Ketones interesting examples

While the usual eonsequence of hydration of enamines is eleavage to a secondary amine and an aldehyde or ketone, numerous cases of stable carbinolamines are known (102), particularly in examples derived from cyclic enamines. The selective terminal hydration (505) of a cross-conjugated dienamine-vinylogous amide is an interesting example which gives an indication of the increased stabilization of the vinylogous amide as compared to simple enamines, which is also seen in the decreased nucleophilicity of the conjugated amino olefin-carbonyl system. 

Non-Kolbe electrolysis of alicyclic p-hydroxy carboxylic acids offers interesting applications for the one-carbon ring extension of cyclic ketones (Eq. 35) [242c]. The starting compounds are easily available by Reformatsky reaction with cyclic ketones. Some examples are summarized in Table 13. Dimethylformamide as solvent and graphite as anode material appear to be optimal for this reaction. 

Another interesting example of a photochemi-cally induced domino process is the combination of the photocyclization of aryl vinyl sulfides with an intramolecular addition as described by Dittami et al. [901 as intermediate a thiocarbonyl ylide can be assumed. The domino-Norrish I-Knoevenagel-allyl-silane cyclization developed by us allows the efficient stereoselective formation of 1,2-trans-subsituted five- and six-membered carbocycles.1911 A photochemical cycloaddition of enamino-aldehydes and enamino-ketones with the intermediate formation of an iminium salt followed by addition to allylsilanes gives access to novel bicyclic heterocy-des. New examples of photochemically induced... 

Since the electroreduction of ketones shown in Scheme 29 has been well established [1-3, 12, 62-65], one more recent interest in the electroreduction of carbonyl compounds is focused on the stereo-selective reduction of ketones. For example, the diastereo-selective cathodic coupling of aromatic ketones has been reported. In the presence of a chiral-supporting electrolyte, a low degree of enantioselectivity has been found [66] (Scheme 30). 

Trifluoromethyl ketones, which are enzyme inhibitors, constitute an interesting example the log P value depends on the equilibrium between hydrate, hemiketal, and ketone and the equilibrium is itself less important than the solvent. The solubihty of each of these forms also depends on the solvent s nature. For these reasons, the observed log P values are often difficult to interpret. 

There are several other photochemical reactions which ketones can undergo in the absence of olefins. Here again, the products obtained depend upon the relative rates of the competing reactions. Some of these reactions are shown in Table XI. Since it is beyond the scope of this review to discuss these reactions in detail, only a few interesting examples germane to the competition with photocycloaddition will be mentioned. 

There is still considerable interest in the products of irradiation of hydrogen abstracting ketones for example, Plank96 has reported that benzophenone in ether gives benzophenone pinacol in higher yield when naphthalene is added. It was suggested that the pinacol is formed from some intermediate which is photochemically destroyed in the absence of a quencher. Challand97 has isolated products from addition of ether to acetophenone. 

The remaining chapters deal with a variety of catalysts for effecting oxidation reactions. Chapter 5 describes three simple protocols for the controlled oxidation of primary or secondary alcohols. The importance of stereocontrolled epoxidation and hydroxylation reactions is reflected by the fact that Chapter 6, directed at this field, is one of the most extensive sections of the book. An interesting example of an enantioselective Baeyer-Villiger reaction is featured in Chapter 7, together with an industrially important ketone to enone conversion. Oxidative carbon-carbon... 

Another interesting example of a practical zinc-ene reaction was reported, that exploited the reversibility of the addition of allylic organozinc reagents to carbonyl compounds120. Indeed, successive treatment of the ft, -unsaturated ketone 168 with w-BuLi and ZnCl2... 

Acylation with cyclopentenyl- or cyclohexenylacetyl chloride gave three-membered ring systems.113 114 An interesting example is the reaction of ethylene with acid chlorides complexed with excess A1C13 to give the 17 p.y-unsaturated ketones 115... 

An interesting example of an indirect acylation of alkenes is the reaction of small olefins (ethylene, propylene, isobutylene) over H-ZSM-5 in the presence of CO at 23°C to give unsaturated ketones.122 In this case the acylium ion is formed in situ from the alkene and CO in the pores of the zeolite at ambient temperature ... 

