Asymmetric Synthesis of alpha-Alkylproline Derivatives from a Chiral Borane-Amine Adduct: Inversion of Enantioselectivity in the Presence of a Crown Ether

Synthesis of optically active 2-substituted azetidine-2-carbonitriles from chiral 1-arylethylamine via α-alkylation of N-borane complexes

The base-promoted α-alkylation of N-((S)-1-arylethyl)azetidine-2-carbonitriles 3via formation of their N-borane complexes 4 was investigated. For example, treatment of diastereomerically pure borane N-((S)-1′-(4′′-methoxyphenyl)ethyl)azetidine-2-carbonitrile complex (1S,2S,1′S)-4b with 1.2 equivalents of LDA at −78 °C followed by 1.3 equivalents of benzyl bromide at −78 °C and warming to room temperature produced α-benzylated (2S,1′S)-5ba in 72% yield and (2R,1′S)-5ba in 2% yield. A mechanism for this diastereoselective α-alkylation was proposed. Our method enables the production of optically active 2-substituted azetidine-2-carbonitriles, such as α-benzylated (S)-10a and (R)-10a, starting from commercially available (S)-(1-(4-methoxyphenyl)ethyl)amine.

Synthesis of optically active 2-substituted azetidine-2-carbonitriles by LDA-promoted diastereoselective α-alkylation of borane N-((S)-1-arylethyl)azetidine-2-carbonitrile complexes was demonstrated.

Asymmetric Synthesis of Cα-Substituted Prolines through Curtin-Hammett-Controlled Diastereoselective N-Alkylation

Asymmetric synthesis of α-substituted proline derivatives has been accomplished by an efficient chirality-transfer method. High diastereoselectivity of the N-alkylation of the proline ester (C→N chirality transfer) was achieved when a 2,3-disubstituted benzyl group was used as the N-substituent. DFT calculations provided a mechanistic rationale for the high degree of stereoselectivity. The generated N-chirality of the quaternary ammonium salt was transferred back to the α-carbon through a stereoselective [2,3]-Stevens rearrangement (N→C chirality transfer) to give α-substituted proline ester.

The aryne Sommelet–Hauser rearrangement

An aryne induced transition-metal-free and mild Sommelet–Hauser rearrangement of tertiary benzylamines for the synthesis of α-aryl amino acid derivatives in moderate to good yields and with broad substrate scope is presented.

Asymmetric Cα-Alkylation of Proline via Chirality Transfers of Conformationally Restricted Proline Derivative: Application to the Total Synthesis of (-)-Amathaspiramide F

An efficient strategy for the asymmetric synthesis of Cα-tetrasubstituted proline derivatives from proline has been established. A nitrogen-fused bicyclic system was devised to control the stereodynamics of proline. Through N-quaternizations with allylic electrophiles followed by [2,3]-rearrangements, the bicyclic proline system delivered enantioenriched Cα-tetrasubstituted prolines. This strategy was applied to the concise total synthesis of (-)-amathaspiramide F.

Base-promoted diastereoselective α-alkylation of borane N-((S)-1'-phenylethyl)azetidine-2-carboxylic acid ester complexes

The base-promoted α-alkylation of N-((S)-1-phenylethyl)azetidine-2-carboxylic acid esters 1 was investigated. The use of diastereomerically pure borane complexes 3 as substrates, which are easily prepared from 1, dramatically improved the yields and diastereoselectivities of α-alkylated products 2. For example, the treatment of tert-butyl ester (1S,2S,1'S)-3a with 2.4 equivalents of lithium bis(trimethysilyl)amide (LiHMDS) at 0 °C followed by 2.6 equivalents of benzyl bromide afforded α-benzylated (2S,1'S)-2aa in 90% yield as almost a single diastereomer. Our method enables the production of optically active α-substituted azetidine-2-carboxylic acid esters starting from commercially available (S)-1-phenylethylamine, which is one of the least expensive chiral compounds.

Asymmetric Lewis acid mediated [1,2]-rearrangement of proline-derived ammonium ylides

The first example of asymmetric Lewis acid mediated [1,2]-rearrangement of N-benzylic proline amides to form quaternary proline derivatives is reported. The presented reaction is shown to proceed with remarkable high C-->N-->C chirality transfer. Various quaternary proline derivatives have been prepared in good to excellent yields and high enantiomeric purity.

Synthesis and catalytic properties of diverse chiral polyamines

Chiral polyamines can be utilized for a variety of potential applications, ranging from asymmetric catalysis to nonviral gene delivery systems for DNA and RNA. They can also be utilized to solubilize carbon nanotubes. Thus, methods for the straightforward synthesis of chiral polyamines are needed. We present herein two synthetic strategies for accessing chiral polyamines. The potential of these chiral amines to catalyze two organic reactions with a high degree of chiral induction was also explored.Text: Chiral polyamines have been utilized for a variety of applications. First, polyamines are polycationic at neutral pH; as such, they interact strongly with both DNA and RNA.1 They can therefore be utilized as effective nonviral gene delivery agents.2 Second, chiral polyamines are efficient catalysts for various organic transformations.3 Polyamines have also been used to solubilize carbon nanotubes.4 Finally, chiral polyamines are excellent ligands for many transition metals.5 Due to their numerous applications, high-yielding synthetic strategies for their preparation are in great demand. We present herein two synthetic strategies for accessing chiral polyamines, and the potential of these chiral amines to catalyze two organic reactions.

