A continuous flow
method for the selective oxidation of secondary
alcohols is reported. The method is based on an Oppenauer-type ruthenium-catalyzed
hydrogen-transfer process that uses acetone as both solvent and oxidant.
The process utilizes a low loading (1 mol%) of the commercially available
ruthenium catalyst [Ru(p-cymene)Cl2]2 and triethylamine as a base and can be successfully applied
to a range of different substrates, with a good level of functional
group tolerance.
Lithiation/electrophile trapping reactions were carried out with the highly enantiomerically enriched complex [Cr(5-bromonaphthalene)(CO)3]. Electrophile quenching with ClPPh2, PhCHO, and (Me3SiO)2 afforded the enantiomerically enriched (>97% ee) planar chiral 5-substituted naphthalene complexes with PPh2, CH(Ph)OH, and OH substituents, respectively. Very mild Pd-catalyzed Suzuki-Miyaura cross-coupling reactions were developed and applied to the highly labile [Cr(5-bromonaphthalene)(CO)3] to give nine new planar chiral aryl-, heteroaryl-, alkynyl-, and alkenylnaphthalene chromium complexes with high enantiomeric purity. The efficient ambient-temperature coupling reactions with borinates prepared in situ were also applied to a number of chlorobenzene complexes and to aryl and vinyl halides.
The total syntheses of the Lythracea alkaloids (+)-vertine and (+)-lythrine are described. Enantioenriched pelletierine is used as a chiral building block and engaged into a two step pelletierine condensation leading to two quinolizidin-2-one diastereomers in a 8 : 1 ratio. The major product is used in the synthesis of (+)-vertine via aryl-aryl coupling and ring closing metathesis to provide a Z-alkene α to the lactone carbonyl function. The same procedure was used for (+)-lythrine after base induced epimerization of the main quinolizidin-2-one diastereomer. Alternative classical ring closure strategies like macrolactonisation or aryl-aryl coupling failed.
A concise total synthesis of the strained pentacyclic alkaloid (+/-)-Vertine has been achieved in eleven steps with the key steps being pelletierine condensation, Suzuki-Miyaura coupling, and ring-closing metathesis.
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