H. Kagoshima et al., AN ACTIVATED GERMANIUM METAL-PROMOTED, HIGHLY DIASTEREOSELECTIVE REFORMATSKY REACTION, Journal of organic chemistry, 63(3), 1998, pp. 691-697
Activated germanium metal, prepared by the reduction of germanium(II)
iodide with potassium metal, was found to promote the Reformatsky reac
tion effectively under mild conditions. In the presence of activated g
ermanium metal, the reactions of alpha-bromo ketones 2a and 2b and alp
ha-bromo imides 2e and 2f with benzaldehyde (1a) proceeded smoothly to
give the corresponding beta-hydroxy carbonyl compounds 3a, 3d, 3e and
3f, respectively, in good yields and with good syn diastereoselectivi
ty. The activated germanium metal-promoted, asymmetric Reformatsky rea
ction of enantiomerically pure-oxazolidinone derivatives 2g-j with var
ious aldehydes 1a-d was also examined; the highest diastereoselectivit
y was achieved when (1S,2R)-2-amino-1,2-diphenylethanol-derived 2j was
used as the Reformatsky donor. The excellent diastereoselectivity cou
ld be explained in terms of the formation of a chairlike, six-membered
transition state between the aldehyde and enolate as in the Zimmerman
-Traxler model. A single recrystallization of the Reformatsky adducts,
followed by hydrolysis and subsequent esterification, led to enantiom
erically pure methyl 3-hydroxy-2-methylalkanoates 10j-m, with almost q
uantitative recovery of the enantiomerically pure 2-oxazolidinone 14.