ABSTRACT
Rhodium-catalyzed intramolecular cyclization of nitrogen-tethered allenols was investigated for the synthesis of functionalized morpholines. By using this strategy, various N-protected 2,5- and 2,6-disubstituted as well as 2,3,5- and 2,5,6-trisubstituted morpholines were obtained via an atom-economic pathway with high to excellent yields, diastereo- and enantioselectivities (up to 99% yield, up to >99 : 1 dr and up to >99.9 ee). The utilities of the synthesized morpholines in ozonolysis, hydration, metathesis and epoxidation reactions were also investigated.
ABSTRACT
Rhodium-catalyzed regio- and enantioselective hydrothiolation of terminal allenes with thioacids is reported for the atom-economic synthesis of chiral branched allylic thioesters. By using a rhodium(I) catalyst system, diversities of terminal allenes and thioacids afforded the corresponding branched thioesters in excellent regioselectivity, high yield, and good enantioselectivity. This method was also explored for Fmoc-protected aminothioacids for diastereoselective synthesis of the corresponding thioesters.
ABSTRACT
A highly regio- and stereocontrolled procedure for the synthesis of vinyl dithiocarbamates via catalyst-free hydrothiolation of unactivated alkynes with in situ prepared dithiocarbamic acids with total atom economy is reported. While unactivated terminal alkynes afforded the Markovnikov adducts with excellent regioselectivity, ethynyltrimethylsilane furnished the E-selective anti-Markovnikov adducts with excellent regio- and stereoselectivity. In addition, the obtained vinyl dithiocarbamates were applied as efficient precursors for the synthesis of secondary thiols and α-hydroxythioamides.
ABSTRACT
A new class of phosphinine/rhodium catalysts for the hydroformylation of terminal and internal alkenes is presented in this study. A series of phosphabenzenes 1-14 has been prepared by condensation of phosphane or tris(trimethylsilyl)phosphane with the corresponding pyrylium salt. Trans-[(phosphabenzene)2RhCl(CO)] complexes 21-25 have been prepared and studied spectroscopically and by X-ray crystal-structure analysis. The hydroformylation of oct-1-ene has been used to identify optimal catalyst preformation and reaction conditions. Hydroformylation studies with 15 monophosphabenzenes have been performed. The catalytic performance is dominated by steric influences, with the phosphabenzene 8/rhodium system being the most active catalyst. Turnover frequencies of up to 45370 h(-1) for the hydroformylation of oct-1-ene have been determined. In further studies, hydroformylation activity toward more highly substituted alkenes was investigated and compared with the standard industrial triphenylphosphane/rhodium catalyst. The reactivity differences between the phosphabenzene and the triphenylphosphane catalyst increase on going to the more highly substituted alkenes. Even tetrasubstituted alkenes reacted with the phosphabenzene catalyst, whereas the triphenylphosphane system failed to give any product. In situ pressure NMR experiments have been performed to identify the resting state of the catalyst. A monophosphabenzene complex [(phosphinine 8)Ir(CO)3H] could be detected as the predominant catalyst resting state.
ABSTRACT
An anti-selective hydroformylation of 2-propylidene-substituted 1,3-dioxanes 16, 17, and 26 with excellent levels of acyclic stereocontrol has been achieved. The basis of this result was a careful substrate design making use of a syn-pentane interaction as the decisive stereochemical control element. Confirmation of this working hypothesis came from conformational analysis studies on alkenic substrate 16 employing 2D NOESY experiments in solution and MACROMODEL/MM3 calculations. This stereoselective, transition metal-catalyzed, C-C bond-forming reaction could serve as a key step for the construction of the all-anti and syn-anti stereotriad building blocks 20, 21, and 31, which should be well-suited for target-oriented polypropionate synthesis. Application of this methodology for the construction of a C5-C11 building block for the synthesis of bafilomycin A1 is described.
Subject(s)
Macrolides , Propionates/chemical synthesis , Anti-Bacterial Agents/chemical synthesis , Biological Factors/chemical synthesis , Biopolymers , Enzyme Inhibitors/chemical synthesis , StereoisomerismABSTRACT
The specific introduction of an appropriately designed reagent-directing group into an organic substrate allows the more efficient use of substrate direction to allow high levels of acyclic stereocontrol in both rhodium-catalyzed hydroformylation and cuprate addition to enoates. This provides access to major building blocks of the polyketide class of natural products. Incorporation of these directed reactions into sequential transformations holds promise for new particularly efficient synthetic methods.
