Divergent and gram-scale syntheses of (-)-veratramine and (-)-cyclopamine.

Veratramine and cyclopamine, two of the most representative members of the isosteroidal alkaloids, are valuable molecules in agricultural and medicinal chemistry. While plant extraction of these compounds suffers from uncertain supply, efficient chemical synthesis approaches are in high demand. Here, we present concise, divergent, and scalable syntheses of veratramine and cyclopamine with 11% and 6.2% overall yield, respectively, from inexpensive dehydro-epi-androsterone. Our synthesis readily provides gram quantities of both target natural products by utilizing a biomimetic rearrangement to form the C-nor-D-homo steroid core and a stereoselective reductive coupling/(bis-)cyclization sequence to establish the (E)/F-ring moiety.

Progress toward a Convergent, Asymmetric Synthesis of Jervine.

Progress toward a convergent approach for the enantioselective synthesis of the Veratrum alkaloid jervine is presented. The two requisite fragments were stereoselectively and efficiently fashioned from economical and readily available reagents. Key reactions include (a) a highly diastereoselective Ireland-Claisen rearrangement to establish the necessary cis-relationship between the amine and methyl group on the tetrahydrofuran E-ring; (b) a diastereoselective selenoetherification reaction that enabled the assembly of the D/E oxaspiro[4.5]decene in the needed configuration; and (c) an enzymatic desymmetrization of an abundant achiral diol en route to a key four-carbon building block as a practical alternative to a protected Roche ester reduction.

Stereoselective Synthesis of (-)-Verazine and Congeners via a Cascade Ring-Switching Process of Furostan-26-acid.

An efficient synthetic strategy for three natural seco-type cholestane alkaloids isolated from the Veratrum plants, based on commercially available naturally occurring and abundant (-)-diosgenin (1), as exemplified in the concise asymmetric synthesis of (-)-verazine (4), (-)-veramiline (5) (proposed structure), and its 22-epimer, (-)-oblonginine (6), is presented. This work highlights the application of a cascade ring-switching process of (-)-diosgenin to achieve the E-ring opening and construction of chiral six-membered lactone challenges in seco-type cholestane alkaloid synthesis. This approach enables the synthesis of related natural and nature-like novel cholestane alkaloids, opening up opportunities for more extensive exploration of cholestane alkaloid biology.

Discovery of potent and novel smoothened antagonists via structure-based virtual screening and biological assays.

The Hedgehog (Hh) signaling pathway plays a critical role in controlling patterning, growth and cell migration during embryonic development. Aberrant activation of Hh signaling has been linked to tumorigenesis in various cancers, such as basal cell carcinoma (BCC) and medulloblastoma. As a key member of the Hh pathway, the Smoothened (Smo) receptor, a member of the G protein-coupled receptor (GPCR) family, has emerged as an attractive therapeutic target for the treatment and prevention of human cancers. The recent determination of several crystal structures of Smo in complex with different antagonists offers the possibility to perform structure-based virtual screening for discovering potent Smo antagonists with distinct chemical scaffolds. In this study, based on the two Smo crystal complexes with the best capacity to distinguish the known Smo antagonists from decoys, the molecular docking-based virtual screening was conducted to identify promising Smo antagonists from ChemDiv library. A total of 21 structurally novel and diverse compounds were selected for experimental testing, and six of them exhibited significant inhibitory activity against the Hh pathway activation (IC50 < 10 µM) in a GRE (Gli-responsive element) reporter gene assay. Specifically, the most potent compound (compound 20: 47 nM) showed comparable Hh signaling inhibition to vismodegib (46 nM). Compound 20 was further confirmed to be a potent Smo antagonist in a fluorescence based competitive binding assay. Optimization using substructure searching method led to the discovery of 12 analogues of compound 20 with decent Hh pathway inhibition activity, including four compounds with IC50 lower than 1 µM. The important residues uncovered by binding free energy calculation (MM/GBSA) and binding free energy decomposition were highlighted and discussed. These findings suggest that the novel scaffold afforded by compound 20 can be used as a good starting point for further modification/optimization and the clarified interaction patterns may also guide us to find more potent Smo antagonists.

Construction of the tricyclic A-B-C core of the Veratrum alkaloids.

Organocatalyzed enantioselective allylation of 2-iodocyclohexenone followed by methylation and oxy-Cope rearrangement delivered enantiomerically enriched 2-methyl 3-allyl cyclohexanone, which engaged in acid-catalyzed Robinson annulation to give the bicyclic enone. Subsequent elaboration of the pendant allyl group into an α-diazo ß-keto ester set the stage for Rh-mediated cyclization to deliver the tricyclic A-B-C core of the Veratrum alkaloids.

