Esterification and Ester Cleavage

Triflic acid has been used in the selective esterification ( (9-acylation) of a series of aminoalcohols in the presence of a crown ether (DB24C8) to prepare rotaxanes.900 The test reaction of diethanolamine with bulky anhydride, crown ether, and triflic acid (molar ratio = 1 2 2 1.5) gave the rotaxane 276 in high yield in a clean reaction [Eq. (5.327)]. /V-Arylmethylaminoalcohols were similarly transformed (85-92% yields). 

Olah et al.902 demonstrated in 1979 that Nafion-H is able to catalyze Diels-Alder reaction of anthracene with a number of dienophile (maleic anhydride [Eq. (5.329)], para-benzoquinone, dimethyl maleate, dimethyl fumarate) in chloroform or benzene 

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 

The formation of polysubstituted cycloheptadienones depicted in Eq. (5.334) constitutes the first example of an acid-induced ketene-diene cycloaddition.908 

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