28 research outputs found
Regio- and Diastereoselective Synthesis of Dihydropyridopyrimidines via Cascade Reactions of 2‑Aminopyridines with Morita–Baylis–Hillman Bromides of Nitroalkenes
The Morita–Baylis–Hillman
(MBH) bromides of nitroalkenes
have been employed as bielectrophiles for the first time. The 1,3-bielectrophilic
reactivity of the MBH bromides has been demonstrated in the synthesis
of 3,4-dihydro-2<i>H</i>-pyrido[1,2-<i>a</i>]pyrimidines.
The reaction of MBH bromides with 2-aminopyridines takes place in
the absence of any reagent in a cascade S<sub>N</sub>2′-6-endo-trig
fashion and is completely regioselective and highly stereoselective.
The products, in their hydrobromide salt form, could be conveniently
isolated and purified by crystallization. The high stereoselectivity
has been rationalized in terms of the greater stability of the transition
state in which the Ar and NO<sub>2</sub> groups are anti to each other
Enantioselective Synthesis of Quaternary α-Aminophosphonates via Conjugate Addition of α-Nitrophosphonates to Enones
Enantioselective Michael addition of <b>α</b>-nitrophosphonates to enones for the synthesis of <b>α</b>-aminophosphonates is reported for the first time. The reaction proceeds in good to high yields and moderate to high selectivity in the presence of a new quinine thiourea catalyst. The quaternary nitrophosphonates were conveniently transformed to cyclic quaternary <b>α</b>-aminophosphonates via in situ reduction–intramolecular cyclization or Baeyer–Villiger oxidation followed by in situ reduction–intramolecular cyclization
Quinine-Derived Thiourea and Squaramide Catalyzed Conjugate Addition of α‑Nitrophosphonates to Enones: Asymmetric Synthesis of Quaternary α‑Aminophosphonates
Conjugate
addition of α-nitrophosphonates to enones was carried
out in the presence of two sets of organocatalysts, viz. a quinine-thiourea
and a quinine-squaramide. The quinine-thiourea provided the products
possessing an α-quaternary chiral center in high enantioselectivities
only in the case of electron rich enones. On the other hand, the quinine-squaramide
was more efficient in that a wide variety of electron rich and electron
poor enones underwent Michael addition of nitrophosphonates to afford
the quaternary α-nitrophosphonates in excellent yields and enantioselectivities.
The hydrogen bonding donor ability of the bifunctional catalyst, as
shown in the proposed transition states, appears primarily responsible
for the observed selectivity. However, a favorable π-stacking
between the aryl groups of thiourea/squaramide and aryl vinyl ketone
also appeared favorable. The reaction was amenable to scale up, and
the enantioenriched quaternary α-nitrophosphonates could be
easily transformed to synthetically and biologically useful quaternary α-aminophosphonates
and other multifunctional molecules
Regio- and Diastereoselective Synthesis of Dihydropyridopyrimidines via Cascade Reactions of 2‑Aminopyridines with Morita–Baylis–Hillman Bromides of Nitroalkenes
The Morita–Baylis–Hillman
(MBH) bromides of nitroalkenes
have been employed as bielectrophiles for the first time. The 1,3-bielectrophilic
reactivity of the MBH bromides has been demonstrated in the synthesis
of 3,4-dihydro-2<i>H</i>-pyrido[1,2-<i>a</i>]pyrimidines.
The reaction of MBH bromides with 2-aminopyridines takes place in
the absence of any reagent in a cascade S<sub>N</sub>2′-6-endo-trig
fashion and is completely regioselective and highly stereoselective.
