Difficulties in the deprotection of 1,2-ketals in nucleosides containing alkylidencarbazoyl groups

The synthesis of unprotected alkylidencarbazoyl nucleoside derivatives 8a-8d is shown. A direct deprotection route from readily available 2',3'-isopropylidene protected nucleosides 5a-5d. prepared from a chemoenzymatic procedure, did not give the selective cleavage of the ketal function without affecting the C=N bond. The next option tried was to look at the previous compound in the retrosynthetic route: 2',3'-protected carbazoyl nucleoside 4. However, in all cases we obtained unsatisfactory results. Stepping further back, the hydrolysis of compound 3a led us to unprotected carbonate nucleoside 9 in quantitative yield. With this compound available, the synthesis towards derivatives 8 was accomplished through a known procedure.