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8307288 
Journal Article 
Chiral N-heterocyclic carbene-catalyzed formal [4+2] cycloaddition of ketenes with enones: highly enantioselective synthesis of trans- and cis-delta-lactones 
Zhang, YR; Lv, H; Zhou, D; Ye, S; , 
2008 
Yes 
Chemistry: A European Journal
ISSN: 0947-6539
EISSN: 1521-3765 
14 
28 
8473-8476 
English 
18688823 
The chiral N-hetrocyclic carbenes (NHC)-catalyzed formal [4+2] cycloaddition of ketones with enones to give δ-lactones was reported. The results show that reaction at 0°C leads to better enantioselectivity and yield, while at -10°C result in slightly better enantioselectivity with lesser yield. It is also noted that the diasterometric ratios and enantiometric excess is improved by a single recrystallization from hexane/2-propanol 9:1. The cis-isomer of δ-lactones could be obtained with high diasteroselectivity and enantioselectivity by the in situ deprotonation-protonation after the NHC-catalyzed cycloaddition. The NHC-catalyzed [4+2] cycloaddition of ketones are possibly initiated by the nucleophilic addition of NHC to ketenes to give triazolium enolates. 
Asymmetric catalysis; Carbenes; Cycloaddition; Ketenes; Lactones