Homoconjugated ketones with extended unsaturation: Wavelength‐selective, regioselective, diastereoselective, and enantiospecific photochemical transformations of methyl 7‐oxospiro[5.5]undeca‐1,3‐and ‐2,4,‐diene‐2‐carboxylate

Abstract

The title molecules were shown to photorearrange with remarkable selectivity. Wavelength variation steers the rearrangement modes, of which the [1,2]-acyl shift was found to be largely regioselective, diastereoselective, and enantiospecific. Chemical intercorrelation of products and mechanistic studies were carried out all along. The potential significance of these photochemical processes in selective synthetic schemes is discussed.Introduction. -We recently described the interesting photochemical behaviour of a certain class of homoconjugated ketones with extended unsaturation, namely 5-acylcyclohexa-1,3-dienes. The spiro[5.5]undeca-l,3-dien-7-one system was found to be of particular interest and versatility [2], and an outline of its irradiation-induced transformations is given in Scheme 1. The wavelength selectivity is evident and remarkable: i) high-energy (254 nm) irradiation brings about an electrocyclic opening of the cyclohexadiene ring to the corresponding conjugated trienone, which is highly photoreactive and leads to a variety of secondary products; ii) at 300 nm, a -cleavage takes place, giving the

Synthetic Aspects of the Di-π-methane Rearrangement

“…The formation of 90 is proposed to take place from the T 1 (π,π*) excited state. 104 A similar study has been carried out on homoconjugated spirocyclobutanones. 105 In this instance a dependence of methyl substitution in the starting enone on the photoproduct distribution was observed.…”

“…Within this area of research an example of enantiospecific ODPM rearrangement has been described for the spiro compound 89 that yields quantitatively the tricyclic derivative 90 both on direct, at wavelengths above 340 nm, and on acetone-sensitized irradiations. The formation of 90 is proposed to take place from the T 1 (π,π*) excited state .…”

“…However, under the same conditions, trimethyl-substituted ketone 93 gives a mixture of products 94 and 95 resulting from a normal oxa-di-π-methane rearrangement and the vinylogous ODPM reaction, respectively . The results of these studies on spiro-homoconjugated ketones, including yields and references, are collected in Table . −

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“…105 The results of these studies on spirohomoconjugated ketones, including yields and references, are collected in Table 13. [103][104][105]…”

“…Furthermore, the ODPM rearrangement of bicyclo[2.2.2]octenones can be carried out in some cases at exceptionally high concentrations (>20%). A variant of this synthetic approach has been used by Demuth and Hinsken 109 to carry out the first synthesis, in enantiomerically pure form, of the angular triquinane (-)-silphiperfol-6-en-5-one (104) A photochemical approach to [3.3.3]propellanes by the ODPM rearrangement has allowed the synthesis of the sesquiterpene hydrocarbon (()-modhephene (113). 111 In this instance the synthesis starts from the readily available diene 114 that is transformed into the bicyclo[2.2.2]octenone (115) through a Diels-Alder cycloaddition using R-chloroacrylonitrile as the ketene equivalent.…”