Studies on the mechanism, selectivity, and synthetic utility of lactone reduction using SmI(2) and H(2)O.
Although simple aliphatic esters and lactones have long been thought to lie outside the reducing range of SmI(2), activation of the lanthanide reagent by H(2)O allows some of these substrates to be manipulated in an unprecedented fashion. For example, the SmI(2)-H(2)O reducing system shows complete selectivity for the reduction of 6-membered lactones over other classes of lactones and esters. The kinetics of reduction has been studied using stopped-flow spectrophotometry. Experimental and computational studies suggest that the origin of the selectivity lies in the initial electron-transfer to the lactone carbonyl. The radical intermediates formed during lactone reduction with SmI(2)-H(2)O can be exploited in cyclizations to give cyclic ketone (or ketal) products with high diastereoselectivity. The cyclizations constitute the first examples of ester-alkene radical cyclizations in which the ester carbonyl acts as an acyl radical equivalent.
Parmar
Dixit
D
School of Chemistry, University of Manchester, Oxford Road, Manchester, M13 9PL, United Kingdom.
Duffy
Lorna A
LA
Sadasivam
Dhandapani V
DV
Matsubara
Hiroshi
H
Bradley
Paul A
PA
Flowers
Robert A
RA
Procter
David J
DJ
D003500:Cyclization; D007454:Iodides / Q000737:chemistry*; D007700:Kinetics; D007783:Lactones / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D010084:Oxidation-Reduction; D012493:Samarium / Q000737:chemistry*; D013237:Stereoisomerism; D014867:Water / Q000737:chemistry*
D007454:Iodides; D007783:Lactones; D014867:Water; D012493:Samarium; C409809:samarium diiodide