully blocked 2'-O-[2-(4-nitrophenyl)ethylsulfonyl]rib 96-102 are achieved from the corresponding 3'-O-(tert-butyl)dimethylsilyl derivatives. Furthermore , various base-protected 5'-O-(monomethoxytrityl)- and 5'-O-(dimethoxy trityl)ribonucleosides, i.e. 59-77, are treated directly with 2-(4-nit rophenyl)ethylsulfonyl chloride forming in all cases a mixture of the 2',3'-di-0- and the two possible 2'- and 3'-O-monosulfonates 107-148 w hich can be separated into the pure components by chromatographic meth ods. The npes group is more labile towards DBU cleavage than the corre sponding base-protecting 2-(4-nitrophenyl)ethyl (npe) and 2-(4-nitroph enyl)ethoxycarbonyl (npeoc) groups allowing selective deblocking which is of great synthetic potential.