Total synthesis of sparstolonin B via a palladium-catalyzed aldehyde α-arylation

A concise and convergent total synthesis of sparstolonin B was developed. A palladium-catalyzed aldehyde α-arylation was utilized to construct the carbon skeleton of the natural product. A subsequent simple one-pot procedure effected global deprotection and closure of the final two rings via an unusual autoredox mechanism for the conversion of a bis-hydroquinone intermediate to the natural product. The 6 step synthetic sequence was realized in 18% overall yield.     

Reaction of phenyl radicals with propyne

The potential energy surface (PES) for the phenyl + propyne reaction, which might contribute to the growth of polycyclic aromatic hydrocarbons (PAHs) under a wide variety of reaction conditions, is described. The PES was characterized at the B3LYP-DFT/6-31G(d) and B3LYP-DFT/6-311+G(d,p) levels of theory. The energies of the entrance transition states, a direct hydrogen-transfer channel and two addition reactions leading to chemically activated C(9)H(9) intermediates, were also evaluated at the QCISD(T)/ 6-311G(d,p) and CCSD(T)/6-311G(d,p) levels of theory. An extensive set of unimolecular reactions was examined for these activated C(9)H(9)(dagger) intermediates, comprising 70 equilibrium structures and over 150 transition states, and product formation channels leading to substituted acetylenes and allenes such as PhCCH, PhCCCH(3), and PhCHCCH(2) were identified. The lowest energy pathway leads to indene, a prototype PAH molecule containing a five-membered ring. The title reaction thus is an example of possible direct formation of a PAH containing a five-membered ring, necessary to explain formation of nonplanar PAH structures, from an aromatic radical unit and an unsaturated hydrocarbon bearing an odd number of carbons. Extensive Supporting Information is available.     

Synthesis of the C1-C21 (C1'-C21') fragment of the dimeric polyketide natural product SCH 351448

[structure: see text] A convergent, stereoselective assembly of the C1-C21 (C1'-C21') fragment of SCH 351448, a 28-membered bis-lactone natural product, has been developed. A highly efficient approach to this fragment assembles 75% of the carbon skeleton and all the stereochemical elements present in the natural product. In addition, an interesting boron ligand effect on the diastereoselectivity of a key aldol reaction with methyl ketone-derived enolborinates is reported.     

Hydrogen-Bonded Complexes of Diaminopyridines and Diaminotriazines: Opposite Effect of Acylation on Complex Stabilities

The association behavior of several 2,4-diamino-s-triazines, 2,6-diaminopyridines, and their acylated derivatives with uracil derivatives was studied. In solution (1)H-NMR and IR spectroscopy were used, and in the solid state as (co)crystals X-ray diffraction was used. Acylation of 2,6-diaminopyridine leads to an increase of the association constant in CDCl(3) of the complexes with N-propylthymine from 84 to 440-920 M(-)(1), whereas acylation of diamino-s-triazines leads to a dramatic fall in the association constant of the complexes with N-propylthymine from 890 to ca. 6 M(-)(1). This phenomenon is related to different conformational preferences of these compounds. The amide groups in bis(acylamino)pyridines prefer a trans conformation, with the carbonyl group anti with respect to the ring nitrogen and coplanar with the aromatic ring. The amides of bis(acylamino)triazines, however, reside predominantly in a cis conformation. Repulsive secondary electrostatic interactions between the cis-amide and uracil carbonyl groups are thought to be responsible for the low association constant of complexes of bis(acylamino)triazines with uracils. The relatively high dimerization constants of bis(acylamino)triazines have been rationalized by the strong tendency to dimerize via quadruple hydrogen bonding.     

Origin of byproducts during the catalytic autoxidation of cyclohexane

The formation of byproducts during the Co(acac)2 and Cr(acac)3-catalyzed cyclohexane autoxidation is compared with the noncatalyzed thermal process. CoII ions seem to cause only a moderate perturbation of the reaction mechanism, causing a fast conversion of the cyclohexyl hydroperoxide via a redox cycle, rather than via abstraction of the alphaH-atom by chain carrying peroxyl radicals. Nevertheless, both the radical propagation and the CoII-induced decomposition of the hydroperoxide cause the formation of cyclohexoxy radicals that are partially transformed to 6-hydroxyhexanoic acid, the major primary byproduct for these systems. However, during the CoII-catalyzed reaction, the concentration of cyclohexanone increases much faster than that of the hydroperoxide, causing the ketone to take over the role of dominant byproduct source. A mechanism for the conversion of cyclohexanone to ring-opened byproducts is put forward. Cr(acac)3 seems to catalyze additional reactions, some of them probably leading directly to byproducts. Indeed, the evolution of (by)products is significantly different from the CoII-catalyzed and the thermal systems, in the sense that they all seem to be primary in origin.