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With trifluoroacetic acid; In ethanol; water;Reflux;
General procedure: One equiv. of carbazate was suspended in EtOH containing 10percent (by vol.) of water(approx. 5 ml per mmol of the carbazate) and 1.05 equiv. of the acetal or ketal wasadded, followed by the addition of 0.05 equiv. of TFA. The reaction mixture was heatedto reflux and the progress of the reaction was monitored by TLC (thin layer chromatographyon silica gel) using ethyl acetate or a mixture of ethyl acetate-light petroleummixture (for the correct eluent, see characterization data of the compounds). After thereaction was complete, the reaction mixture was cooled to about 45C and three (3)equiv. of acetic acid was added, followed by the dropwise addition of a THF solutionof three (3) equiv. of NaBH3CN (approx. 1 ml of THF per 1.5 mmol of NaBH3CN).The reaction mixture was stirred at approx. 45C for 80 min. Subsequently, the reactionmixture was cooled to room temperature, acidified using 0.5 M HCl aqueous solution(6 ml of 0.5 M HCl per 1 mmol of starting carbazate) and stirred until the liberation ofhydrogen ceased. The reaction mixture was neutralized (to pH 8) by the dropwise additionof a saturated NaHCO3 solution (4 ml per 1 mmol of carbazate). The volatileswere removed under reduced pressure at approx. 40C. The residue was dissolved inethyl acetate, washed with saturated NaHCO3, twice with water and finally with saturatedaqueous sodium chloride. The combined aqueous washes were extracted twicewith ethyl acetate (25 ml per 1 mmol of expected product) and the extracts werewashed with saturated aqueous sodium chloride and combined with the organic phase. After drying over anhydrous Na2SO4 and evaporation of the solvent under reduced pressure,the residue was purified by column chromatography on silica gel using ethyl acetate-light petroleum (1:1 or 1:2) or ethyl acetate as eluent. For specific informationabout the eluent used to monitor the progress of the reaction and for purification, seethe Rf data for each compound.
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