Unsaturated carbonyl compounds Dienones

Despite the increasing information on the photochemistry of 2,4-dienones and other unsaturated ketones, as well as on the ring-chain valence isomerism of halogen-substituted pyran and dihydi opyran systems,the data are still very scarce. The intermediate formation of pyrans valence-isomeric with unsaturated carbonyl compounds in the pyridine syntheses based on reactions of ammonia with aldehydes or ketones, advocated by various authors (cf. Section II,B,2,f), is still rather speculative. (See also Section II,B,2,e for the valence isomerism of 5-chloro-2,4-dienones with pyrylium chlorides.)... 

A simple two-step protocol for the generation of a terminal diene is to add allyl magnesium bromide to an aldehyde or a ketone and subsequent acid or base catalysed dehydration (equation 34)72. Cheng and coworkers used this sequence for the synthesis of some indole natural products (equation 35)72a. Regiospecific dienones can be prepared by 1,2-addition of vinyllithium to a,/l-unsaturated carbonyl compounds and oxidative rearrangement of the resulting dienols with pyridinium dichromate (equation 36)73. 

Cyclobutane formation via light-induced [2 + 2] cycloaddition is probably one of the best studied photochemical reactions and has been reviewed thoroughly up to 1972 (Houben-Weyl, Vols. 4/5 a and 4/5 b). The most important types of C —C double-bond chromophores undergoing such reactions arc alkenes, 1,3-dienes, styrenes, stilbenes, arenes, hetarenes, cycloalk-2-enones, cyclohexa-2,4(and 2,5)-dienones, 1,4-benzoquinones, and heteroanalogs of these cyclic unsaturated carbonyl compounds. For p notocyciodimerizations see Houben-Weyl, Vol. 4/5 a, p 278 and for mixed [2 + 2] photocycloadditions of these same chromophores to alkenes see Section 1.3.2.3. 

A review of photo-cycloadditions of dienones and quinones has been published.41 The first example of a Lewis acid-catalysed 2 + 2-cycloaddition of styrene with naphthoquinone has been reported.42 FMO methods have been used to investigate the effect of substituents on the regiochemistry of the 2 + 2-photo-cycloaddition of a, fi-unsaturated carbonyl compounds with substituted alkenes.43 Evidence has been presented for the presence of a triplet exciplex intermediate in the photo-cycloaddition of 4,4-dimethylcyclohexenone to 1,1-diphenylethylene.44 The intramolecular 2-1-2-photo-cycloaddition of 2-acyloxy-3-hexenoylcyclohexenones (26) is highly diastereo-selective yielding the tricyclic adduct (27) (Scheme 10).45... 

As with other intramolecular ene reactions, this reaction is best suited to the preparation of cyclopentanes, but can also be used for the preparation of cyclohexanes. The reaction cannot be used for the formation of cyclopropanes or cyclobutanes since the unsaturated carbonyl compound is more stable than the ene adduct. 8,e-Unsaturated ketones (167) do not give cyclobutanes (171) by enolization to give (170) followed by a type I reaction but instead give cyclohexanones (169) by enolization to give (168) followed by a type II reaction. Alkynes can replace alkenes as the enophile. Enols can be prepared from pyrolysis of enol esters, enol ethers and acetals and from -keto esters and 1,3-dicaibonyl compounds. Tlie reaction is well suited to the preparation of fused or bridged bicyclic and spirocyclic compounds. Tandem ene reactions in which two rings are formed in one pot from dienones have also been described. The examples discussed below 2-i63 restricted to those published since Conia and Le Perchec s 1975... 

Dimethyloxysulphonium-methylide has long been known to add to the olefinic group of a 3-unsaturated carbonyl compounds, and additional examples have been reported. Cyclopropane formation also has been reported from stabilized oxysulphonium ylides such as (85) and those carrying two stabilizing groups on the methylide carbon. The dienone (91) underwent double addition with (89) to form the syn-isomer only, but the... 

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