Benzopyrylium-4-olates

Suzuki, T. Furuta, T. Hiramilsu, and M. Kuzuya, Tetrahedron Lett., 4107 (1974). 

Thermolysis of indenone oxides (151) is an equally useful route to the betaines (150). Dimethyl acetylenedicarboxylate and compound 151 (R = R = Ph) at 175 C give adduct 153, and cyclohexanone at 150°C gives adduct 154. Similarly, 1,3-dipolar adducts (e.g., 155) have been obtained using a wide variety of olefins—including cis- and trans-1,2-dichloroethylene, dimethyl maleate, dimethyl fumarate, maleic anhydride, cis- and rra i-stilbene, ra i-dibenzoylethylene, tran. -l,2-dicyanoethylene, A -phenylmaleimide, vinylene carbonate, acenaphthylene, and norbor-nadiene. With cis olefins the endo adduct (155) is usually the predominant isomer. Diphenylcyclopropenone gives compound 156 by spontaneous elimination of carbon monoxide from the initial adduct (157). Adduct 156 

Prolonged thermolysis of compound 151 (R = R = Ph) results in the formation of a pair of dimeric products which have been formulated as compounds 160 (47%) and 161 (4%) and which can be regarded as being formed by addition of the indenone carbonyl group to the betaine. Precedent for this type of addition is formation of the cyclohexanone adduct 154. Evidence for structure 160 is provided by the observation that acid hydrolysis gives a monohydrate which is formulated as compound 162—this product 162 displayed. . . ultraviolent absorption Isomer 161 does not form a monohydrate. 

It is notable that only dimer 160 is produced photochemically, and it is possible that this product (160) is kinetically preferred whereas the thermodynamically preferred isomer (161) is formed only at elevated temperatures. 

Nilsen and Undheim have prepared the unsubstituted betaine 166 by oxidation of 4-acetoxyisochromene (164). Thus, compound 164 with tetrachloro-l,2-benzoquinone (TBQ) or 2,3-dichloro-5,6-dicyano-l,4-ben-zoquinone (DDQ) forms adducts 163 and 165 which in trifluoroacetic acid give 2-benzopyrylium-4-olate (166). Compound 166 cannot be isolated because dimerization readily occurs. Surprisingly, the TBQ adduct (165) gives the endo dimer (168) (19%) together with a small amount of the exo 

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Interaction of a carbonyl group with an electrophilic metal carbene would be expected to lead to a carbonyl ylide. In fact, such compounds have been isolated in recent years 14) the strategy comprises intramolecular generation of a carbonyl ylide whose substituent pattern guarantees efficient stabilization of the dipolar electronic structure. The highly reactive 1,3-dipolar species are usually characterized by [3 + 2] cycloaddition to alkynes and activated alkenes. Furthermore, cycloaddition to ketones and aldehydes has been reported for l-methoxy-2-benzopyrylium-4-olate 286, which was generated by Cu(acac)2-catalyzed decomposition of o-methoxycarbonyl-m-diazoacetophenone 285 2681... 

The most important pyran betaines are the heteroaromatic pyrylium-3-olate (258) and 2-benzopyrylium-4-olate (259) systems. Numerous analogues of both are known, and it has been shown that these have a high propensity to participate in cycloadditions (72JOC3838)>... 

The catalytic asymmetric dipolar cycloaddition reactivity of in rt/ -formed 2-benzopyrylium-4-olates (e.g., 139) with a variety of dipolarophiles including a-alkoxy ketones 140, cr-keto esters 141, and acryloyl oxazolidinones 142 has been extensively studied (Scheme 16) <20000L3145, 2002JA14836, 2003S1413, 2005JOC47, 2006T9218>. 

It is an interesting facet of the chemistry of 2-benzopyrylium-4-olates (150) that they appear to form two types of dimer (cf. 160 and 167) depending upon the nature of the ring substituents. 

Significant levels of enantioselectivity were obtained in 1,3-dipolar cycloadditions of 2-benzopyrylium-4-olate generated from the Rh2(OAc)4-catalyzed decomposition... 

In contrast to the diastereoselectivity of the reaction with benzyloxyacetaldehyde derivatives, the Sc(III)-(5, 5)-PyBOX-catalyzed cycloadditions of 2-benzopyrylium-4-olate with methyl and benzyl pyruvate showed high exoselectivity (Scheme 7.26 and Table 7.20). This is probably attributed to the unfavorable dipolar interactions between the carbonyl groups of 2-benzopyrylium-4-olate and the ester in the eniio-approach. However, the maximum enantiomeric excess of the exo-adduct was only 56% ee when (S,S)-PyBOX-TPSm was used as a ligand (Figure 7.3 and Table 7.20, entry 3). After several attempts to increase the enantioselectivity, both diastereo- (up to exo endo = 96 4) and enantioselectivities (up to 87% ee (exo)) were determined to improve in the Sc(III)-(5,5)-PyBOX-i-Pr-catalyzed reaction (up to 94% yield) when pyruvic acid was used as an additive (entries 5, 6, 8, and 9). By the examinations of some... 

FIGURE 7.2 Relationship between metals and enantio- and diastereoselectivities in PyBOX-i-Pr-M (OTf)3-catalyzed reactions of 2-benzopyrylium-4-olate with benzyloxyacetaldehyde. 

TABLE 7.21 Asymmetric Cycloaddition Reactions of 2-Benzopyrylium-4-olate with a-Ketoesters Catalyzed by Sc(III)-PyBOX-i-Pr-TFA Complex"... 

SCHEME 7.27 Asymmetric cycloaddition reactions of 2-benzopyrylium-4-olate with 3-acryloyl-2-oxazolidinone catalyzed by PyBOX-Ph-Yb(IH) complex. 

SCHEME 7.28 Asymmetric cycloaddition reactions of 2-benzopyrylium-4-olates with 3-(2-alkenoyl)-2-oxazolidinones catalyzed by (4S,5S)-PyBOX-4,5-Ph2-Yb(in) complex. 

TABLE 7.23 Asymmetric Cycloaddition Reactions of 3-Acyl-2-benzopyrylium-4-olates with Vinyl Ethers Catalyzed hy (R)-BINIM-4Me-2QN-Ni(II) Complex"... 

FIGURE 7.4 Relationship between the ionic radius of the lanthanoid metals and the enantio- or diastereoselectivities for the cycloadditions of 2-benzopyrylium-4-olate with butyl vinyl ether catalyzed by chiral (45, 55)-PyBOX, 5-Ph2-M(OTf)3 complexes. 

For the BINlM-Ni(ll)-catalyzed reactions of cyclohexyl vinyl ether, the use of an epoxyindanone as the 3-acyl-2-benzopyrylium-4-olate precursor revealed that the chiral Lewis acid can function as a catalyst for asymmetric induction (Scheme 7.29). Thus, slow addition (over a period of 1 h) of epoxyindanone into a solution of cyclohexyl vinyl ether and the Ni(ll) catalyst in dry CH2CI2 under reflux conditions gave eniio-cycloadduct (60% yield) with 86% ee. This result suggests that the asymmetric induction is effectively catalyzed by the (7 )-BINIM-4Me-2QN-Ni(II) complex, and without the participation of Rh2(OAc)4, which may be involved only in the generation of the carbonyl ylides for reactions of diazocarbonyl compounds as substrates [66]. 

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