Ketones, unsaturated hydroxylation

Haloalkanes are readily oxidized to the corresponding aldehydes or ketones. The best yields are attained with secondary alcohols and unsaturated hydroxyl groups [5]. a-Nitroketones, which are valuable intermediates in organic synthesis, are... 

A-ring unsaturated ketones and the sensitive 11 j5-hydroxyl group will survive conditions of the Mattox rearrangement. 

N,O-acetal intermediate 172, y,<5-unsaturated amide 171. It is important to note that there is a correspondence between the stereochemistry at C-41 of the allylic alcohol substrate 173 and at C-37 of the amide product 171. Provided that the configuration of the hydroxyl-bearing carbon in 173 can be established as shown, then the subsequent suprafacial [3,3] sigmatropic rearrangement would ensure the stereospecific introduction of the C-37 side chain during the course of the Eschenmoser-Claisen rearrangement, stereochemistry is transferred from C-41 to C-37. Ketone 174, a potential intermediate for a synthesis of 173, could conceivably be fashioned in short order from epoxide 175. 

The general mechanistic features of the aldol addition and condensation reactions of aldehydes and ketones were discussed in Section 7.7 of Part A, where these general mechanisms can be reviewed. That mechanistic discussion pertains to reactions occurring in hydroxylic solvents and under thermodynamic control. These conditions are useful for the preparation of aldehyde dimers (aldols) and certain a,(3-unsaturated aldehydes and ketones. For example, the mixed condensation of aromatic aldehydes with aliphatic aldehydes and ketones is often done under these conditions. The conjugation in the (3-aryl enones provides a driving force for the elimination step. 

Amino alcohols (33) were transformed to ketones on copper.360 361 The transformation involves the dehydrogenation of the hydroxyl group, the elimination of dimethylamine, and the hydrogenation of the unsaturated ketone (Scheme 4.111). 

Compound 12 contains a vinylic hydroxyl group and thus is expected to be unstable at higher temperatures in solution. Under the conditions of matrix isolation, however, it is perfectly stable and a third photochemical activation is required to induce the tautomerization to the unsaturated ketone 13. 

Other functional groups which have a heteroatom rather than a hydroxyl group capable of directing the hydrogenation include alkoxyl, alkoxycarbonyl, carboxylate, amide, carbamate, and sulfoxide. The alkoxy unit efficiently coordinates to cationic iridium or rhodium complexes, and high diastereoselectivity is induced in the reactions of cyclic substrates (Table 21.3, entries 11-13) [25, 28]. An acetal affords much lower selectivity than the corresponding unsaturated ketone (Table 21.3, entries 14 and 15) [25]. 

The N-anthracenylmethyl ammonium catalysts 11 and 12 also proved to be very effective in the asymmetric epoxidation of a,p-unsaturated ketones utilizing aqueous sodium165,661 or potassium1671 hypochlorite solution as an oxidant. Protection of the hydroxyl group in the N-anthracenylmethyl ammonium salts may be essential to attain high enanti-oselectivities in the epoxidation165"671, and use of... 

---