3 8 of isomer 8a with mp 116 118 u00b0 c and u03b1 d 20 u2212 84 3 u00b0 c 1 mecn 1 h nmr cdcl 3 u03b4 7 06 8 04 m 14h 5 40 5 67 m 2h 5 22 5 39 m 1h 5 00 5 15 m 1h 4 61 m 1h 4 02 4 17 m 1h 3 78 3 87 m 1h 3 30 3 50 m 1h 2 45 2 93 m 7h 2 18 2 28 m 1h 1 53 2 03 m 8h pyridinium p toluenesulfonate 20 mg was added to a stirred solution of the compound 8a 0 44 g obtained in previous step in methanol 20 ml at room temperature the mixture was stirred at 40 50 u00b0 c for 3 4 hours tlc monitoring and evaporated under reduced pressure the residue was diluted with dichloromethane 30 ml the solution was washed with saturated aqueous solution of sodium bicarbonate 10 ml and brine 10 ml dried over sodium sulfate filtered and evaporated to give 0 35 g 93 4 of oily residue the residue was crystallized from a mixture of hexane and ether to give compound 6a as white crystals.