Modified Guanidines as Potential Chiral Superbases. 3. Preparation of 1,4,6-Triazabicyclooctene Systems and 1,4-Disubstituted 2-Iminoimidazolidines by the 2-Chloro-1,3-dimethylimidazolinium Chloride-Induced Cyclization of Guanidines with a Hydroxyethyl Substituent
Simple preparation methods of modified guanidines have been explored as potential chiral superbases. Thus, 3,7,8-trisubstituted and 3,6,7,8-tetrasubstituted 1,4,6-triazabicyclooctene systems were prepared from (1S,2S)-1,2-diphenylethylenediamine through stepwise 2-chloro-1,3-dimethylimidazolinium chloride (DMC)-induced cyclizations of protected thioureas to the corresponding 2-iminoimidazolidines and then of 2-(2-hydroxyethylimino)imidazolidines to the bicyclic systems. Linear guanidines with a 2-hydroxyethyl functional group were prepared by the reaction of carbodiimides with 2-amino alcohols. Reaction of linear-type guanidines with DMC followed by base treatment afforded 1,4-disubstitued 2-iminoimidazolidines. Furthermore, another type of 1,4,6-triazabicyclooctene was also prepared through double DMC-induced cyclization of guanidines with two a-hydroxyethyl substituents.
N-acylimidazolidin-2-ones: new chiral auxiliaries for carboxylic acid alkylation
Chiral N-acylated imidazolidin-2-ones, readily available from aminoethanols in three steps through the nucleophilic opening of oxazoline intermediates, have been demonstrated to undergo highly diastereoselective benzylations and methylations via their sodium enolates. In most cases, the resulting products are highly crystalline, and the chiral auxiliaries can be readily recycled. (C) 1997 Elsevier Science Ltd.