Synthesis of A B C-ring subunit of C-nor-D-homo-steroidal alkaloids: towards the total synthesis of cyclopamine.

A practical approach to the synthesis of the A, B and C-ring subunit of cyclopamine has been developed. This synthetic tactic highlights the utility of mandelate acetal-mediated resolution of the fused ring ketone (±)-4 and IBX-mediated oxidation cascades from 12 to 9. The availability of advanced intermediates from enantiomerically pure (+)-4 and 2 could provide efficient access to biologically active and structurally diverse C-nor-D-homo-steroidal alkaloids such as cyclopamine.

A convenient and efficient one-pot conversion of peimisine into cyclopamine.

A convenient and efficient one-pot synthesis of cyclopamine from peimisine is described. The key steps involve one-pot hydrazination and subsequent Bamford-Stevens reaction. The mild reaction conditions, high overall yield as well as an easy purification indicate this process can potentially be used for the scale-up preparation of cyclopamine.

Enhancement of cyclopamine via conjugation with nonmetabolic sugars.

The Veratrum alkaloid cyclopamine, an inhibitor of cancer stem cell growth, was used as a representative scaffold to evaluate the inhibitory impact of glycosylation with a group of nonmetabolic saccharides, such as d-threose. In a five-step divergent process, a 32-member glycoside library was created and assayed to determine that glycosides of such sugars notably improved the GI50 value of cyclopamine while metabolic sugars, such as d-glucose, did not.

A piperidine chiron for the Veratrum alkaloids.

A Veratrum piperidine chiron was prepared over 11 steps (7.9% yield) from (-)-citronellal. Three methods for the installation of the propargylic side chain onto a cyclic enamide are presented.

A new cyclopamine glucuronide prodrug with improved kinetics of drug release.

We prepared a new glucuronide prodrug of cyclopamine designed to target selectively the Hedgehog signalling pathway of cancer cells. This prodrug includes a novel self-immolative linker bearing a hydrophilic side chain that can be easily introduced via"click chemistry". With this design, the prodrug exhibits reduced toxicity compared to the free drug on U87 glioblastoma cells. However, in the presence of ß-glucuronidase, the prodrug conducts to the quick release of cyclopamine thereby restoring its antiproliferative activity.

Exo-cyclopamine--a stable and potent inhibitor of hedgehog-signaling.

The combination of theoretical and computational studies with organic synthesis and biological investigations has led to exo-cyclopamine. This stable and highly potent derivative of cyclopamine promises big potential as an experimental drug against several types of human cancer.

Synthesis of all diastereomers of the piperidine--alkaloid substructure of cyclopamine.

All four diastereomers of the trisubstituted piperidine-alkaloids of the veratramine and jervine type were synthesized with complete stereocontrol starting from enantiopure citronellic acids. The flexible, high-yielding, and scalable route described here will facilitate convergent syntheses and give access to analogues of cyclopamine and other biologically active and diverse steroid alkaloids.

Design and synthesis of inhibitors of Hedgehog signaling based on the alkaloid cyclopamine.

The synthesis and biological evaluation of structurally simplified, metabolically stable cyclopamine-like Sonic Hedgehog (SHH) signaling inhibitors, i.e., 5, is described in four chemical steps from commercially available steroidal precursors. Biological evaluation of this cyclopamine analogue in two different systems establishes the high potency of 5 as a SHH signaling inhibitor. This approach provides important new lead structures for the development of new cancer chemotherapeutic agents based on the inhibition on SHH signaling.

Semisynthetic cyclopamine analogues as potent and orally bioavailable hedgehog pathway antagonists.

Herein is reported the synthesis of a novel class of hedgehog antagonists derived from cyclopamine. The acid sensitive D-ring of cyclopamine was homologated utilizing a sequence of chemoselective cyclopropanation and stereoselective acid-catalyzed rearrangement. Further modification of the A/B-ring homoallylic alcohol to the conjugated ketone led to the discovery of new cyclopamine analogues with improved pharmaceutical properties and in vitro potency (EC 50) ranging from 10 to 1000 nM.

Synthesis and anticancer activity studies of cyclopamine derivatives.

A diversity-oriented synthesis has been developed for facile construction of a library of carbohydrate-cyclopamine conjugates. The synthetic protocol is suitable for generating cyclopamine derivatives with various structural motifs for exploring the desired activity. From this initial library, we have observed one derivative that exhibits improved activity against lung cancer cell as compared to cyclopamine.