Formation of Chiral C(sp3)−C(sp) Bond by Allylic Substitution of Secondary Allylic Picolinates and Alkynyl Copper Reagents
摘要:
To establish allylic substitution of secondary allylic alcohol derivatives with alkynyl copper reagents, allylic esters bearing the (2-pyridine)CO2-, (2-pyrazine)CO2-, (EtO)(2)PO2-, C6F5CO2-, o-(Ph2P)-C6H4CO2-, MeOCO2-, or AcO-group were examined. First, picolinate (R-1 = Me, R-2 = CH2OPMB) was subjected to reaction with (TMS-C C)(2)CuLi center dot LiBr at 0 degrees C. Although no substitution took place, MgBr, (3 equiv) was found to promote the reaction to produce the anti S(N)2' product in 93% yield with 94% regioselectivity and 99% chirality transfer. In contrast, substitution of the other esters with the copper reagent in the presence of MgBr, were less reactive ((2-pyrazin)CO2-) or marginally reactive (other cases). Generality of the substitution using picolinates was established with five picolinates (R-1 = Me, Ph(CH2)(2), PMBO(CH2)(3); R-2 = Me, CH2OPMB, CH2OTBS, C5H11, c-C6H11) and seven alkynyl copper reagents (R-3 = TMS, Ph, p-TBSOC6H4, p- and o-MeOC6H4, P-MeC6H4, p-FC6H4), furnishing anti S(N)2' products in 61-93% yields with high regioselectivity (usually > 90%) and high chirality transfer (usually > 95%). In addition, transformation of the products was briefly studied.
Has a nice ring to it: A concise and modular totalsynthesis of the naturally occurring antibiotic virginiamycin M2 is described. A Barbier‐type cyclization was used to close the 23‐membered macrocycle and deliver virginiamycin M2 in 19 steps from a chiral organosilane.
Total Synthesis of (−)-Virginiamycin M<sub>2</sub>: Application of Crotylsilanes Accessed by Enantioselective Rh(II) or Cu(I) Promoted Carbenoid Si–H Insertion
作者:Jie Wu、James S. Panek
DOI:10.1021/jo202119p
日期:2011.12.16
A stereoselective synthesis of the antibiotic (−)-virginiamycin M2 is detailed. A convergent strategy was utilized that proceeded in 10 steps (longest linear sequence) from enantioenriched silane (S)-15. This reagent, which was prepared via a Rh(II)- or Cu(I)-catalyzed carbenoid Si–H insertion, was used to introduce the desired olefin geometry and stereocenters of the C1–C5 propionate subunit. A modified
Reversible 1,3-<i>anti</i>/<i>syn</i>-Stereochemical Courses in Copper-Catalyzed γ-Selective Allyl–Alkyl Coupling between Chiral Allylic Phosphates and Alkylboranes
The stereochemical courses of the copper-catalyzed allyl-alkyl coupling between enantioenriched chiral allylic phosphates and alkylboranes were switchable between 1,3-anti and 1,3-syn selectivities by the choice of solvents and achiral alkoxide bases with different steric demands. The reactions with gamma-silylated allylic phosphates allow efficient synthesis of enantioenriched chiral allylsilanes with tertiary or quaternary carbon stereogenic centers. Cyclic and acyclic bimodal participation of alkoxyborane species in an organocopper addition-elimination sequence is proposed to account for the phenomenon of the anti/syn-stereochemical reversal.