Cycloaddition with cyclohexadienones

As one would expect, conjugated ketones are mostly poor hydrogen abstractors. This is a fortunate circumstance, because it leaves room for a variety of reactions that are quite useful from the preparative point of view. These include rearrangements [24], addition reactions and 2+2 cycloaddition with alkenes. An example of rearrangement of a cross-conjugated cyclohexadienone is shown above. This gives a cyclopropylcyclopentenone that opens up in the reaction medium (see Scheme 2.13). 

The cycloaddition of picryl azide with phenoxyallene took place at the C1-C2 double bond of the allene exclusively to give the triazoline intermediate 97 [89]. This intermediate underwent a facile Claisen rearrangement to yield cyclohexadienone 98, which rapidly tautomerized to phenol 99. 

An example of 2,4,6-triphenylpyrylium-3-olate (65 R = R = R = Ph, R = H) reacting as a 1,3-dipole was first provided by Suld and Price who obtained a maleic anhydride adduct (C25HigO5). Subsequently, an extensive study of the cycloadditions of this species has been published by Potts, Elliott, and Sorm. With acetylenic dipolarophiles, compound 65 (R = R = R = Ph, R = H) gives 1 1 adducts that have the general structure 74 and that isomerize to 6-benzoyl-2,4-cyclohexadienones (76) upon thermolysis. This thermal rearrangement (74 -> 76) has been interpreted in terms of an intermediate ketene 75. The 2,3-double bond of adduct 74 (R = Ph) is reduced by catalytic hydrogenation. Potential synthetic value of these cycloadducts (74) is demonstrated by the conversion of compound 74 (R = Ph) to l,2,3,4,6-pentaphenylcyclohepta-I,3,5-triene (79 R= Ph) via the alcohol 78 (Scheme 1). ... 

Cycloaddition of the diene 20 with dienophiles bearing a phenyl sulfoxide substituent leads, after elimination of phenyl sulfenic acid and hydrolysis, to a 4,4-disubstituted cyclohexadienone or a substituted phenol product. For example, an elegant synthesis of disodium prephenate 34 makes use of this chemistry... 

An asymmetric synthesis of tricyclic compounds through the desym-metrization of cyclohexadienones was developed to provide a wide variety of cyclohexadienones with tetrahydrofuran motifs via a [4 + 2] cycloaddition (14AGE8184). 

Rhodium(I)-catalysed 2 + 2 + 2-cycloadditions of ene-allene (96) with allene (97) yielded trans-fused hydrindanes (98) and decalins with high regio- and stereo-selectivities (Scheme 31). A Ni-phosphine complex catalysed the 2 + 2 + 2- (g) cycloaddition of diynes with substituted ketenes to produce 2,4-cyclohexadienones in high yields. No decarbonylation products were observed. The chiral A-heterocyclic carbene-catalysed enantioselective 2 + 2 + 2-cycloaddition of ketenes with CS2 produced l,3-oxatian-6-ones in good yields and with high enantioselectivities. 2" ... 

A gold-catalysed 3 + 2-cycloaddition-hydrolytic Michael addition-retro-aldol reaction of propargyl esters tethered to cyclohexadienones gives tetrahydrofuranones, diox-atricycloundecenones, and furofurans (Scheme 112). The product cyclohexenones or cyclohexanones with a y-quaternary centre result from multiatom transpositions... 

A tandem photorearrangement-cyeloaddition of cyclohexadienones tethered with various alkenes (28) results in the formation of polycyclic frameworks. With allynyl ether-tethered substrates, [3 + 2] cycloaddition occurs, leading to strained alkenes, which can be further employed for the elaboration of molecular complexity. ... 

Samuel provided the first example that the oxyallyl intermediate in the Type A rearrangement could be trapped intermolecularly with a diene in a (3-t4]-cycloaddition. Samuel found that the apparent lifetime of simple 4,4-dialkyl substituted cyclohexadienones was too short to permit intermolecular capture. However, irradiation of Schuster s 4-methyl-4-trichloromethyl-2.5-cyclohexadienone 1 (Rj = CHj, Rj = CCI3) in the presence of either furan or cyclopentadiene produced 1 1 adducts 7 (X = O or CHj). This type of [4-1-3]-cycloaddition reaction has long been considered as evidence for an oxyallyl transient and is compatible with either a biradical or zwitterionic species. ... 

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