Photocycloadditions dioxinones

The role of the dioxinone chromophore on the selectivity in the first bond formation, was examined by comparison to the enone analogs 149 and 151, lacking the /1-oxygen found in the dioxinone chromophore. Irradiation of 149 and 151 afforded only the fraws-fused photoproducts 150 and 152 respectively, which indicates that in contrast to dioxinones, the first bond formation in the enone photocycloadditions can take place from either the a- or /1-carbon of the enone via six-membered ring of the corresponding diradical intermediate (Scheme 32). 

Systematic study on the diastereofacial selectivity in the intramolecular photocycloaddition of alkenes to chiral dioxinones was recently reported by Haddad and coworkers129 on compounds of type 298. Preferred pyramidalization in the direction of the less exposed side (the axial methyl at the acetal center) described in structure 298b, and first bond formation at this position (found to be the case in dioxinones 143 and 146, Scheme 31), are essential features for obtaining selective photocycloadditions of alkenes to chiral dioxinones from this side, leading to the kinetically favored products. In such cases the preferred approach is not necessarily from the more exposed side (Figure 6). 

General and stereoselective synthesis of spiroethers and less thermodynamically stable spiroketals have recently been developed by Hadded and coworkers129,130. The key step is the intramolecular photocycloaddition of chiral dioxinones of type 305 to dihydropyrones. Subsequent fragmentation of the produced four-membered ring provides, after oxidative enlargement of the cyclic ketone, the thermodynamically less stable spiroketal 310 (R = H) as was demonstrated on photoproduct 308 (Scheme 66). 

An intramolecular [2+2] photocycloaddition of allyl ethers with dioxinones followed by a base-induced fragmentation leads to substituted tetrahydropyran-4-ones <1997TL5579>. A one-pot scandium triflate catalyzed diastereoselec-tive cyclization between aldehydes and (3-hydroxy dioxinones 1046 followed by alkoxide addition to the resulting bicycles 1047 leads to 3-carboxy-substituted tetrahydropyran-4-ones 1048 with high levels of diastereoselectivity as a mixture of keto/enol tautomers (Scheme 268, Table 49) <20050L1113>. 

Explanations for the outcome of the photocycloaddition reactions have been proposed. Seebach et al. suggested that carbon atom C-6 is pyramidalized in the excited triplet state in the opposite direction as compared to the ground state [125]. Sato proposed that different dioxinone conformations are responsible which vary depending on the reaction type [126]. 

If five-membered ring Formation is not possible (n > 3) a six-membered ring is the next most favored. The major product is typically the straight adduct. An example of intramolecular photocycloaddition in dioxinone proceedng with high regioselectivity is shown in Table 5, entry 15127. 

I, 5-dicarbonyl compounds and their interesting and varied chemistry, e.g., the formation of cyclohexenones from aldol condensation of the products. As an extension of the de Mayo reaction (the photocycloaddition of enolated /i-diketones to double bonds) enol esters, enol ethers, vinylogous esters and amides, and dioxinone have been employed as the enone components. Some intermolecular examples have already been discussed in Section 1.6.1.4.2.1.6. (cf. Table 4, entries 2 and 3) and some intramolecular systems are collected in Table 5, entries 10,... 

Photocycloadditions of dioxinones have also been reported by Sato et al., particularly with chiral-2-spirocyclic dioxinones. Both inter- and intramolecular versions have shown high regio-and stereoselectivity (Equation (7)) <90H(30)217,90H(3i)2ii5>. 

Fig. 6.49. Stereoselectivity of photocycloaddition, nucleophilic addition to and hydrogenation of dioxinones...

Owing to the interest in cycloadducts of dioxinones in the synthesis of natural products, the intermolecular 2 -i- 2 photocycloaddition reaction of numerous dioxinones has been generalized [159]. In an attempt to control the regioselectivity of the cycloaddition and the configuration of the asym-... 

Dioxinones (194) can be prepared as enantiomerically pure compounds. The intramolecular 2 -I- 2 photocycloaddition of 194 usually leads to a mixture of two regioisomers, 195 and 196, and even to only isomer when m = 2 and w = 0. The principle merit of this approach resides in the ability of the ester and ketalic groups in the photoadducts to be cleaved easily into adducts similar to those provided by intermolecular photocycloaddition of dioxinones with the corresponding alkenes [159d]. Only one stereoisomer... 

An intramolecular dioxinone photocycloaddition is used in the synthesis of (+)-valeranone. Irradiation of dioxinone 146 gives a high yield of cycloadduct 147, which undergoes reduction followed by spontaneous retro-2i dio fragmentation. The resulting diketoaldehyde intermediate 149 is cyclized to enone 150 via aldol condensation. This enone is converted to (+)-valeranone in a few steps. 

Photocycloaddition of dioxinone 228, with the correct C-l/C-3 relative stereochemical relationship for taxane construction, leads to the formation of a single diastereomer 229. Fragmentation under basic conditions and treatment of the resulting keto acid with diazomethane generates the keto ester 230 and its C-15 epimer (3 1 ratio) in quantitative yield. 

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