Protonated glyoxylate imines

Bronsted-acid-catalyzed Diels-Alder reactions are not frequent because of the proton sensitivity of many dienes and cycloadducts, especially when long reaction times and high temperatures are required. Examples in aqueous medium involving imines activated by protonation as dienophiles and a proton-promoted Diels-Alder reaction of glyoxylic acid with cyclopentadiene are considered in Section 6.1. 

Protonated imines are effective dienophiles. Thus in the reaction of methyl glyoxylate with the hydrochloride 127 of alanine methyl ester in the presence of cyclopentadiene, a mixture of hydrochlorides of the exo- and ewrfo-adducts 128-131 was formed (equation 69). The diastereomeric ratio of the exo-compounds was 83 1761. 

By treatment with NaH, isoxazolidine 196 underwent N-O bond cleavage to afford 198. The process is believed to occur through deprotonation of the carbamate nitrogen followed by proton transfer with simultaneous N-O bond cleavage to imine 197, and subsequent loss of methyl glyoxylate (Equation 23) <2001JOC6046>. 

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