Triflic acid Diels-Alder reactions

Trifluoromethanesulfonic acid (triflic acid) in toluene greatly activates the Diels-Alder reaction of benzaldehydes with dimethylated 1,3-butadienes [89] (Table 4.22). With mono-methylated 1,3-butadienes the reaction gives less... 

Table 4.22 Diels-Alder reactions catalyzed by triflic acid...

Similarly, the diethyl acetal of acrolein undergoes facile Diels-Alder reactions in the presence of triflic acid, although yields are only moderate. 2-Vinyl-l,3-dioxolane is recommended as the reagent of choice because of higher yields.2... 

Corey, E. J. Shibata, T. Lee, T. W. Asymmetric Diels-Alder Reactions Catalyzed by a Triflic Acid Activated Chiral Oxazaborolidine. J. Am. Chem. Soc. 2002,124,3808-3809. 

A low amount (1 mol%) of triflic acid is sufficient to carry out the hetero Diels-Alder reaction of aromatic aldehydes with simple dienes to furnish 3,6-dihyro-2/7-pyran derivatives in moderate to good yields692 [Eq. (5.248)]. The strongly deactivated para-methoxybenzaldehyde and pentanal gave the products in very low yields. 

Efficient synthetic methods have been developed for the synthesis of 4-functionalized quinolines mediated by triflic acid.703 Electron-rich, highly reactive ethynyl ketene-S,5-acetals react readily with arylamines and aldehydes in an aza-Diels-Alder reaction to afford the desired products [Eq. (5.256)]. Arylimines and ethynyl ketene-S,5-acetals react similarly (60-70% yields). 

The aza-Diels-Alder reaction in Eq. (5.331) catalyzed by triflic acid has been carried out to synthesize adduct 278a, an intermediate in the total synthesis of microfungal alkaloid ( )-lapatin.904 Whereas the reaction of the substituted azadienes led to the exclusive formation of exo compounds, the unsubstituted parent compound gave a 1 1 mixture of the isomers. 

Gorman and Gassman905 have shown that undecatetraenes undergo cyclization (intramolecular Diels-Alder reaction) in the presence of triflic acid to provide bicyclo[4.3.0]nonyl [Eq. (5.332)], bicyclo[4.4.0]decyl, and bicyclo[5.4.0]undecyl [Eq. (5.333)] ring systems, depending on the methyl-substitution pattern. On the basis of a comparative study with varied tetraenes, they concluded that product formation, at least in some cases, could be best interpreted by a stepwise... 

Scheme 51 presents an entirely different mode of catalyzed [4 + 2] cycloadditions. Thus irimediylsilyl triflate or triflic acid apparently transform vinyl orthoesters and acrolein acetals to powerful transient allyl cation dienophiles (214) and (219), respectively. The ionic Diels-Alder reactions (213) (216)... 

Catalysts derived from triflic acid decompose at appreciable rates at or above 0 °C, which limits the utility of these reagents in Diels-Alder reactions. Switching to triflimide as the acid source resulted in protonation of 45 to produce 53.16 H NMR from -80 to 23 °C showed the formation of three species including 53 and two diastereomeric tetracoordinated boron species in a ratio of 1 1.2. Additionally, 53 was found to have greater thermal stability and superior catalytic efficiency compared to 46/47 for less reactive dienes. This was illustrated in the Diels-Alder reaction of 54 with 55 to produce adduct 56. 

Cationic Lewis acids show improved acidity and hence activity. For instance, even at —94°C, relatively unreactive dienes such as butadiene (8.29) react with bromoacrolein (8.05) with excellent yields and good selectivities in the presence of the oxazaborinane (8.30). The Lewis acidity of oxazoborolidines can also be enhanced by protonation using strong Bronsted acids. The triflate salt (8.32) generated by protonation of the corresponding oxazaborolidine with triflic acid shows high activity and wide scope catalysing the Diels-Alder reaction of acyclic... 

Corey et al. have shown the asymmetric Diels-Alder reactions catalyzed by a triflic acid activated chiral oxazaborolidine (eq 45). Triflic acid has also been found to be an efficient catalyst (1 mol %) for the hetero-Diels-Alder reaction between aromatic aldehydes and unactivated dienes. ... 

In a related reaction, the Danishefsky diene 1434 cyclizes with ethyl pyruvate 1435 in the presence of catalytic amounts of the asymmetric Lewis acid catalyst 1436, at -72 °C in THF, to give the Diels-Alder adduct 1437, in 85% yield and 91% ee, and the ring-opened product 1438, which cyclizes, however, with triflic acid to give 1437 [11] (Scheme 9.9). 

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