Several interesting examples of solid inclusion complexes with ketone guests which undergo the Norrish II reactions have been examined. They illustrate the breadth of reaction cavity types and resultant selectivities that can be expected in such systems. 

Solid inclusion complexes of photoreactive ketones with cyclodextrins as hosts (Figure 6) provide interesting examples of how a fairly stiff, somewhat heterogeneous (in terms of polarity, size, and guest orientation) reaction... 

Another important class of cycloaddition reactions is the formation of oxetane rings between a photoexcited carbonyl compound and an unsaturated molecule. These reactions also occur probably through an exciplex although these exciplexes are non-fluorescent as they are formed from the triplet state of the ketone or aldehyde. The formation of the four-membered oxetane ring is an interesting example of a typical photochemical reaction... 

A further interesting example in the context of natural products is shown, in the transformation of the p-lactam 102 into the p-amino ketone 103, Scheme 34, which upon carbonyl reduction provides the amino lactone 104, the cyclized form of the Ai-terminal amino acid residue found in the antibiotic family of nikkomycins, Fig. 6 [101]. 

Three further interesting examples, which delineate the utility of [3-lactams in synthesis, have been described recently. In an approach to C-linked glycosyl amino acids, Scheme 37, addition of the lithium dianion 112 to the corresponding N-Boc (3-lactam 111 provides the (3-amino ketone 113 [104],... 

An interesting example of heterocyclization involving both the aziridine fragment and the carbonyl group is a reaction of aziridinyl ketones with ammonia and carbonyl compounds, giving rise to 3,5a-dihydro-l//-azireno[l,2-c]imidazoles. This reaction will be considered in more detail in Sects. 1.2 and 1.3. 

The ruthenium-catalyzed addition of C-H bonds in aromatic ketones to olefins can be applied to a variety of ketones, for example acetophenones, naphthyl ketones, and heteroaromatic ketones. Representative examples are shown in the Table 1. Terminal olefins such as vinylsilanes, allylsilanes, styrenes, tert-butylethy-lene, and 1-hexene are applicable to this C-H/olefin coupling reaction. Some internal olefins, for example cyclopentene and norbornene are effective in this alkylation. The reaction of 2-acetonaphthone 1 provides the 1-alkylation product 2 selectively. Alkylations of heteroaromatic ketones such as acyl thiophenes 3, acyl furans, and acyl pyrroles proceed with high yields. In the reaction of di- and tri-substitued aromatic ketones such as 4, which have two different ortho positions, C-C bond formation occurs at the less congested ortho position. Interestingly, in the reaction of m-methoxy- and m-fluoroacetophenones C-C bond formation occurs at the congested ortho position (2 -position). 

An interesting example is the hydroiminoacylation reaction, a good alternative to hydroacylation reactions, using aldimines as a synthetic equivalent to aldehydes (Scheme 4) [4]. The rhodium-catalyzed hydroiminoacylation of an olefin with aldimines produced a ketimine which could be further acid-hydrolyzed to give the ketone. The reaction proceeded via the formation of a stable iminoacylrhodi-um(III) hydride (this will be discussed in the mechanism section), production of which is facilitated by initial coordination of the rhodium complex to the pyridine moiety of the aldimine. This hydroiminoacylation procedure opened up the direct... 

An interesting example of the transfer of center chirality to helicity is the work by Ogawa et al., based on an asymmetric aromatic oxy-Cope rearrangement to provide nonracemic [5]helicenes (Fig. 15.8) [75]. The starting material with center chirality, bicyclo[2,2,2]ketone (-)-21 (>98% ee), was obtained by enzymatic resolution. In the annelation step, the phenanthrene derivative was subjected to aromatic oxy-Cope rearrangement, to afford a pentacyclic product in 47 % yield. The corresponding [5]helicene 22 was obtained in 7 % overall yield (> 98 % ee) after six steps. 

Formation of the Wieland-Miescher ketone via an enantioselective Robinson annulation represents but one interesting example.91... 

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