Total Synthesis of (+)-Hyacinthacine A1, (+)-7a-epi-Hyacinthacine A1, 6(R)-Hydroxy-hyacinthacine A1 and 6(S)-Hydroxy-7a-epi-hyacinthacine A1

The total synthesis of natural (+)-hyacinthacine A1 (6), (+)- 7a-epi-hyacinthacine A1 (7) and their 6-hydroxy analogues 21 and 16 was achieved using a nitrone cycloaddition strategy with D-ribose-derived cyclic nitrone 8 as the dipole and tert-butyl acrylate as the dipolarophile. After separation of the adducts, reductive cleavage of the N–O bond followed by lactam reduction and deprotection afforded the two nonnatural hydroxy analogues in excellent yields. The synthesis of (+)-hyacinthacine A1 (6) and of (+)-7a-epi-hyacinthacine A1 (7), which required deoxygenation at C(6), was accomplished by DIBAL-H reduction of mesylate derivatives of the pyrrolizidinols 20 and 15, respectively. Evaluation of the synthesized compounds against a panel of 12 commercially available glycosidases showed that hyacinthacine A1 (6) and its (6R)-hydroxy analogue 21 are good inhibitors of amyloglucosidase; the non-natural compound 21 is also a strong inhibitor of β-glucosidase, while 6 showed only moderate inhibition. The non-natural (+)-7a-epi-hyacinthacine A1 (7) is a moderate inhibitor of amyloglucosidase and α-L-fucosidase; the presence of the (6S)-hydroxy group in 16 led to diminished activity.