Unified, Radical-Based Approach for the Synthesis of Spiroketals

The Xanthate Route to Ketones: When the Radical Is Better than the Enolate

The alkylation of enolates is one of the backbones of ketone chemistry, yet in practice it suffers from numerous limitations due to problems of regiochemistry (including O- versus C-alkylation), multiple alkylations, self-condensation, competing elimination, and incompatibility with many polar groups that have to be protected. Over the years, various solutions have been devised to overcome these difficulties, such as the employment of auxiliary ester or sulfone groups to modify the p K_a of the enolizable hydrogens, the passage by the corresponding hydrazones, the use of transition-metal-catalyzed redox systems to formally alkylate ketones with alcohols, etc. Most of these hurdles disappear upon switching to α-ketonyl radicals. Radicals are tolerant of most polar functions, and radical additions to flat sp² centers are generally easier to accomplish than enolate substitution at tetrahedral sp³ carbons. The main stumbling block, however, has been a lack of generally applicable methods for the generation and intermolecular capture of α-ketonyl radicals. We have found over the past years that the degenerative exchange of xanthates represents in many ways an ideal solution to this problem. It overcomes essentially all of the difficulties faced by other radical processes because of its unique ability to reversibly store reactive radicals in a dormant, nonreactive form. The lifetime of the radicals can therefore be significantly enhanced, even in the concentrated medium needed for bimolecular additions, while at the same time regulating their absolute and relative concentrations. The ability to perform intermolecular additions to highly functionalized alkene partners opens up numerous possibilities for rapid and convergent access to complex structures. Of particular importance is the elaboration of ketones that are prone to self-condensation, such trifluoroacetone, and of base-sensitive ketones, such as chloro- and dichloroacetone, since the products can be used for the synthesis of a myriad fluorinated and heteroaromatic compounds of relevance to medicinal chemistry and agrochemistry. The formal distal dialkylation of ketones, also of utmost synthetic interest, is readily accomplished, allowing convenient access to a wide array of useful ketone building blocks. Cascade processes can be implemented and, in alliance with powerful classical reactions (aldol, alkylative Birch reductions, etc.), furnish a quick route to complex polycyclic scaffolds. Furthermore, the presence of the xanthate group in the adducts can be exploited to obtain a variety of arenes and heteroarenes, such as pyrroles, thiophenes, naphthalenes, and pyridines, as well as enones, dienes, and cyclopropanes. Last but not least, the reagents and most of the starting materials are exceedingly cheap, and the reactions are safe and easy to scale up.

Use of a benzyl ether as a traceless hydrogen donor in the anti-Markovnikov hydrofunctionalization of alkenes with xanthates

A new protocol for the anti-Markovnikov hydrofunctionalization of alkenyl alcohol O-Bn ethers was developed using xanthates as functionalizing agents in the presence of lauroyl peroxide as a radical initiator and a stoichiometric oxidant. The benzyl group serves as a traceless hydrogen donor in the remote radical hydrogen atom transfer event during the process.

A Radical Bidirectional Fragment Coupling Route to Unsymmetrical Ketones

A powerful strategy for the regioselective bidirectional synthesis of unsymmetrically substituted ketones is described, relying on the fact that the exchange of a xanthate is much faster than the radical addition to an unactivated alkene. The use of an alkene as the formal "alkylating" agent associated with the tolerance for numerous functional groups and the mildness of the experimental conditions removes many of the problems associated with the classical ionic and transition-metal-based approaches.

Some aspects of radical chemistry in the assembly of complex molecular architectures

This review article describes briefly some of the radical processes developed in the authors' laboratory as they pertain to the concise assembly of complex molecular scaffolds. The emphasis is placed on the use of nitrogen-centred radicals, on the degenerate addition-transfer of xanthates, especially on its potential for intermolecular carbon-carbon bond formation, and on the generation and capture of radicals through electron transfer processes.

Synthesis of [n,5]-spiroketals by ring enlargement of donor-acceptor-substituted cyclopropane derivatives

Exocyclic enol ethers served as starting materials for the synthesis of [n,5]-spiroketals (n = 5, 6). A metal-mediated cyclopropanation using ethyl diazoacetate afforded spiroannelated cyclopropane derivatives bearing an ester group. A reduction of the corresponding ester by LiAlH(4), followed by subsequent oxidation using hypervalent iodine reagents, produced [n,5]-spiroketals in moderate to good yields. The key step within this three-step sequence is the ring enlargement of the three-membered ring with an oxygen donor and a carbonyl acceptor group into the five-membered enol ether system. Catalytic amounts of the Lewis acid Yb(OTf)(3) facilitate the ring enlargement and increase the yield of the corresponding spiroketal in many cases. When Yb(OTf)(3) was used, our experiments revealed an open transition state rather than a concerted mechanism because the stereochemistry of the spirocenter was not conserved during the ring enlargement. As a result, the thermodynamically more favored anomeric [n,5]-spiroketal was observed as the major product. All the structures were established unambiguously by NOESY experiments.