Alkaloids as phase-transfer-catalysts

Arai and co-workers have used chiral ammonium salts 89 and 90 (Scheme 1.25) derived from cinchona alkaloids as phase-transfer catalysts for asymmetric Dar-zens reactions (Table 1.12). They obtained moderate enantioselectivities for the addition of cyclic 92 (Entries 4—6) [43] and acyclic 91 (Entries 1-3) chloroketones [44] to a range of alkyl and aromatic aldehydes [45] and also obtained moderate selectivities on treatment of chlorosulfone 93 with aromatic aldehydes (Entries 7-9) [46, 47]. Treatment of chlorosulfone 93 with ketones resulted in low enantioselectivities. 

Based on prior results where Ricci used Cinchona alkaloids as phase-transfer-catalysts, the group proceeded to look at hydrophosphonylation of imines [48], Employing the chiral tertiary amine as a Brpnsted base, a-amino phosphonates products were synthesized in high yields and good selectivities. 

Enantioselective synthesis of a-amino acids by phase-transfer catalysis with achiral Schiff base esters using Cinchona alkaloids as phase-transfer catalysts 04ACS506. 

The asymmetric Darzens reaction of a-chlorocycloalkanones using quatemized cinchona alkaloids as phase-transfer catalyst and LiOH as base in BU2O at room temperature is usually high yielding. However, the moderate ee is disappointing. 

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