The products, in their hydrobromide salt form, could be conveniently
isolated and purified by crystallization. The high stereoselectivity
has been rationalized in terms of the greater stability of the transition
state in which the Ar and NO<sub>2</sub> groups are anti to each other
Synthesis of Fused Bromofurans via Mg-Mediated Dibromocyclopropanation of Cycloalkanone-Derived Chalcones and Cloke–Wilson Rearrangement
A convenient two-step sequence for the conversion of
alkylidenecycloalkanones
to bromofurans is reported. The steps involve Mg-mediated diastereoselective
dibromocyclopropanation of alkylidenecycloalkanone followed by acidic
alumina-mediated regioselective ring expansion of the cyclopropyl
ketone. The scope of the reaction was investigated using alkylidenecycloalkanones
derived from tetralone, indanone, and benzosuberone to afford 2-aryl-3-bromofurans
fused to various benzocycloalkanes. Representative examples of stereoconvergent
dibromocyclopropanation and total aromatization of the furobenzocycloalkane
are also reported
Quinine-Derived Thiourea and Squaramide Catalyzed Conjugate Addition of α‑Nitrophosphonates to Enones: Asymmetric Synthesis of Quaternary α‑Aminophosphonates
Conjugate
addition of α-nitrophosphonates to enones was carried
out in the presence of two sets of organocatalysts, viz. a quinine-thiourea
and a quinine-squaramide. The quinine-thiourea provided the products
possessing an α-quaternary chiral center in high enantioselectivities
only in the case of electron rich enones. On the other hand, the quinine-squaramide
was more efficient in that a wide variety of electron rich and electron
poor enones underwent Michael addition of nitrophosphonates to afford
the quaternary α-nitrophosphonates in excellent yields and enantioselectivities.
The hydrogen bonding donor ability of the bifunctional catalyst, as
shown in the proposed transition states, appears primarily responsible
for the observed selectivity. However, a favorable π-stacking
between the aryl groups of thiourea/squaramide and aryl vinyl ketone
also appeared favorable. The reaction was amenable to scale up, and
the enantioenriched quaternary α-nitrophosphonates could be
easily transformed to synthetically and biologically useful quaternary α-aminophosphonates
and other multifunctional molecules
Enantioselective Synthesis of Quaternary α-Aminophosphonates via Conjugate Addition of α-Nitrophosphonates to Enones
Enantioselective Michael addition of <b>α</b>-nitrophosphonates to enones for the synthesis of <b>α</b>-aminophosphonates is reported for the first time. The reaction proceeds in good to high yields and moderate to high selectivity in the presence of a new quinine thiourea catalyst. The quaternary nitrophosphonates were conveniently transformed to cyclic quaternary <b>α</b>-aminophosphonates via in situ reduction–intramolecular cyclization or Baeyer–Villiger oxidation followed by in situ reduction–intramolecular cyclization
(3 + 3) Annulation of Nitroallylic Acetates with Stabilized Sulfur Ylides for the Synthesis of 2‑Aryl Terephthalates
A novel (3 + 3) annulation
approach has been developed for the
synthesis of 2-aryl terephthalates from nitroallylic acetates and
stabilized sulfur ylides. The 2-aryl terephthalates, which are also
biaryls bearing a terephthalate moiety, are formed through a cascade
of reactions such as a γ-selective S<sub>N</sub>2′ reaction,
γ-selective intramolecular Michael addition, and two eliminations
in the presence of Cs<sub>2</sub>CO<sub>3</sub> in CH<sub>3</sub>CN
at room temperature. The products are formal precursors of farnesyltransferase
inhibitors and are also potential monomers in polymer chemistry
(3 + 3) Annulation of Nitroallylic Acetates with Stabilized Sulfur Ylides for the Synthesis of 2‑Aryl Terephthalates
A novel (3 + 3) annulation
approach has been developed for the
synthesis of 2-aryl terephthalates from nitroallylic acetates and
stabilized sulfur ylides. The 2-aryl terephthalates, which are also
biaryls bearing a terephthalate moiety, are formed through a cascade
of reactions such as a γ-selective S<sub>N</sub>2′ reaction,
γ-selective intramolecular Michael addition, and two eliminations
in the presence of Cs<sub>2</sub>CO<sub>3</sub> in CH<sub>3</sub>CN
at room temperature. The products are formal precursors of farnesyltransferase
inhibitors and are also potential monomers in polymer chemistry
Synthesis of Fused Bromofurans via Mg-Mediated Dibromocyclopropanation of Cycloalkanone-Derived Chalcones and Cloke–Wilson Rearrangement
A convenient two-step sequence for the conversion of
alkylidenecycloalkanones
to bromofurans is reported. The steps involve Mg-mediated diastereoselective
dibromocyclopropanation of alkylidenecycloalkanone followed by acidic
alumina-mediated regioselective ring expansion of the cyclopropyl
ketone. The scope of the reaction was investigated using alkylidenecycloalkanones
derived from tetralone, indanone, and benzosuberone to afford 2-aryl-3-bromofurans
fused to various benzocycloalkanes. Representative examples of stereoconvergent
dibromocyclopropanation and total aromatization of the furobenzocycloalkane
are also reported