2-Benzopyrylium-4-olates cycloaddition

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>. 

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... 

SCHEME 7.26 As)Tnmetric cycloaddition reactions of 2-benzopyrylium-4-olate with several... 

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

SCHEME 7.28 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. 

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