Protonated C-acyl iminium ions as heterodienophiles

Grieco et al. have also reported the use of C-acyl iminium ions as heterodienophiles in the aqueous Diels-Alder reaction with cyclopentadiene [8]. This work details the cyclocondensation of C-acyl A-alkyl iminium ions 18 and C-acyl A-unsubstituted iminium ions 19 with cyclopentadiene in water  

In a typical protocol, the C-phenacyl A-benzyl iminium ion 20 is generated from a 2.0M aqueous solution of benzylamine hydrochloride (l.Oequiv) containing phenylglyoxal (1.2equiv). Addition of neat cyclopentadiene (2.0 equiv) affords a heterogeneous mixture, which after 22 h at ambient temperature gives rise to an 88% yield of adducts 21 and 22 in a 3/2 ratio  

From (SHsoleucine whero Ri= R2sMe, R2=H From (R)-phonylglycine whom Rib RssMo, RssH  

The A -unsubstituted iminium ion 23 generated in situ from an aqueous saturated ammonium chloride solution and phenylglyoxal reacts with cyclopentadiene providing an 89% yield of the exo/endo adducts 24 and 25 (1/2 ratio)  

Use of pyruvic aldehyde in place of phenylglyoxal resulted in the corresponding Diels-Alder adducts in similar proportion and yield. 

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