Amidinium ions synthesis

Schuster et al. reported the accelerating effect of the amidinium ion on the Diels-Alder reaction [ 15]. Reaction of the diene 59 and diketone 60a or 60b in the presence of lipophilic amidinium ion 63 (1 equiv.) gave 61 and 62 (2.5 3.2 1), with a 100-fold rate increase compared to the uncatalysed conditions. When the reaction was run in the presence of chiral amidinium compound 64, 61 and 62 were obtained in 70 94% yield (ca 3 1) with 11 50% ee. The Diels-Alder adduct 61b is a key intermediate for synthesis of (—)-norgestrel (Scheme 7.12). The reaction enhancement effect of amidinium ion can be explained in terms of the hydrogen bond mediated interaction with diketone. 

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