Total Synthesis of (+)‐Hyacinthacine A₁, (+)‐7a‐<i>epi</i>‐Hyacinthacine A₁, (6<i>R</i>)‐6‐Hydroxyhyacinthacine A₁ and (6<i>S</i>)‐6‐Hydroxy‐7a‐<i>epi</i>‐hyacinthacine A₁

Abstract

Abstract The total synthesis of natural (+)‐hyacinthacine A 1 ( 6 ), (+)‐7a‐ epi ‐hyacinthacine A 1 ( 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 non‐natural hydroxy analogues in excellent yields. The synthesis of (+)‐hyacinthacine A 1 ( 6 ) and of (+)‐7a‐ epi ‐hyacinthacine A 1 ( 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 A 1 ( 6 ) and its (6 R )‐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 A 1 ( 7 ) is a moderate inhibitor of amyloglucosidase and α‐ L ‐fucosidase; the presence of the (6 S )‐hydroxy group in 16 led to diminished activity.