Chemo-, regio-, and stereoselectivity of F-ring opening reactions in the cephalostatin series

Performance of Green Desymmetrization Methods toward Bioactive Cephalostatin Analogues

Since the discovery of cephalostatins, which have shown remarkable activity against human cancer cells, they have attracted the attention of researchers to target the synthesis of such impressive, complicated molecules using the green desymmetrization approach. In the current review, we report the progress in the desymmetrization of symmetrical bis-steroidal pyrazines (BSPs) as an approach toward potentially active anti-- cancer agents, namely cephalostatins/ ritterazines. The achievement of synthesizing a gram-scaled prodrug with comparable activity to the potent natural cephalostatins using green methods is our primary target. These synthetic methods can be scaled up based on the symmetrical coupling (SC) of two steroidal units of the same type. Our secondary target is the discovery of new green pathways that help in structural reconstruction programming toward the total synthesis of at least one potentially active family member. The strategy is based on functional group interconversions with high flexibility and brevity using green selective methods. The introduction of controlling groups using nontrivial reconstruction methodologies forms the backbone of our work. After certain modifications to the symmetrical BSP starting material, the resulting analogs underwent several chemoselective transformations through three main routes in rings F, D, and C. One of these routes is the chemoselective spiroketal opening (ring-F). The second route was the functionalization of the Δ14,15 bond (ring-D), including chlorination/dechlorination, in addition to epoxidation/ oxygenation processes. Finally, the introduction of the C-11 methoxy group as a directing group on ring-C led to several chemoselective transformations. Moreover, certain transformations on C-12 (ring-C), such as methylenation, followed by hydroboration- oxidation, led to a potentially active analog. The alignment of these results directs us toward the targets. Our efforts culminated in preparing effective anti-cancer prodrugs (8, 24, 30, and 31), which are able to overcome cancer drug resistance (chemoresistance) by inducing the atypical endoplasmic reticulum-mediated apoptosis pathway, which works through the release of Smac/Diablo and the activation of caspase-4.

Novel double functional protection of cephalostatin analogues using a gas-free chlorination method

Herewith, we report on a method that allows to simultaneously protect both the ∆^14,15 bond and the carbonyl group of the symmetrical bis-steroidal diketone 2. We found that environmentally friendly and gas-free chlorination is ideally suited to achieve this goal. This method was discovered during our efforts to methoxylate 2 in a solution of dichloromethane and basic methanol in the presence of diacetoxy iodobenzene. Unexpectedly, the ∆^14,15 bonds were chlorinated once as well as twice in a statistical manner. Interestingly, the singly dichlorinated desymmetrized product is an ideal precursor for conduction a series of position selective transformations. Importantly, the carbonyl group present in the nonchlorinated hemisphere can be selectively reduced, olefinated or oximated, while the other carbonyl group stays unaltered. A structurally related “monomeric” steroid derivative undergoes ∆^14,15 chlorination and 11-position methoxylation under same conditions. These findings represent a powerful entry for preparing new nonsymmetrical cephalostatin derivatives.

The Cephalostatins. 24. Isolation, Structure, and Cancer Cell Growth Inhibition of Cephalostatin 20

For the purpose of advancing knowledge of the structural variations available in the natural cephalostatins contained in the marine worm Cephalodiscus gilchristi, the isolation and structure of the 20th member (1) has been accomplished (10(-7) % yield). In turn cephalostatin 20 (1) proved to be enough for an initial SAR study comprising six important human cancer cell lines. A parallel objective was aimed at the possible discovery of a natural cephalostatin with a more accessible structure for total synthesis and/or synthetic modifications, but with powerful cancer cell growth inhibition.

Cephalostatins and ritterazines

This review article is a tribute to the numerous chemists whose relentless effort for the last quarter of a century resulted in the isolation, identification, and finally the chemical synthesis of a family of bis-steroidal pyrazine alkaloids of marine origin. In the task of defeating cancer, the search for bioactive substances among the naturally occurring compounds is, without any doubt, a preferential approach. The remarkable contribution of Petitt, Fusetani, and their coworkers allowed to discover this family of marine alkaloids that emerge as potential therapeutic anticancer agents, although there is still a long way to go. The challenging and dangerous task of collecting living organisms from deep-waters was followed by a laborious isolation, elucidation of the complicated structures and biological tests. The outcome of this paramount effort was the identification of 45 compounds that stand, to date, as some of the most potent anticancer agents. The intriguing structures of the isolated alkaloids drew the attention of synthetic chemists, valiant enough to undertake the challenging task of synthesizing some of the most active members of the family. Fuchs, Heathcock, Winterfeldt, Suarez, Shair, and their associates pioneered in the establishment of feasible synthetic routes for the preparation of some of the naturally occurring compounds and a large number of synthetic analogs, allowing to establish SAR criteria that have guided the design of new synthetic analogs. Numerous analogs have been prepared to investigate the mechanism of action of bis-steroidal pyrazines, e.g. cephalostatin analogs bearing a strained spiroketal moiety. However, the mechanism of action and the biological target of these compounds remain far from being understood. Therefore, the rational design of simpler, yet highly active analogs seems at the current stage elusive. It is still 1 to clear why these compounds need to be dimeric to show high biological activity. Furthermore, it is not known whether the central pyrazine ring is simply a linker or has some additional function. This could be tested by examining the biological activity of steroidal dimers with other linkers, e.g. with a benzene ring. Such analogs have been actually prepared but without functional groups necessary for biological activity. The clinical trials of cephalostatins have got stuck due to a shortage of material. There is an urgent need to provide highly active, yet not too complex analogs, which could be available in substantial amounts for advanced pharmacological studies.

The cephalostatins. 22. synthesis of bis-steroidal pyrazine pyrones (1)

Cephalostatin 1 (1), a remarkably strong cancer cell growth inhibitory trisdecacyclic, bis-steroidal pyrazine isolated from the marine tube worm Cephalodiscus gilchristi, continues to be an important target for practical total syntheses and a model for the discovery of less complex structural modifications with promising antineoplastic activity. In the present study, the cephalostatin E and F rings were greatly simplified by replacement at C-17 with an α-pyrone (in 12), typical of the steroidal bufodienolides, and by a dihydro-γ-pyrone (in 16). The synthesis of pyrazine 12 from 5α-dihydrotestosterone (nine steps, 8% overall yield) provided the first route to a bis-bufadienolide pyrazine. Dihydro-γ-pyrone 16 was synthesized in eight steps from ketone 13. While only insignificant cancer cell growth inhibitory activity was found for pyrones 12 and 16, the results provided further support for the necessity of more closely approximating the natural D-F ring system of cephalostatin 1 in order to obtain potent antineoplastic activity.

The cephalostatins. 21. Synthesis of bis-steroidal pyrazine rhamnosides (1)

The synthesis of bis-steroidal pyrazines derived from 3-oxo-11,21-dihydroxypregna-4,17(20)-diene (4) and glycosylation of a D-ring side chain with α-L-rhamnose have been summarized. Rearrangement of steroidal pyrazine 10 to 14 was found to occur with boron triflouride etherate. Glycosylation of pyrazine 10 using 2,3,4-tri-O-acetyl-α-L-rhamnose iodide led to 1,2-orthoester-α-L-rhamnose pyrazine 17b. By use of a persilylated α-L-rhamnose iodide as donor, formation of the orthoester was avoided. Bis-steroidal pyrazine 10 and rhamnosides 17b and 21c were found to significantly inhibit cancer cell growth in a murine and human cancer cell line panel. Pyrazine 9 inhibited growth of the nosocomial pathogen Enterococcus faecalis.