Enone From alkene

This section contains alkylations of ketones and protected ketones, ketone transpositions and annulations, ring expansions and ring openings and dimerizations. Conjugate reductions and Michael alkylations of enone are listed in Section 74 (Alkyls from Alkenes). 

Conjugate reductions and reductive alkylations of enones are listed in Section 74 (Alkyls from Alkenes). 

In cycloadditions of enones to alkenes novel strategies have been adopted for ring expansion of the cycloadducts, either by the choice of appropriate alkenes, e.g. 2-(trimethylsiloxy)buta-1,3-diene,70 vmv-2-trimethylsiloxybuten-2-oales71 or 3,3-dimethylcyclopropene,72 or by using 3-oxo-l-cyeloalkene-l-carboxylates as enones.73 Asymmetric [2 + 2] photocycloaddition of cyclopent-2-enone to a (+ )-dihydrofuran acetonide constitutes the cornerstone of the synthetic strategy in the first total synthesis of the novel antitumor metabolite ( )-echinosporin.74 The cycloaddition product 25 from treatment of 2-(2-carbomethoxyethyl)-2-cyclopentenone (24) with ethene has been used as a precursor for the preparation of tricyclo[4.2.0.01,4]octane.75... 

Collins reagent is used for the introduction of carbonyl groups at allylic positions." This transformation of alkenes into enones is much slower than the oxidation of alcohols, requiring a great excess of Cr03 2Py and prolonged reaction times. Consequently, alcohols can be oxidized to aldehydes and ketones by Collins reagent without interference from alkenes. 

A different competitive reaction on the singlet 1,4-biradical level is observed for 2-acylcyclohex-2-enones 25, which on irradiation in the presence of 2,3-dimethylbut-2-ene afford both l-acylbicyclo[4.2.0]octan-2-ones 26 as well as 3,4,4a,5,6,7-hexahydro-8i7-isochromen-8-ones 27 (Sch. 9). These concurrent photochemical [4 + 2]-cycloadditions represent the first examples of dihydropyran formation from enones and alkenes to be reported in the literature [48]. 

Introduction of chiral auxiliaries in the starting materials is very attract for applications to organic synthesis. However, to be of synthetic interest, chiral auxiliaries have to be inexpensive, readily introduced on the starting ma rial, inert in the conditions of irradiation, and readily removed from the photo ducts. Even if the first requirements can be easily satisfied with chiral ket esters, and amides, it is often difficult to avoid side reactions involving auxiliary [63]. In order to control all the asymmetric centers created in the in molecular photocycloadditions of cyclic enones with alkenes, esters of c alcohols were first considered. Although menthyl and bomyl derivatives ga only low de, 8-phenylmenthyl esters produced a far better asymmetric inducti [64]. The facial selectivity was found to depend on the syn/anti nature of t cycloadducts and the structure and location of the chiral auxiliary on either tl enone or the alkenyl moiety. More surprisingly the selectivity also depe strongly on the nature of the solvent (Scheme 21). 

Cyclobutane formation from alkenes does not occur thermally. It is allowed by the Woodward-Hoffmann rules in the excited state and so is a photochemical reaction. The reaction occurs cleanly when one alkene is conjugated, e.g, as an enone, and so absorbs u.v. light. 

Allylic oxidation. Allylic alcohols are oxidized to enones by Pd(OAc)2 in wet DMF. Allylic esters are obtained from alkene/carboxylic acid mixtures on treat-... 

These two mechanisms, the reversible biradical intermediate and the intermediate exciplex [25] have both been useful for analysis of the regioselectivity and stereoselectivity observed in [2 -I- 2] photocycloaddition between enones and alkenes. Hoffman et al. [26], for example, describes his stereoselective photocycloadditions as arising from a biradical from the triplet excited enone which may or may not involve exciplex formation.. Ground state trans-cycloalkenones have also been proposed as the reactive intermediates which lead to [2 + 2] cycloadducts [27]. The distance between the reacting partners is clearly an issue. If the n systems are not sufficiently close, then photocycloaddition will not occur [28]. 

For the photoadducts derived from cyclic enones and alkenes, stereochemistry at the ring junction is influenced by the structure and especially by the ring size of the starting reagents. For steric reasons, only cis-fused cycloadducts can be formed on photocycloaddition of cyclopentenones (n = 0). From cyclohexenone derivatives (n = 1), cis- and transfused adducts can be isolated, even if the cis-fused structure is thermodynamically more stable. This indicates that trans-fused cycloadducts result from a kinetic rather than a thermodynamic control. Fortunately, trans-fused cycloadducts can be epimerized easily to the more stable cis stereoisomers (Scheme 11). 

The importance of steric hindrance on facial selectivity is illustrated first for asymmetric cycloalkenes. High facial selectivity is observed with cycloalkenes having one stereogenic center in an allylic position. Photocycloaddition of cyclopentenone 25 with cycloalkene 26 led to a mixture of regio-isomers 27 and 28, resulting in a selective cycloaddition of the excited enone from the diastereoface opposite the large isopropyl group [69], Similar results are observed with alkenes 32, 35, and 38 [70-73] (Scheme 13). 

This rule is strictly observed with cyclopentenone derivatives 106, 108, and 110, and 6-alkenylcyclohexenones such as 112 and 118 [124-127, 129]. However, facial selectivity may decrease with more mobile enones, which can support more twisted transition states, and especially with 4-alkenylcyc-lohexenones such as 115 [11 lb, 128]. When the asymmetric center is present in a cycloalkenyl chain, a selective approach of the alkene by the excited enone from the same side as the tether is observed. Furthermore, there is high facial selectivity on the enone system and only one intramolecular cycloadduct is obtained with 120, 122, 124, 126, and 128 [131-136] (Scheme 21). 

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