Studies on sugar nitro-olefins. Part 7. Synthesis of 3-(alditol-1-yl)-1,2,3,5,6,7-hexahydro- and -1,5,6,7-tetrahydro-indol-4-ones. X-Ray molecular structure of (3S)-3-(1,2,3,4,5-penta-O-acetyl-D-galacto-pentitol-1-yl)-1,2,3,5,6,7-hexahydroindol-4-one