ISSN:
0009-2940
Keywords:
Diastereoselectivity
;
Protonation
;
Pyrazines
;
dialkoxydihydro-
;
Chemistry
;
Inorganic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Stereoselective Protonation of Carbanions, 2[1]. - Diastereoselective Protonation of Schöllkopf's Bislactim Ether AnionsThe anions of the bis-lactim ethers 5-7 serve as models for a systematic investigation of diastereoselective protonation. From 5Li, 6Li, and 7Li in THF at -78°C 30 proton sources (—OH, —NH, —SH, —CH) of different acidity and bulkiness produce the following cis/trans ratios: 5 8:92 → 37:63; 6 15:85 → 48:52; 7 25:75 → 59:41 (equilibria ≈ 50:50). Substitution of R1 = iPr in 5 by Me (→ 6) decreases the stereoselectivity slightly, but substitution of OMe in 6 by OEt (7) much stronger (Table 1). Most of the added Lewis acids show week or even leveling effects, whereas with 5K a dramatic change is observed with cis/trans-5 = 27:73 → 89:11 (Table 2). By variation of solvent and several additives only minor effects on the cis/trans ratio are observed (Table 3). Backed by 13C—NMR spectra of the ion pairs and literature data protonation of 5Li-7Li is supposed to occur on a dimer, which will mainly react to give trans-5-7. The prevailing formation of cis-5 from 5K is explained by the reaction of the monomeric ion aggregat.
Additional Material:
7 Tab.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/cber.19941270125
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