Total Synthesis of (+)-Hyacinthacine A1 Using a Chemoselective Cross-Benzoin Reaction and a Furan Photooxygenation-Amine Cyclization Strategy

We report the shortest synthesis of glycosidase inhibitor (+)-hyacinthacine A₁ using a highly chemoselective N-heterocyclic carbene-catalyzed cross-benzoin reaction as well as a furan photooxygenation-amine cyclization strategy. This is the first such cyclization on a furylic alcohol, an unprecedented reaction due to the notorious instability of the formed intermediates. The photooxygenation strategy was eventually incorporated into a three-step one-pot process that formed the requisite pyrrolizidine framework of (+)-hyacinthacine A₁.

Corrected Structure of Natural Hyacinthacine C1 via Total Synthesis

Hyacinthacines C₁ and C₄ are natural products that were isolated from Hyacinthoides non-scripta and Scilla socialis in 1999 and 2007, respectively. Despite their different ¹H NMR and ¹³C NMR spectroscopic data, these compounds have been assigned the same structures, including absolute configurations. This work details the total synthesis of natural (+)-hyacinthacine C₁, whose structure is confirmed as being the C-6 epimer of that reported. The synthetic strategy focused on inverting the configuration at C-1 of the final hyacinthacines via operating the inversion at the corresponding carbon atom in three previously synthesized intermediates. To do this, the advanced intermediates were subjected to Swern oxidation, followed by a stereoselective reduction with L-Selectride. This approach led to the synthesis of (+)-5 -epi-hyacinthacine C₁ (15), the corrected structure for (+)-hyacinthacine C₁ (19), (+)-6,7-di- epi-hyacinthacine C₁ (23), and (+)-7- epi-hyacinthacine C₁ (29). Glycosidase inhibition assays revealed that (+)-hyacinthacine C₁ (19) proved the most active, with IC₅₀ values of 33.7, 55.5, and 78.2 μM, against the α-glucosidase of rice, human lysosome, and rat intestinal maltase, respectively.

Indium(iii)-catalyzed tandem synthesis of 2-alkynyl-3,3-dichloropyrrolidines and their conversion to 3-chloropyrroles

A convenient route to 2-alkynyl-3,3-dichloropyrrolidines was described by addition of terminal acetylenes to 2,2,4-trichloroaldimines using In(OTf)₃ as a catalyst. In the presence of a base these pyrrolidines were transformed into 3-chloropyrroles.

Pyrrolizidine alkaloids

This review covers pyrrolizidine alkaloids isolated from natural sources. Topics include: aspects of structure, isolation, and biological/pharmacological studies; total syntheses of necic acids, necine bases and closely-related non-natural analogues.