Hydroxy group interactions in stannylated carbohydrates. Structures and thermal stabilities of 5-deoxy-5-C-(Ph3Sn)-1,2-O-isopropylidene-??-D-xylofuranose, 5-deoxy-5-C-(IPh2Sn)-1,2-O-isopropylidene-??-D- and -L-xylofuranose, and 5-deoxy-5-C-(I2PhSn)-1,2-O-isopropylidene-??-D- and -L-xylofuranose