2-benzopyrylium-4-olate

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... 

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... 

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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