An application of borane as a protecting group for pyridine

Efforts to couple 4 with 12 employing base mediation are problematic due to the formation of 6. To circumvent this issue, 12 was converted to the pyridine borane complex (13). Alkylation of 4 with 13 provided 3 after removal of the borane under acidic conditions and saponification of the ester.

Stereoselective Synthesis of Quaternary Proline Analogues

This review describes available methods for the diastereoselective and asymmetric synthesis of quaternary prolines. The focus is on the preparation of alpha-functionalized prolines with the pyrrolidine moiety not embedded in a polycyclic frame. The diverse synthetic approaches are classified according to the bond which is formed to complete the quaternary skeleton.

Diastereocontrolled Synthesis of Enantioenriched 3,4-Disubstituted β-Prolines

Enantioenriched 3,4-disubstituted beta-prolines have been prepared with a high diastereocontrol through a carbometalation reaction or through a domino Michael addition/carbometalation reaction.

Preparation and Structural Characterization of Three Types of Homo- and Heterotrinuclear Boron Complexes: Salen{[B−O−B][O2BOH]}, Salen{[B−O−B][O2BPh]}, and Salen{[B−O−B][O2P(O)Ph]}

Three types of homo- and heterotrinuclear boron complexes have been obtained in moderate to good yields from reactions of salen-type ligands with boric acid and combinations of boric acid with phenylboronic and phenylphosphonic acid. The products are air-stable and have relatively high melting points (>290 degrees C) but are poorly soluble or insoluble in common organic solvents. They have been characterized as far as possible by elemental analysis, mass spectrometry, IR, (1)H, (11)B, and (31)P NMR spectroscopy, and X-ray crystallography. Furthermore, theoretical calculations have been performed for representative examples to permit a complete comparison of the different structure types. A detailed analysis of the molecular structures showed that the complexes are constructed around a central B(3)O(3) or B(2)PO(3) ring. The salen ligands are attached to two boron atoms of these rings, which have therefore tetrahedral coordination geometries. The complexes contain seven- and eight-membered heterocycles of the B(2)C(n)ON(2) (n = 2, 3) type with chair or twisted-chair and boat-chair or chair-chair conformations, respectively. In the homotrinuclear complexes one of the three boron atoms is three-coordinate and can therefore still act as Lewis acid, thus making these products interesting for catalytic applications, e.g. in asymmetric synthesis. Depending on the substitutents attached to the boron atoms, these complexes show a relationship with either trimetaboric acid, boroxine, or the tetraborate dianion found in Borax.

A mild and general solid-phase method for the synthesis of chiral polyamines. Solution studies on the cleavage of borane-amine intermediates from the reduction of secondary amides

A mild oxidative workup protocol using iodine in an acetic acid-acetate buffer solution is described for the cleavage of borane-amine adducts arising from the borane-promoted reduction of polyamides supported onto practical trityl-based resins. Chiral polyamines with diverse side-chain functionalities can be generated as free bases without premature release from the solid support and with essentially no racemization using this method. A series of model oligomeric secondary diamides 6 containing various alpha-amino acid residues (Val, Phe, Tyr, Ser, Cys, Met, Gln, Trp) provided triamine products 8 in high yields and good to excellent purity. On the other hand, a substrate containing a tertiary amide (15) formed a rather unusual triaminoborane intermediate that required more stringent workup conditions to liberate the polyamine product 20. The reduction of oligomeric tertiary amides such as 9 was found sluggish, but these compounds could nonetheless be obtained in high purity from in situ reductive amination of the corresponding secondary amines. Control studies, carried out in solution with model secondary amide 23, confirmed the efficiency of the buffered iodine solution and highlighted several advantages (no heating necessary, no need for strong bases or acids) over existing methods for the cleavage of borane-amine adducts. A possible mechanism involving all buffer components (iodine, acetic acid, and acetate ion) is proposed in which borane-amine adducts are transformed first to the monoiodoborane-amine and then to the corresponding acetoxyborane-amine adduct of much weaker coordination affinity. The latter would dissociate readily and get trapped by the acetic acid to provide the desired secondary amine. This reduction/oxidative workup protocol is useful as a general method for the facile solid-phase synthesis of polyamines for eventual release in solution and use in various applications. It is also potentially very useful toward the synthesis and screening of bead-supported libraries of free oligoamines assembled through split-pool methods.

Palladium and Raney nickel catalyzed methanolic cleavage of stable borane-amine complexes

[figure: see text] Palladium and Raney nickel were found to catalyze the methanolysis of borane-amine adducts. Hence, strongly complexed amines can now be liberated by simple treatment with Pd/C or Raney Ni in methanol. The method is applicable to primary, secondary, tertiary, and aromatic amines, and the mildness of the reaction conditions allows preservation of otherwise labile functional groups.