P-Unsaturated Enones

Scheme 9.5. Conjugate Addition of AUylic Silanes to a, p-Unsaturated Enones...

Dicarbonyl donors are excellent Michael donors in asymmetric conjugate addition to a,p-nnsatnrated ketones. Wang and co-workers [79] applied chiral Cinchona-thiourea catalyst 131 to various carbon donors in the addition to aromatic enones. A diverse array of nucleophiles, mainly 1,3-dicarbonyls proceeded smoothly in the conjugate addition to a,p-unsaturated enone 132 (Scheme 29). 

Scheme 6.137 Mechanistic proposal for 121-catalyzed asymmetric Michael additions of 1 H-benzotriazole to a.P Unsaturated enones.

Conjugate additions of carbamates to a, P-unsaturated enones require - apart from metal halide - TMSC1 as a stoichiometric additive [96]. The addition of ethyl carbamate to cyclohexenone (41h) requires only 50mol% TMSC1, which was an exceptionally low amount compared with other Michael acceptors. With 10 mol% of the catalyst, the yield of 3-aminocyclohexenone derivative 66 was good (93%) [97], Aza-Michael reactions also proceed in aqueous media with good results if Co(II),... 

Although lower-order cuprate reagents will often engage in displacement reactions with alkyl halides, such reactions are usually slow. They are generally much less facile than 1,4-addition reactions to a,P-unsaturated enones or enoates. The latter processes are particularly facile when trimethylsilyl chloride is employed as an additive. It was Corey and Boaz10 who first recognised the accelerating effect of trimethylsilyl chloride on cuprate addition reactions to a,p-unsaturated carbonyls. Buszek therefore capitalised on Corey s earlier observations in his reaction of 10 with lithium dimethylcuprate to obtain 15. 

The formation of a,p-unsaturated enones from enol acetates has been applied to the synthesis of 2,3-disubstituted 2-cyclopentenones, including jasmone homologs. The yields of the anodic oxida on are usually in the range of 80-90%. 

Ito, Y., Suginome, M. Palladium-catalyzed or -promoted oxidation via 1,2- or 1,4-elimination oxidation of silyl enol ethers and related enol derivatives to a,P-unsaturated enones and other carbonyl compounds. Handbook of Organopalladium Chemistry for Organic Synthesis 2002, 2, 2873-2879. 

Details of the photochemical behaviour of the p, - unsaturated enones (186) has been reported. This supplements the report made earlier in note form. The reaction described involves the photoconversion of the enones (186) into the imines (187) and (188) by a path which is thought to involve a 1,5-benzoyl migration. The terminus for this is a pendant phenyl group. Such a process has not previously been described. An... 

The same 43/water biphasic system efficiently works for the Michael addition of indoles to a,P-unsaturated enones, the Mannich reaction in its one-pot three-component version, the allyl stannylation of carbonyl compounds and the asymmetric hydroxymethylation of silylenolethers. The last reaction (Figure 45) is carried out in the presence of the chiral bipyridine 45. ... 

For acyclic allylic alcohols, very little a,p-unsaturated enone formation was observed besides epoxidation. Chemoselectivity was much less for cyclic allylic alcohols, for which oxidation of fhe allylic alcohol group competed significantly with epoxidation. In the case of 2-cyclohexenol as the substrate, the enone was even found to be the main product. A comparative sandwich POM-catalyzed epoxidation study of various (subsfifufed) cycloalkenols revealed that the enone versus epoxide chemoselectivity is controlled by the C=C-C-OH dihedral angle Ma in the allylic alcohol substrate. The more this dihedral angle deviates from fhe optimum C=C-C-OW dihedral angle otw for allylic acohol epoxidation, the more enone is formed (Fig. 16.5). 

The first approach to this problem was to attach the chiral auxiliary to the diene by a vinylogous trans-esterification reaction of chiral alcohols with the 3-alkoxy enones (Scheme 21). This reaction was used to construct a variety of chiral dienes including the menthol and phenmenthol dienes (55a) and (55b) by transesterification followed by enol silylation using the Simchen procedure (RsSiOTf taN). However, these dienes exhibit poor diastereofacial selectivity despite the precedent for high stereochemical control in homo Diels-Alder reactions by the use of similar auxiliaries on a,P-unsaturated enones. ... 

The generated chromium-complexed benzyl radical species could be trapped with a,p-unsaturated enones and esters giving cyclization products. Thus, treatment of o-substituted benzaldehyde chromium complexes with Sml2 in the presence of a,p-unsaturated ester gave y-lactones with high diastereoselectivity (Eq.9) [9]. 

Scheme 3.26 Enantioselective transfer hydrogenation of ot,P-unsaturated enones.

Scheme 2.77 Enantioselective organocatalytic conjugate addition of malonates to acyclic a,p-unsaturated enones catalyzed by 106...

List and Muller reported a Brpnsted acid catalyzed asymmetric 6k electrocyclization of benzylideneacetone-derived phenylhydrazone. These compounds were prepared in situ from a,p-unsaturated enones 244 and phenylhydrazine 245, to give 2-pyrazolines 247 in high yields and enantioselectivities. Scheme 3.79 [1(X)]. 

SCHEME 5.41. Michael addition of malonates to a,p-unsaturated enones. 

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