Thesis (M.Sc. in Pharm.)--Chulalongkorn University, 2006

Five labdane diterpenoids, ent-3-oxomanoyl oxide (1), ent-1,2-dehydro-3-oxomanoyl oxide (2), ent-1,2-dehydro-12[alpha]-hydroxy-3-oxomonoyl oxide (3), ent-1[beta]-hydroxy-3-oxo-manoyl oxide (4) and ent-3[alpha]-hydroxymanoyl oxide (5) were isolated from the stem bark of Croton roxburghii N.P. Balakr, and they were derivatized to give 8 derivatives, ent-1,2-dehydro-3-oxo-manoyl oxide-14,15-oxirane (6), ent-1,2-dehydro-12[alpha]-hydroxy-3-oxo-manoyl oxide-14,15-oxirane (7), ent-1[beta]-hydroxy-3-oxo-manoyl oxide-14,15-oxirane (8), ent-3[alpha]-hydroxy-manoyl oxide-14,15-oxirane (9), ent-3-oxo-manoyl oxide-14(R), 15-diol (10), ent-3[alpha]-Hydroxy-manoyloxide-14(R),15-diol (11), ent-3[alpha],14(R), 15-triacetyl-manoyl oxide (12) และ ent-3[beta]-hydroxy-manoyl oxide (13). All of the isolated compounds and their derivatives showed moderate [alpha]-glucosidase inhibitory activity. Assay for [alpha]-glucosidase inhibitory activity showed that 3[beta]-C substituted (13) increased the [alpha]-glucosidase inhibitory activities more than 3[alpha]-C substituted (5). Epoxidation of the exocyclic double bond (6-9) makes the inhibitory activity reduced.