•Synthesis of ritterostatin, a hybrid analog of ritterazine and cephalostatin.•Ritterostatin synthesis involves reductive/oxidative modifications of hecogenin acetate.•Evaluation of the impact of C23 hydroxyl group on bioactivity of ritterostatin.Cephalostatins, ritterazines and their hybrid bis-steroidal pyrazine analogs such as 25-epi-rittereostatin GN1N show unusually high potency against a wide range of cancer cell lines. Herein, we report the synthesis and bioactivity of 23-deoxy-25-epi ritterostatin GN1N, which lacks the 23-hydroxyl group of 25-epi rittereostatin GN1N. The less oxygenated bis-steroidal pyrazine was ∼50- to 1000-fold less potent than 25-epi ritterostatin GN1N, highlighting the importance of the 23-hydroxyl group for the antiproliferative activity of the cephalostatin/ritterazine class of drugs.Download high-res image (154KB)Download full-size image

N7-Alkyl-2′-deoxyguanosines are major adducts in DNA that are generated by various alkylating mutagens and drugs. However, the effect of the N7 alkylation on the hydrogen-bonding patterns of the guanine remains poorly understood. We prepared N7-methyl-2′-deoxyguanosine (N7mdG)-containing DNA using a transition-state destabilization strategy, developed a novel polβ-host–guest complex system, and determined eight crystal structures of N7mdG or dG paired with dC, dT, dG, and dA. The structures of N7mdG:dC and N7mdG:dG are very similar to those of dG:dC and dG:dG, respectively, indicating the involvement of the keto tautomeric form of N7mdG in the base pairings with dC and dG. On the other hand, the structure of N7mdG:dT shows that the mispair forms three hydrogen bonds and adopts a Watson–Crick-like geometry rather than a wobble geometry, suggesting that the enol tautomeric form of N7mdG involves in its base pairing with dT. In addition, N7mdG:dA adopts a novel shifted anti:syn base pair presumably via the enol tautomeric form of N7mdG. The polβ-host–guest complex structures reveal that guanine-N7 methylation changes the hydrogen-bonding patterns of the guanine when paired with dT or dA and suggest that N7 alkylation may alter the base pairing patterns of guanine by promoting the formation of the rare enol tautomeric form of guanine.

Human DNA polymerase β (polβ) inserts, albeit slowly, T opposite the carcinogenic lesion O6-methylguanine (O6MeG) ∼30-fold more frequently than C. To gain insight into this promutagenic process, we solved four ternary structures of polβ with an incoming dCTP or dTTP analogue base-paired with O6MeG in the presence of active-site Mg2+ or Mn2+. The Mg2+-bound structures show that both the O6MeG·dCTP/dTTP–Mg2+ complexes adopt an open protein conformation, staggered base pair, and one active-site metal ion. The Mn2+-bound structures reveal that, whereas the O6Me·dCTP–Mn2+ complex assumes the similar altered conformation, the O6MeG·dTTP–Mn2+ complex adopts a catalytically competent state with a closed protein conformation and pseudo-Watson–Crick base pair. On the basis of these observations, we conclude that polβ slows nucleotide incorporation opposite O6MeG by inducing an altered conformation suboptimal for catalysis and promotes mutagenic replication by allowing Watson–Crick-mode for O6MeG·T but not for O6MeG·C in the enzyme active site. The O6MeG·dTTP–Mn2+ ternary structure, which represents the first structure of mismatched polβ ternary complex with a closed protein conformation and coplanar base pair, the first structure of pseudo-Watson–Crick O6MeG·T formed in the active site of a DNA polymerase, and a rare, if not the first, example of metal-dependent conformational activation of a DNA polymerase, indicate that catalytic metal-ion coordination is utilized as a kinetic checkpoint by polβ and is crucial for the conformational activation of polβ. Overall, our structural studies not only explain the promutagenic polβ catalysis across O6MeG but also provide new insights into the replication fidelity of polβ.

In this Letter, we report unexpected formation of E-ring opened steroidal alkaloid from primary amine presumably via a hypoiodite-mediated iodoamination followed by fragmentation of sterically congested iodospiroaminal.

Highlights•16,22-Diketocholesterol is a novel ligand for oxysterol-binding protein Osh4.•We have determined X-ray structure of 16,-22-diketocholesterol bound to Osh4.•Our diketocholesterol–Osh4 structure may represent a structural intermediate between open and closed conformations.We have synthesized 16,22-diketocholesterol, a novel ligand for oxysterol-binding protein Osh4, and determined X-ray structure of the diketocholesterol in complex with Osh4. The X-ray structure shows that α7 helix of Osh4 assumes open conformation while the rest of Osh4, closed conformation, implying this diketocholesterol–bound Osh4 structure may represent a structural intermediate between the two conformations.Download full-size image

•23-Deoxy-25-epi north unit of cephalostatin 1 was prepared from hecogenin acetate.•23-Deoxy-25-epi north 1 synthesis involves multiple reductions and oxidations.•Key steps include stereoselective 5/5-spiroketalization.An efficient synthesis of the 23-deoxy-25-epi north unit of cephalostatin 1 has been achieved in 17 steps via reductive and oxidative functionalizations of hecogenin acetate with an overall yield of 3.8%. This synthesis features transetherification-mediated E-ring opening, D-ring oxidation, hemiketalization-mediated E-ring closure, and stereoselective 5/5-spiroketalization.Download high-res image (183KB)Download full-size image