Xanthen-9-ylidene protecting groups in glycerol chemistry

Abstract

The preparation of racemic, (S)- and (R)-1,2-O-(xanthen-9-ylidene)glycerol 17a, 20a and 23a and racemic, (S)- and (R)-1,2-O-(2,7-dimethylxanthen-9-ylidene)glycerol 17b, 20b and 23b is reported. The racemic derivatives 17a and 17b are converted into their stearate esters, which are then treated with dichloroacetic acid and pyrrole under mild conditions to give racemic 1-O-stearoylglycerol 25 in good yield. The xanthen-9-ylidene and 2,7-dimethylxanthen-9-ylidene residues are incorporated into 9,9-di(pyrrol-2-yl)xanthene 36 and 2,7-dimethyl-9,9-di(pyrrol-2-yl)xanthene 37. These by-products are easily removed by treatment with iron(III) chloride in diethyl ether solution. What are believed to be enantiomerically pure (R)- and (S)-1-O-stearoylglycerol 28 and 5 are similarly prepared in good yields from (S)- and (R)-1,2-O-(xanthen-9-ylidene)glycerol 20a and 23a.