Deletion of the C26 Methyl Substituent from the Bryostatin Analogue Merle 23 Has Negligible Impact on Its Biological Profile and Potency

Highly Diastereoselective Chelation-Controlled 1,3-anti-Allylation of (S)-3-(Methoxymethyl)hexanal Enabled by Hydrate of Scandium Triflate

En route to the total synthesis of (+)-Neopeltolide, we explored Lewis acid-assisted diastereoselective allylation of MOM-protected 3-hydroxylhexanal with β-(2,2-diethoxyethyl)-substituted (allyl)tributylstannane. The hydrated form of scandium triflate was found to be essential for attaining high 1,3-anti-diastereoselectivity (d.r. 94:6), while the use of anhydrous catalyst resulted in a modest diastereocontrol (d.r. 76:24). The preferred 1,3-anti-selectivity in this transformation can be rationalized in the framework of the Reetz chelate model of asymmetric induction. The 1,3-anti-configuration of the product was confirmed by its conversion into the known C7-C16 building block of (+)-Neopeltolide. We also report an improved protocol for the synthesis of β-(2,2-diethoxyethyl)-substituted (allyl)tributylstannane, which can be utilized as a cost-efficient bipolar isoprenoid-type C5-building block in the synthesis of natural compounds.

Preclinical and Clinical Studies on Bryostatins, A Class of Marine-Derived Protein Kinase C Modulators: A Mini-Review

Bryostatins are complex macrolactones isolated from marine organisms Bryozoan Bugula neritina. They are potent modulators of protein kinase C isozymes (PKCα: ki = 1.3-188 nM), and are one of the most extensively investigated marine natural products in clinical trials. Although ~21 natural bryostatins have been isolated, however only bryostatin-1 (1) has received much interest among medicinal chemists and clinicians. The structure-activity relationship of bryostatins has been well established, with the identification of key pharmacophoric features important for PKC modulation. The low natural abundance and the long synthetic route have prompted medicinal chemists to come-up with simplified analogs. Bryostatin skeleton comprises three pyran rings connected to each other to form a macrocyclic lactone. The simplest analog 27 contains only one pyran, which is also able to modulate the PKCα activity; however, the cyclic framework appears to be essential for the desired level of potency. Another simplified analog 17 ("picolog") exhibited potent and in-vivo efficacy against lymphoma. Bryostatin-1 (1) has shown an acceptable intravenous pharmacokinetic profile in mice and displayed promising in-vivo efficacy in mice models of various cancers and Alzheimer's disease. Bryostatin-1 was investigated in numerous Phase I/II oncology clinical trials; it has shown minimal effect as a single agent, however, provided encouraging results in combination with other chemotherapy agents. FDA has granted orphan drug status to bryostatin-1 in combination with paclitaxel for esophageal cancer. Bryostatin-1 has also received orphan drug status for fragile X syndrome. Bryostatin-1 was also investigated in clinical studies for Alzheimer's disease and HIV infection. In a nutshell, the natural as well as synthetic bryostatins have generated a strong hope to emerge as treatment for cancer along with many other diseases.

Function-Oriented Synthesis: Design, Synthesis, and Evaluation of Highly Simplified Bryostatin Analogues

Using a function-oriented synthesis strategy, we designed, synthesized, and evaluated the simplest bryostatin 1 analogues reported to date, in which bryostatin's A- and B-rings are replaced by a glutarate linker. These analogues, one without and one with a C26-methyl group, exhibit remarkably different protein kinase C (PKC) isoform affinities. The former exhibited bryostatin-like binding to several PKC isoforms with K_i's < 5 nM, while the latter exhibited PKC affinities that were up to ∼180-fold less potent. The analogue with bryostatin-like PKC affinities also exhibited bryostatin-like PKC translocation kinetics in vitro, indicating rapid cell permeation and engagement of its PKC target. This study exemplifies the power of function-oriented synthesis in reducing structural complexity by activity-informed design, thus enhancing synthetic accessibility, while still maintaining function (biological activity), collectively providing new leads for addressing the growing list of therapeutic indications exhibited by PKC modulators.

Unlocking the Drug Potential of the Bryostatin Family: Recent Advances in Product Synthesis and Biomedical Applications

Bryostatins are a class of naturally occurring macrocyclic lactones with a unique fast developing portfolio of clinical applications, including treatment of AIDS, Alzheimer's disease, and cancer. This comprehensive account summarizes the recent progress (2014-present) in the development of bryostatins, including their total synthesis and biomedical applications. An emphasis is placed on the discussion of bryostatin 1, the most-studied analogue to date. This review highlights the synthetic and biological challenges of bryostatins and provides an outlook on their future development.