15 research outputs found
RECENT CASES
Hydride abstraction from monocationic
hydride bridged salts [HÂ(H<sub>2</sub>BâL)<sub>2</sub>]<sup>+</sup> [BÂ(C<sub>6</sub>F<sub>5</sub>)<sub>4</sub>]ÂŻ (L = Lewis
base) generates an observable
primary borenium cation for L = <i>i</i>Pr<sub><i>2</i></sub>NEt, but with L = Me<sub>3</sub>N, Me<sub>2</sub>NPr, or several <i>N</i>-heterocyclic carbenes, highly reactive dicationic dimers
are formed
Stille Coupling of an Aziridinyl Stannatrane
An aziridinyl stannatrane <b>8</b> couples with aryl or alkenyl
halides RX under modified Stille conditions to afford substituted
aziridines. Efficient coupling at room temperature is possible in
the best examples in the presence of (<sup><i>t</i></sup>Bu<sub>3</sub>P)<sub>2</sub>Pd and CuOPÂ(O)ÂPh<sub>2</sub> (CuDPP)
Enantiocontrolled Synthesis of a Tetracyclic Aminal Corresponding to the Core Subunit of Diazonamide A
A chiral
benzylic ether serves as an auxiliary for oxindole carboxylation
(dr 5.2:1.0) that sets C<sub>10</sub> configuration in a potential
diazonamide precursor. The chiral substituent allows diastereomer
separation and departs during a subsequent acid-catalyzed ring closure
to form a tetracyclic aminal. With suitable N-protection, crystallization
affords the aminal with 98â99% ee
Reactivity of Aziridinomitosene Derivatives Related to FK317 in the Presence of Protic Nucleophiles
The syntheses and reactivity of <i>N</i>-TBDPS
and <i>N</i>-trityl protected derivatives of an aziridinomitosene
corresponding
to FK317 are described. New reactivity patterns were observed for
these highly sensitive and functionally dense heterocycles under mild
nucleophilic conditions approaching the threshold for degradation.
Thus, the silyl or trityl protected aziridinomitosene reacted with
Cs<sub>2</sub>CO<sub>3</sub>/CD<sub>3</sub>OD to give isomeric products
where substitution occurred at C(10) and CÂ(9a) (mitomycin numbering)
providing a CD<sub>3</sub> ether and a CD<sub>3</sub> hemiaminal,
respectively. These findings show that heterolysis at C(10) is faster
than at aziridine C(1), in contrast to the behavior of typical aziridinomitosenes
in the mitomycin series. The labile <i>N</i>-TBDPS hemiaminal
and the more stable <i>N</i>-trityl hemiaminal resemble
the mitomycin K substitution pattern. A reagent consisting of CsF
in CF<sub>3</sub>CH<sub>2</sub>OH/CH<sub>3</sub>CN desilylated a simple <i>N</i>-TBDPS aziridine but caused nucleophilic cleavage at C(1)
as well as C(10) without cleavage of the <i>N-</i>TBPDS
group in the fully functionalized penultimate aziridinomitosene. The
high reactivity of the C(10) carbamate with nucleophiles precludes
the use of deprotection methodology that requires <i>N</i>-protonation for fully functionalized aziridinomitosenes in the FK317
series
Amine-Directed Hydroboration: Scope and Limitations
Iodine activation induces intramolecular hydroboration of homoallylic and bis-homoallylic amine boranes with good to excellent control of regiochemistry compared to control experiments using excess THFâ˘BH<sub>3</sub>. Deuterium labeling and other evidence confirm that the iodine-induced hydroboration reaction of homoallylic amine boranes occurs via an intramolecular mechanism equivalent to the classical 4-center process and without competing retro-hydroboration. Longer carbon chain tethers result in lower regioselectivity, whereas the shorter tether in allylic amines results in a switch to dominant intermolecular hydroboration. Regioselectivity in THFâ˘BH<sub>3</sub> control experiments is higher for the allylic amine boranes compared to the iodine activation experiments, whereas the reverse is true for homoallylic amine borane activation
PâDirected Borylation of Phenols
Internal borylation occurs upon activation of aryl di-isopropylphosphinite boranes with HNTf<sub>2</sub> to give heterocyclic intermediates that can be reductively quenched to afford <b>6</b> or treated with KHF<sub>2</sub> to give the phenolic potassium aryl trifluoroborate salts <b>10</b>. The latter salts are useful for Pd-catalyzed coupling with aryl iodides under Molander conditions, provided that precautions are taken to remove the KNTf<sub>2</sub> byproduct from the preceding KHF<sub>2</sub> step
PâDirected Borylation of Phenols
Internal borylation occurs upon activation of aryl di-isopropylphosphinite boranes with HNTf<sub>2</sub> to give heterocyclic intermediates that can be reductively quenched to afford <b>6</b> or treated with KHF<sub>2</sub> to give the phenolic potassium aryl trifluoroborate salts <b>10</b>. The latter salts are useful for Pd-catalyzed coupling with aryl iodides under Molander conditions, provided that precautions are taken to remove the KNTf<sub>2</sub> byproduct from the preceding KHF<sub>2</sub> step
Synthesis of a Nonracemic <i>C</i><sub>2</sub>âSymmetric Tetrahydro-1,4-azaborine and Evaluation of Hydroboration Enantioselectivity
Tetrahydro-1,4-azaborines
were accessed by hydroboration of <i>N</i>,<i>N-</i>diprenylÂtolueneÂsulfonamide <b>4</b>. Conversion
to the methylborinates <b>11</b> and <b>12</b> followed
by heating with l-alanine and crystallization
afforded (<i>R</i>,<i>R</i>,<i>S</i>)-<b>13</b> (27%). Reduction of borinic acid (<i>R</i>,<i>R</i>)-<b>18</b> with Soderquistâs KH*
gave (<i>R</i>,<i>R</i>)-<b>19</b>, and
hydride abstraction by TMSCl in the presence of alkenes resulted in
hydroboration, 84â86% ee for (<i>Z</i>)-alkenes,
but (<i>E</i>)-alkenes or 1,1-disubstituted alkenes gave
<5% ee