✍ Scribed by Angel Mazón; Carmen Nájera
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The enantiomerically pure glycine derivative tert-butyl (S)-2-(tert-butyl)-3-methyl-4-oxo-l-imidazolidinecarboxylate (Boc-BMI) reacts with (Z)-l,4-dichloro-2butene and 3-chloro-2-(chloromethyl)-1-propene to give the bicyclic intermediates 5 and 6, respectively. These dialkylated systems are hydrolyzed to the corresponding heterocyclic or-amino acids (S)-baikiain (L-4,5-didehydropipecolic acid) and (S)-4-methylene proline which are obtained in 96 and 90% ee, respectively. (~) 1997 Elsevier Science Ltd (-)-Baikiain (S)-I (L-4,5-didehydropipecolic acid) 1,2 and racemic 4-methyleneproline (2) 3 are natural cyclic amino acids present in rhodesian teak (Baikiaea plurijuga), Caesalpina tinctoria, red algae and in seeds of Ioquat (Eriobotrya japonica), respectively. Enantiomerically pure (S)-I 4 and (S)-25 have different biological activities as structurally modified prolines. The former is usually obtained from natural sources and the latter has been recently prepared by EPC-synthesis starting from trans-4-hydroxy-L-proline 5a'6 or (S)-pyroglutamic acid 7 and by a radical cyclization of a (S)-serine derivative. 8
The dialkylation of chiral glycine enolates with dielectrophiles is an appropriate strategy for the convergent asymmetric synthesis of cyclic ¢x-amino acids. 9 Sch611kopf's bis-lactim ether 31° and Seebach's imidazolidinones (Boc-BMI) 4 II are good chiral building blocks for the preparation of saturated heterocyclic or-amino acids containing azetidine, pyrrolidine, piperidine and perhydroazepine rings. 12 We describe here the application of the (S)-Boc-BMI derivative to the synthesis of the unsaturated heterocyclic ¢x-amino acids (S)-baikiain 1 and (S)-4-methyleneproline 2.
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