共查询到20条相似文献,搜索用时 0 毫秒
1.
2.
3.
4.
Stereoselectivity of 9,9′-Spirobifluorene Crown Ethers towards α-Amino Alcohols Crown ethers I-VI were tested by partition experiments for their stereoselectivity towards α-amino alcohols 1–10. The stereoselectivity depends in a regular way on both the absolute and relative configuration of the crown ether and α-aminoalcohol. Comments are made on some high stereoselectivities. 相似文献
5.
6.
7.
8.
9.
10.
11.
Gyrgy Frter 《Helvetica chimica acta》1980,63(6):1383-1390
The Stereoselectivity of the α-Alkylation of (+)-(1R, 2S)-cis-Ethyl-2-hydroxy-cyclohexanecarboxylate In continuation of our work on the stereoselectivity of the α-alkylation of β-hydroxyesters [1] [2], we studied this reaction with the title compound (+)- 2 . The latter was prepared through reduction of 1 with baker's yeast. Alkylation of the dianion of (+)- 2 furnished (?)- 4 in 72% chemical yield (Scheme 1) and with a stereoselectivity of 95%. Analogously, (?)- 7 was prepared with similar yields. Oxidation of (?)- 4 and (?)- 7 respectively furnished the ketones (?)- 6 (Scheme 3) and (?)- 8 (Scheme 4) respectively, each with about 76% enantiomeric excess (NMR.). It is noteworthy that yeast reduction of rac- 6 (Scheme 3) is completely enantioselective with respect to substrate and product and gives optically pure (?)- 4 in 10% yield, which was converted into optically pure (?)- 6 (Scheme 3). The alkylation of the dianionic intermediate shows a higher stereoselectivity (95%) from the pseudoequatorial side than that of 1-acetyl- or 1-cyano-4-t-butyl-cyclohexane (71% and 85%) [9] or that of ethyl 2-methyl-cyclohexanecarboxylate (82%). The stereochemical outcome of the above alkylation is comparable with that found in open chain examples [1] [2]. Finally (+)-(1R, 2S)- 2 was also alkylated with Wichterle's reagent to give (?)-(1S, 2S)- 9 in 64% yield. The latter was transformed into (?)-(S)- 10 and further into (?)-(S)- 11 (Scheme 5). (?)-(S)- 10 and (?)-(S)- 11 showed an e.e. of 76–78% (see also [11]). Comparison of these results with those in [11] confirmed our former stereochemical assignment concerning the alkylation step. 相似文献
12.
13.
14.
15.
16.
17.
18.
C. H. Eugster R. Buchecker Ch. Tscharner G. Uhde G. Ohloff 《Helvetica chimica acta》1969,52(6):1729-1731
19.