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The first structural characterization of a mononuclear, EPR-active, oxosulfido-Mo(V) compound related to the very rapid form of xanthine oxidase (VR-XnO) is reported. The compound, [CoCp2][TpiPrMoVOS(2-OC6H4CO2Et)] [Cp = cyclopentadienyl; TpiPr = hydrotris(3-isopropylpyrazol-1-yl)borate], exhibits a distorted octahedral geometry with Mo=O and Mo=S distances of 1.761(5) and 2.215(2) Å, respectively, and an O=Mo=S angle of 107.33(14)°. Significantly, the MoV=S distance is much shorter than the value of 2.36 Å reported for oxosulfido-Mo(V) compounds (Singh, R.; et al. Inorg. Chem. 1989, 28, 8) but close to the range established for VR-XnO by protein crystallography. The methyl and phenyl esters were also prepared but the latter is highly reactive and undergoes an intramolecular, radical-based cyclization/elimination reaction to form [CoCp2][TpiPrMoIVO{2-OC6H4C(O)S-κO,κS}]. This study provides the first definitive measurement of the MoVS bond distance in an unambiguously characterized oxosulfido-Mo(V) compound and supports the presence of a short (ca. 2.22 Å) Mo=S bond in VR-XnO. It also demonstrates that the MoV=S moiety participates in radical-based reactions that are facilitated by the facile redox interplay of Mo and S and by substrates susceptible to radical eliminations. [ABSTRACT FROM AUTHOR]

Palladium-mediated hydrostannylation of alkynoyl compounds is combined with Stille-Scott cross-coupling (reductive-coupling) to give one-pot access to divinyl and aryl vinyl ketones, which undergo Nazarov cyclization to give cyclopentenones upon treatment with acid. This reaction sequence has been studied with a variety of different substitution patterns, including the use of oxazolidinone auxiliaries to achieve torquoselectivity in the Nazarov cyclization. Through a combination of good yields and moderate to good levels of stereochemical induction, this approach affords efficient, convergent, and asymmetric access to a variety of different cyclopentanoid systems. [ABSTRACT FROM AUTHOR]

Reductive coupling of α,β-unsaturated acid chlorides A with alkynoyls B provides convergent access to Nazarov cyclization precursors, α-carboxy divinyl ketones C. Cyclization of C gives an intermediate oxyallyl cation intermediate D, which can be trapped with tethered arenes (Ar). The resultant products can be further cyclized through nucleophilic displacement of suitable leaving groups X by tethered OH groups to give lactones (in a subsequent step). Where X is a suitable chiral auxiliary (e.g., oxazolidinone) this strategy affords access to homochiral cyclopentanoids. [ABSTRACT FROM AUTHOR]

The early stages of the retro-Diels-Alder reaction are clearly apparent in the structures of the cycloadducts formed between furan or 5-trimethylsilylcyclopentadiene with maleic anhydride and N-methylmaleimide. The degree of lengthening of the C-C bonds that break in this reaction is clearly related to the known reactivity of these cycloadducts toward this reaction. In the structures of the cycloadducts 21 and 22 derived from 2-methoxyfuran, the early stages of an alternative fragmentation reaction are apparent, consistent with the reactivity of these compounds in solution. [ABSTRACT FROM AUTHOR]

The intramolecular Diels-Alder reaction has been used as a powerful method to access the tricyclic core of the eunicellin natural products from a number of 9-membered-ring precursors. The endo/exo selectivity of this reaction can be controlled through a remarkable organocatalytic approach, employing MacMillan's imidazolidinone catalysts, although the mechanistic origin of this selectivity remains unclear. We present a combined experimental and density functional theory investigation, providing insight into the effects of medium-ring constraints on the organocatalyzed intramolecular Diels-Alder reaction to form the isobenzofuran core of the eunicellins. [ABSTRACT FROM AUTHOR]

Oxazolidinones are powerful promoters of the Nazarov reaction, enabling the cyclization of conventionally resistant substrates to be achieved under mild conditions. They exert excellent regio- and torquoselective control in both the conventional Nazarov reaction giving cyclopentenones and in the “interrupted” Nazarov reaction, giving more highly substituted multistereocenter containing products. [ABSTRACT FROM AUTHOR]

Computational, solution phase, and crystal structure analysis of 2- and 4-organoselenylmethyl-substituted pyridinium ions (10a-c and 11a-c) provides strong evidence for C-Se hyperconjugation (σC-Se-π*) between the C-Se σ-bond and the π-deficient aromatic ring and a through-space interaction (nSe-π*) between the selenium p-type lone pair and the π-deficient aromatic ring. There is also a weak anomeric-type interaction (nSe-σ*CC) involving the selenium p-type lone pair electrons and the polarized CH2-C(Ar) σ-bond. NBO analysis of calculated cations with varying electron demand (B3LYP/6-311++G**) show that C-Se hyperconjugation (σC-Se-π*) is the predominant mode of stabilization in the weakly electron-demanding pyridinium ions (10d, 11d, 14, and 15); however, the through-space (nSe-π*) interaction becomes more important as the electron demand of the β-Se-substituted carbocation increases. The anomeric interaction (nSe-σ*CC) is relatively weak in all ions. [ABSTRACT FROM AUTHOR]