diastereoselective syntheses, enantioselective syntheses diastereoselective syntheses, enantioselective syntheses (incl. cis/trans-isomerism) O 0031
-034Synthetic Methodology for the Construction of Structurally Diverse Cyclopropanes.-Practical and efficient routes for the stereoselective conversion of homoallylic alcohols like (I) and (VII) to diastereomerically pure cis-and trans-1,2-disubstituted, and 1,2,3-trisubstituted cyclopropanes are developed. One route involves ring closing metathesis reaction of bisolefins like (III) to give after cleavage of the 7-membered ring the silylated cyclization precursors like (V). The other more efficient approach provides the cyclopropanes in just two steps by cross-metathesis reaction of the homoallylic alcohol and by activation of the cyclization precursors like (XV) with inexpensive SOCl 2 at room temperature. The cyclopropane product chirality is ultimatively controlled by the choice of homoallylic alcohol starting material.
N-heterocyclic olefins (NHOs), IPrCH2 (1) and SIPrCH2 (2) (IPrCH2 = {N(2,6-iPr2C6H3)CH}2CCH2 and SIPrCH2 = {N(2,6-iPr2C6H3)CH2}2CCH2), react with HSiCl3 and afford IPrCH(SiHCl2) (3) and SIPrCH(SiHCl2) (4), respectively. Compounds 3 and 4 have been isolated in almost quantitative yield. Interestingly, treatment of the silylene IPr·SiCl2 with 1 also affords 3, where silylene insertion into a C–H bond is observed. Computational analysis shows a high energy barrier for silylene insertion, therefore a protonation–deprotonation mechanism is more likely.
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