Aldehyde-enone

Krische et al. demonstrated intramolecular reaction with Co(dpm)2 (5mol%) and PhSiH3 (120 mol %) as a hydride donor (Scheme 8) [14-16]. Addition of aldehyde-enone 17 to a solution of the Co catalyst and phenylsi-lane resulted in the formation of the corresponding aldol cyclization product... 

Under similar conditions, employing a cationic Rh complex (10mol%) and hydrogen (1 atm), the aldehyde-enone 17 was subjected to the cycli-zation to give the cyclic aldol product 18 in 89% with czs-selectivity up to 10 1 (Scheme 19) [31]. Use of (p-CF3Ph)3P as ligand accelerated the reaction... 

While C-C bond formation via metallation and alkyl or aryl halides is well known (COMC (1982) 5.4.2.7.5), the C-C bond formation with TBAF and aldehyde, enones, or enals is new (Figure 9).318 Carboranyl groups can actually be used as protecting groups for aldehydes and ketones due to the facile cleavage of the cage C-C(OH) bond with... 

This is a strange compound. The exocyclic (arrowed) alkene is an enol at one end but an enamine and a vinyl sulfide at the other. All three atoms, O, N, and S, have lone pairs making this a very electron rich alkene. The nitrogen atom dominates so that conjugate addition 192 to enones works well. The product 194 loses the catalyst 189 to give a 1,4-diketone 195. The reaction is simplicity itself the aldehyde, enone, catalyst and weak base (usually amine or sodium acetate) are heated in ethanol. 

The following series of allylic and 0-stannyl ketyl reactions is intended to provide insight into the reactivity and potential of these interesting synthetic intermediates. Various cyclizations, strained ring scissions, and electrophile trapping reactions will be demonstrated on diverse substrates such as ketones, aldehydes, enones and a-ketocyclopropanes. All of the chemistry described herein utilizes mild and neutral reaction conditions to accomplish a wide range of synthetic transformations. 

As already indicated, carbonyl compounds such as ketones, aldehydes, enones, and quinones possess the property to act as effective electron acceptors in the excited state for generating radical anions in the presence of electron-donating partners such as alkenes, aromatics, ruthenium complexes, amines, and alcohols. We will not consider the reactivity of enones and quinones, but we will focus our attention on the behavior of the radical anions formed from ketones and aldehydes. Four different processes can occur from these radical anions including coupling of two radical anions and/or coupling of the radical anion with the radical cation formed from the donor, abstraction of hydrogen from the reaction media to produce alcohols, cyclization, in the case of ce-unsaturated radical anions, and fragmentation when a C -X bond (X=0, C) is present (Scheme 18). 

As mentioned above, although some examples of intramolecular MBH reactions have been reported in the literature, this aspect is still in its infancy. Most known reports are based on the cyclizations of combinations of enone-enone, enone-acrylate, enone-aldehyde, unsaturated thioester-aldehyde, enone-allylic carbonate frameworks, etc. More recently, Krafft et al. have developed a novel, entirely organo-mediated intramolecular MBH reaction by using allyl chloride 277 as an alternative electrophile to afford densely functionalized cyclic enones 278. This reaction tolerates modification of the enone and the use of primary and secondary allylic chlorides and generates both five-and six-membered rings in excellent yields. Both mono- and disubstituted alkenes are formed with excellent selectivity in the absence of a transition metal catalyst (Scheme 1.100). ... 

Compared with unsaturated aldehydes, enones tend to be less reactive Michael acceptors. However, the addition of nitrogen-centered nucleophiles to enones can be promoted by certain primary amine catalysts by means of an iminium activation mechanism. Specifically, 9-amine-9-deoxy-epi cinchone derivatives (catalysts 9,36/37), in the presence of organic acids, have proven their usefiilness in catalyzing the addition of carbamate derivatives [75], protected hydroxylamine derivatives [76], and A/-heterocycles to acyclic [77, 78] and cychc unsaturated ketones [79] (Scheme 11.21). In general, aromatic ketones were less reactive and led to lower yields. 

Applications Based on Deprotonation of Vinyl Selenides. Deprotonation (by KDA) of phenyl vinyl selenide and prompt treatmrait with electrophiles such as representatives of the compound classes alkyl halides, aldehydes, enones, and epoxides gives the expected products. Scheme 13 shows a typical example and illustrates the preservation of stereochemistry about the double bond. Vinyl selenides can be hydrolysed to ketones. ... 

Trimethylsilyldiethylamine-promoted intramolecular 1,4-conjugate addition has also been reported. Reaction of an aldehydic enone, such as that in eq 26, with trimethylsilyldiethylamine in acetonitrile at room temperature provides a cyclic product in 73% yield with 7 3 diastereoselectivity. 

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