Carbonyl groups ketyl radicals from

When saturated steroidal ketones are reduced in ammonia, an alcohol is usually present to act as a proton donor and high yields of steroidal alcohols are obtained. Under these conditions, reduction probably proceeds by protonation of the radical-anion (or ketyl) (61), which results from a one electron addition to the carbonyl group, followed by addition of a second electron and proton. Barton has proposed that reduction proceeds via protonation of the dianion (62) arising from addition of two electrons to the carbonyl group. This proposal implies that the ketyl (61) undergoes addition of a second electron in preference to undergoing protonation by the... 

Sml2 can be used for SET reagent to carbonyl groups. Thus, eq. 4.13 shows the initial SET from Sml2 to a carbonyl compound (26) to generate a ketyl radical, a nucleophilic radical, which then reacts with electron-deficient ethyl acrylate through SOMO-LUMO interaction to form y-lactone (28) [34-36]. 

Ketyl Radical Anions from Carbonyl Groups... 

Pestalotiopsin A and 6-epitaedolidol are structurally related caryophyllene-type sesquiterpenes. In 2003, Procter reported the use of a Sml2-mediated 4-exo-trig carbonyl-alkene cyclisation to construct the core of pestalotiopsin A.57 Treatment of cyclisation substrate 33 with Sml2 in THF, MeOH and 2,2,2-trifluoroethanol gave cyclobutanol products 34 and 35 in good yield and with moderate diastereoselectivity. The major diastereoisomer is believed to arise from a cyclisation in which coordination to the silyl ether group directs addition of the ketyl radical anion to the alkene (Scheme 7.17).57... 

Stannyl ketyl radicals were generated from an amide carbonyl group and when incorporated in cinnamic enamides, indolizidinone rings can be assembled [37]. Du and Curran have shown that a-thioaminoalkyl radicals such as 24 resulting from the addition of a tris(trimethylsilyl)radical onto N-aryl thiocarbamates, thioamides or thioureas 22, can be exploited in the context of the synthesis of carbocyclic and heterocyclic fused quinolines 23 (Scheme 8) [38]. 

Attempts to extend this work to the keto-oxime substrate 54 derived from D-glucosamine with an JV-phthalimido group resulted in the formation of a completely different product (Scheme 40). In this case, cyclization was initiated by reduction of the phthalimido carbonyl group to its corresponding ketyl radical anion followed by cyclization onto the ketone, providing an a-hydroxylactam 55 which was proposed to be a potentially useful scaffold for diversity-oriented synthesis. 

A mechanistic scheme for the titanium-mediated coupling of two carbonyl groups involves an initial one-electron transfer from titanium metal to the aldehyde or ketone carbonyl group to produce radical anions (ketyl species), which then dimerize to a Ti-pinacolate. Hydrolysis of the pinacolate generates the vicinal diol. 

This work has led to more studies on the reaction mechanism of reduction by hydrosulfite, Chung [17] proposed that the mechanism proceeds by stepwise electron transfer from the reducing agent to the carbonyl group to form a ketyl radical intermediate that can then abstract a hydrogen atom from the medium, dimerize to pina-col, or undergo further reduction (Figure 13.2). 

A novel application of the radical cascade for construction of the indolizidinone skeleton focused on the initial formation of (9-stannyl ketyls. The tributyl tin radical was found to react with the carbonyl group of 507 to give ketyl 508 (Scheme 87) (04T8181). Consecutive 6-endo- and 5-exo-trig cyclizations then furnished stannyl enol ether 510. Eventual hydrolysis of the enol ether provided indolizidinone 511 in 36% yield as a Id-mixture of diastereomers. Again, the predominant isolation of the thermodynamic favored products derived from a 6-endo-trig cyclization can be... 

Mixtures of photoinitiators have been actively studied. Michler s Ketone and benzoyl peroxide have been shown to effectively induce the photopolymerisation of methyl methacrylate through the formation of an initial complex shown in scheme 3 7, Although the exact initiating radical does not appear to be ascertained it is almost certainly the arylalkylamino radical from the Michler s Ketone. In the interaction of benzil and thioxanthone with triethylamine in the photoinduced polymerisation of acrylic monomers their is a competition between reverse electron transfer and ketyl radical formation . As the carbonyl concentration increases the bimolecular termination rates due to radical recombination increases. The same workers also studied the same system but replaced the ketone initiators with pyrene . Their inability to identify pyrene end groups indicated that the active initiating species arise from a complex between the pyrene and the triethylamine. 

In such a structure there must be considerable resonance interaction between the carbonyl group and the electron associated with the ionized OH adduct. Under these conditions, therefore, an absorption spectrum similar to the normal ketyl anion would not be unlikely. If then, the absorption at 4400 A. at high pH includes a component caused by the species arising from oxidative attack by OH or O" radicals as well as that caused by the ketyl anion, it is not surprising that adding N20 has little effect, even if the conversion of e aq to OH was complete during the pulse. 

Ascorbic acid is highly reactive to all the primary water radicals, because of its carbonyl group and double bond. Reaction with ei or H reduces ascorbic acid to a ketyl radical, while reaction with OH oxidizes it to the relatively unreactive tricarbonyl radical ion [9]. Aside from a possible reaction with cytochrome-c (or ferrimyoglobin), the radical ion is most likely to undergo a complex disproportionation reaction that regenerates the ascorbic acid and produces dehydroascorbic acid, which has essentially the same vitamin activity. These reactions need to be considered, because ascorbic acid is added to foods to fortify them, to facilitate curing meats, and to enhance antioxidants. 

Following the observation that only the carbonyl group was electrochemically reduced under the experimental conditions used, a radical formed via protonation of the ketyl radical anion resulting from a one-electron transfer was suggested as the key intermediate responsible for the coupling (Scheme 26). 

Interestingly, the ketyl radicals generated from the carbonyl group at the side chain of 6-7-dimethoxy tetraline chromium complex 15 attacked on the chromi-um-complexed arene ring with loss of the methoxyl group to afford the cycliza-tion chromium complex 16 as a single diastereomer (Eq. 12) [12]. 

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