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Deoxygenations of alcohols, i.e., processes that replace a hydroxyl group with hydrogen at a saturated carbon, find applications in both total synthesis and the systematic modifications of natural products. They may also be employed to introduce deuterium or tritium in a site‐specific manner. Reductive methods that involve ionic or highly polarized reagents or intermediates can be limited in their applicability: for example, competing reaction pathways including cationic rearrangements and anionic eliminations may be encountered in sterically hindered systems with substrates bearing heteroatoms close to the center undergoing reduction. As evidenced by developments over the last few decades, methods that involve the generation and direct quenching via hydrogen atom abstraction of the derived, carbon‐centered radical typically show the greatest tolerance for the presence of other functional groups and for variations in both the steric acid and the electronic environment in the vicinity of the center undergoing deoxygenation. Derivatization of the hydroxyl is a prerequisite, the determinant factors for efficient formation of the deoxygenated product lies in the ability of the combination of the substrate and reagents to induce homolysis of the C‐O bond coupled with the induction of homolysis to rapidly reduce a free radical by hydrogen donation, thereby propagating an efficient chain process. A high‐yielding way to realize this sequence was first described by Barton McCombie using the free‐radical chain reaction of O ‐thioacyl derivatives of secondary alcohols with tri‐ n ‐butylstannane. This chapter provides a detailed description and comparison of the combinations of substrates and reagents that will bring about these processes and provides a summary and evaluation of alternative deoxygenation methods. Mechanistic and stereochemical issues set out the scope and limitations of these processes with respect to both the thioacylation and reduction steps and exemplify some applications to both total synthesis and the modification of natural products.

A series of potent and selective cholecystokinin-B/gastrin receptor antagonists based on the dibenzobicyclo[2.2.2]octane (BCO) skeleton which have recently been described were found to show species-dependent behavior when examined in rat and dog models. We now report the discovery of compounds in which the BCO skeleton has been replaced with bicyclic, heteroaromatic frameworks, such as a 5,6-disubstituted indole or benzimidazole. These new ligands maintain the affinity and selectivity profile of the previous compounds in vitro but show a much more consistent behavior pattern in vivo. Representative examples of this class of compound have been shown to inhibit pentagastrin-stimulated acid secretion when administered intravenously at doses of 0.1 mumol kg-1 or less.

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X-ray crystallographic analysis of red crystals formed on mixing octacarbonyldicobalt(0) and BINAP, combined with NMR evidence obtained from a catalytic asymmetric Pauson-Khand reaction, suggests that this complex is a precatalyst to this reaction and leads to a new hypothesis for the role of axially chiral diphosphanes in the catalytic asymmetric Pauson-Khand reaction.The Pauson-Khand reaction (PKR), the [2 + 2 + 1] cyclocarbonylation of an alkyne and an alkene to form a cyclopentenone, is of considerable synthetic interest not only because cyclopentenones are useful building blocks for more elaborate structures but also because they are important biologically active compounds in their own right. For example, the cyclopentenone prostanoids have recently started to attract much interest, and this area of research is now providing promising candidates for, inter alia, antiinflammatory and antiviral pharmaceuticals. 1 While most applications of the PKR to date have used stoichiometric amounts of cobalt, there are now several catalytic versions of the reaction available that use a range of metals. 2 Initial exploratory attempts to introduce asymmetry into the reaction using titanium, 3 rhodium, 4 and iridium 5 catalysts have been encouraging, providing good enantioselectivities and turnover numbers.The feasibility of an asymmetric cobalt-catalyzed PKR has been demonstrated. 6 Using octacarbonyldicobalt(0) (20 mol %) as the cobalt source, a range of chiral diphosphanes (20 mol %) were tested on standard intramolecular substrates. While very modest enantioselectivities were observed for DIOP, DuPHOS, and planar chiral ferrocene diphosphanes, high selectivities were observed with the axially chiral ligand BINAP. For example, cyclocarbonylation of enyne 1a (eq 1) gave the product cyclopentenone 2a in 53% yield and 90% ee in 14 h. It was proposed that the phosphorus atoms of the BINAP ligand bridge the two cobalt atoms, while cyclopentenone formation occurs with participation of both cobalt centers 6 according to the generally accepted Pauson-Khand mechanism. 7 As a result of our interest in the cobalt-catalyzed PKR, 8 we initiated a study of its asymmetric version. After detailing our modified conditions for the cobaltcatalyzed asymmetric PKR based on axially chiral diphosphanes, we wish to report herein (a) the isolation of a hexacarbonyldicobalt(0) complex in which BINAP binds to just one of the two cobalts and (b) evidence that suggests that this complex is a precatalyst to asymmetric catalytic PKRs.We initially examined the cyclocarbonylation of enyne 1a in the presence of (S)-BINAP. Optimization of this reaction led to the use of 3.75 mol % of Co 4 (CO) 12 as the cobalt source, 9 which in our hands proved more robust and reliable than Co 2 (CO) 8 , together with 7.5 mol % of (S)-BINAP. After Co 4 (CO) 12 and (S)-BINAP were premixed, operating at 75°C under 1.05 atm of carbon monoxide for 5 h gave a 70% yield of cyclopentenone 2a and 89% ee (Table 1, entry 1). A survey of five other diphosphanes (Table 1, entries ...