RECENT CASES

Hydride abstraction from monocationic hydride bridged salts [H­(H₂B–L)₂]⁺ [B­(C₆F₅)₄]¯ (L = Lewis base) generates an observable primary borenium cation for L = <i>i</i>Pr_<i>2</i>NEt, but with L = Me₃N, Me₂NPr, 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 (^<i>t</i>Bu₃P)₂Pd and CuOP­(O)­Ph₂ (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 C₁₀ 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 98–99% 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 Cs₂CO₃/CD₃OD to give isomeric products where substitution occurred at C(10) and C­(9a) (mitomycin numbering) providing a CD₃ ether and a CD₃ 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 CF₃CH₂OH/CH₃CN 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 THF•BH₃. 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 THF•BH₃ 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

P‑Directed Borylation of Phenols

Internal borylation occurs upon activation of aryl di-isopropylphosphinite boranes with HNTf₂ to give heterocyclic intermediates that can be reductively quenched to afford <b>6</b> or treated with KHF₂ 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 KNTf₂ byproduct from the preceding KHF₂ step

P‑Directed Borylation of Phenols

Internal borylation occurs upon activation of aryl di-isopropylphosphinite boranes with HNTf₂ to give heterocyclic intermediates that can be reductively quenched to afford <b>6</b> or treated with KHF₂ 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 KNTf₂ byproduct from the preceding KHF₂ step

Oxygen-Directed Intramolecular Hydroboration

Oxygen-Directed Intramolecular Hydroboratio

Synthesis of a Nonracemic <i>C</i>₂‑Symmetric Tetrahydro-1,4-azaborine and Evaluation of Hydroboration Enantioselectivity

Tetrahydro-1,4-azaborines were accessed by hydroboration of <i>N</i>,<i>N-</i>diprenyl­toluene­sulfonamide <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 Soderquist’s KH* gave (<i>R</i>,<i>R</i>)-<b>19</b>, and hydride abstraction by TMSCl in the presence of alkenes resulted in hydroboration, 84–86% ee for (<i>Z</i>)-alkenes, but (<i>E</i>)-alkenes or 1,1-disubstituted alkenes gave <5% ee

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