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We describe a synthetic strategy for the angelimicin family of anthraquinoid natural products that involves converting a central highly oxygenated decalin intermediate to the AB and A'B' subunits. Herein, we report the synthesis of the bicyclic A'B' subunit that complements our earlier route to the tricyclic AB framework. The differentiating tact in the two syntheses focused on controlling the Suárez radical fragmentation of lactol precursors by modulating the substrate's structural rigidity. A more flexible lactol gave the tricyclic AB framework, whereas a more rigid substrate led to the bicyclic A'B' precursor, presumably through divergent pathways from the radical produced in the initial fragmentation step. These results establish a versatile advanced synthetic precursor for the angelimicins, and on a more general note, illustrate strategies for applying the Suárez fragmentation to diverse and complex molecular frameworks.

We report a facile, high-yield synthesis and characterization of discrete, ternary porphyrin–metal–polyoxometalate (por–M–POM) complexes where a group IV transition-metal ion is bound both to the porphyrin core and to the lacunary site of a Keggin POM, PW11O39–7. The remarkably robust complexes exploit the fact that HfIV and ZrIV are 7–8 coordinate and reside outside the plane of the porphyrin macrocycle, thus enabling the simultaneous coordination to meso-tetraphenylporphyrin (tpp) or meso-tetra(4-pyridyl)porphyrin (tpyp) and to the defect site in the Keggin framework. The physical properties of the (tpp)Hf(PW11O39)[tba]5, (tpyp)Hf(PW11O39)[tba]5, and (tpp)Zr(PW11O39)[tba]5 complexes are similar because the metal ions have similar oxidation states and coordination chemistry. This architecture couples the photonic properties of the porphyrin to the POM because the metal ion is incorporated into both frameworks. Thus, the ternary complexes can serve as a basis for the characterization of HfIV and ZrIV porphyrins bound to oxide surfaces through the group IV metal ions. The Hf(por) and Zr(por) bind strongly to TiO2 nanoparticles and indium tin oxide (ITO) surfaces, but significantly less binds to crystalline SiO2 or TiO2 surfaces. Together, the strong binding of the metalloporphyrins to the POM, nanoparticles, and the ITO surfaces, and paucity of binding to crystalline surfaces, suggests that the three-to-four open coordination sites on the Hf(por) and Zr(por) are predominantly bound at surface defect sites. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Alkyl, aryl, and metallocenyl azides are tolerated for the preparation of the Julia-Kociensky substrates.