- Synthesis of Chiral Tertiary Boronic Esters: Phosphonate-Directed Catalytic Asymmetric Hydroboration of Trisubstituted Alkenes
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Highly enantioselective rhodium-catalyzed hydroboration of allylic phosphonates by pinacolborane affords chiral tertiary boronic esters. The β-borylated phosphonates are readily converted to chiral β- and γ-hydroxyphosphonates and aminophosphonates and to phosphonates bearing a quaternary carbon stereocenter. The utility of the latter is illustrated by the synthesis of (S)-(+)-bakuchiol methyl ether.
- Chakrabarty, Suman,Takacs, James M.
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- A facile asymmetric synthesis of Δ3-2-Hydroxybakuchiol, Bakuchiol and ent-Bakuchiol
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A facile asymmetric synthesis of Δ3-2-Hydroxybakuchiol, Bakuchiol, and ent-Bakuchiol is described through one common intermediate bearing a chiral all-carbon quaternary carbon center, which involves stereoselectively unconjugated alkylation of the α,β-unsaturated imide bearing Evans' auxiliary, Takai-Utimoto reaction, Negishi reaction, and Heck reaction as key steps.
- Xu, Qian-Qian,Zhao, Qun,Shan, Guang-Sheng,Yang, Xi-Cheng,Shi, Qi-Yuan,Lei, Xinsheng
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p. 10739 - 10746
(2013/12/04)
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- Synthesis of quaternary carbon stereogenic centers through enantioselective Cu-catalyzed allylic substitutions with vinylaluminum reagents
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Catalytic enantioselective allylic substitution (EAS) reactions, which involve the use of alkyl- or aryl-substituted vinylaluminum reagents and afford 1,4-dienes containing a quaternary carbon stereogenic center at their C-3 site, are disclosed. The C-C bond-forming transformations are promoted by 0.5-2.5 mol % of sulfonate bearing chiral bidentate N-heterocyclic carbene (NHC) complexes, furnishing the desired products efficiently (66-97% yield of isolated products) and in high site (>98% SN2′)- and enantioselectivity [up to 99:1 enantiomer ratio (er)]. To the best of our knowledge, the present report puts forward the first cases of allylic substitution reactions that result in the generation of all-carbon quaternary stereogenic centers through the addition of a vinyl unit. The aryl- and vinyl-substituted vinylaluminum reagents, which cannot be prepared in high efficiency through direct reaction with diisobutylaluminum hydride, are accessed through a recently introduced Ni-catalyzed reaction of the corresponding terminal alkynes with the same inexpensive metal-hydride agent. Sequential Ni-catalyzed hydrometalations and Cu-catalyzed C-C bond-forming reactions allow for efficient and selective synthesis of a range of enantiomerically enriched EAS products, which cannot be accessed by previously disclosed strategies (due to inefficient vinylmetal synthesis or low reactivity and/or selectivity with Si-substituted derivatives). The utility of the protocols developed is demonstrated through a concise enantioselective synthesis of natural product bakuchiol.
- Gao, Fang,McGrath, Kevin P.,Lee, Yunmi,Hoveyda, Amir H.
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supporting information; experimental part
p. 14315 - 14320
(2010/12/19)
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- Stereoselective total synthesis of natural (S)-bakuchiol and its enantiomer
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A practical stereoselective synthesis of (S)-bakuchiol (1) and its enantiomer is reported. The important intermediate, (R)-configured β-siloxy aldehyde 5, was obtained in three steps from the easily available material geraniol (2) via the key step of Yamamoto's rearrangement of epoxy silyl ethers. (S)-Bakuchiol (1) and its enantiomer, (R)-bakuchiol (17), were finally obtained in different synthetic sequences with overall yields of 51% (ten steps) and 40% (nine steps) from geraniol (2), respectively.
- Du, Xiao-Long,Chen, Hong-Li,Feng, Hui-Jin,Li, Yuan-Chao
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experimental part
p. 371 - 378
(2009/02/07)
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