Ketones chemoselective/stereoselective

In transforming bis-ketone 45 to keto-epoxide 46, the elevated stereoselectivity was believed to be a consequence of tbe molecular shape — tbe sulfur ylide attacked preferentially from tbe convex face of the strongly puckered molecule of 45. Moreover, the pronounced chemoselectivity was attributed to tbe increased electropbilicity of the furanone versus the pyranone carbonyl, as a result of an inductive effect generated by tbe pair of spiroacetal oxygen substituents at tbe furanone a-position. ... 

The solvent is very important for the hydrogenation of ketones. One of the most important factors in the liquid-phase hydrogenation of ketones is whether the medium is acidic, neutral, or basic, and a great deal of work has gone into attempting to understand chemoselectivity and stereoselectivity based on combinations of the metal catalyst and the reaction medium. 

The stereoselective reduction of the ketone function of 9 leads to a direct entry to selectively protected aldopentoses ( inversion strategy ) (Borysenko et al. 1989), which greatly expand the potential of this new protocol (Scheme 5). Following Evans protocol the tetramethylammo-nium triacetoxyborohydride-mediated reduction provides the yyn-diol 15 constituting a protected D-ribose (95%, >96% de). The anti-selective reduction to 17 was obtained after silyl protection of the free hydroxyl group of 9 to the OTBS-ether 16 using L-selectride. The aldopentose 18 was then accessible via chemoselective acetal cleavage followed by in situ cyclization (47% over two steps, >96% de). 

Hetero-Diels-Alder reactions of other a, -clhylenic trifluoromethyl ketones provide a good alternative access to trifluoromethyl-substitutcd dihydropyrans (Tabic 7). These cycloadditions occur with inverse electron demand with electron-rich heterodienophiles such as vinyl ethers. They are performed under mild thermal conditions or at room temperature and are chemoselective and rcgioselective. but generally not stereoselective. 

Reductive eliminations of halohydrin derivatives can also be used to prepare functionalized alkenes. Trihalomethanes react chemoselectively with aldehydes in the presence of ketones in a reaction initiated by CrCb. The ( )-isomer of the alkenyl iodide was formed stereoselectively in the example in equation (5). 

A more versatile reducing agent is samarium diiodide, which promotes chemoselective cyclizations of functionalized keto aldehydes in a stereodefined manner to form 2,3-dihydrocyclopentane carboxylate derivatives in good yields and with diastereoselectivities of up to 200 1 (equation 38)7 The reaction proceeds via selective one-electron reduction of the aldehyde component and subsequent nucleophilic attack on the ketone moiety. Stereochemical control is established by chelation of the developing diol (19) with Sm " " which thereby selectively furnishes cis diols (equation 39). The stereoselective M/-cyclization of 1,5-diketones to cis cyclopentane-1,2-diols using TiCU/Zn has been used to prepare stereodefined sterically hindered acyclic 1,2-diols when a removable heteroatom, such as sulfur or selenium, is included in the linking chain (equation 40). 

---