Selectivities in reactions of organolithium reagents with unprotected 2-halobenzoic acids

[reaction: see text] Exposing 2-fluorobenzoic acid (1a) to 2.2 equiv of LTMP at ca. -78 degrees C leads to deprotonation at the 3-position whereas 2-chloro/bromobenzoic acids (1b,c) are lithiated adjacent to the carboxylate. The resulting dianions 3Li-1a and 6Li-1b,c are trapped as such by chlorotrimethylsilane. In the absence of internal quench, 6Li-1b,c isomerize to the more stable 3Li-1b,c. The latter eliminate lithium halide and set free benzyne-3-carboxylate (2) that reacts regioselectively with LTMP to give 3-tetramethylpiperidinobenzoic acid (3).     

Directed metalation and regioselective functionalization of 3-bromofuran and related heterocycles with NaHMDS

A mild and regioselective functionalization protocol for 3-bromofuran and analogs has been developed. Selective metalation and functionalization of C2 can be achieved as a result of the directing effect of the adjacent electron-withdrawing bromo group. In addition, the C5 position can also be selectively functionalized by blocking the C2 position via silylation or by simply controlling the reaction temperature. These functionalized compounds bearing a C3 bromo substituent may be further elaborated by utilizing a Suzuki–Miyaura cross-coupling procedure.     

Ring-selective functionalization of N,N'-diarylureas by regioselective N-alkylation and directed ortho metalation

[reaction: see text]. Unsymmetrical N,N'-diarylureas may be alkylated regioselectively at the more sterically congested nitrogen atom. The resulting mono-N-alkylated ureas undergo directed metalation (ortholithiation) with sec-BuLi to yield, on electrophilic quench, products functionalized regioselectively at the ring bearing the alkylated nitrogen atom.     

Sequential functionalization of pyrazole 1-oxides via regioselective metalation: synthesis of 3,4,5-trisubstituted 1-hydroxypyrazoles

A range of 3,5-diarylated and 3,4,5-triarylated 2-(4-methoxybenzyl)pyrazole 1-oxides have been prepared by regioselective deprotonation at C-5 or bromine-magnesium exchange at C-3 or C-4 followed by transmetalation with ZnCl(2) and palladium(0)-catalyzed cross-coupling. Furthermore, the metalated pyrazole 1-oxides could be trapped with electrophiles. The sequential metalation/functionalization of the pyrazole 1-oxides may follow the order C-5, C-3, C-4, or alternatively the order C-3, C-5, C-4. The 4-methoxybenzyl group of the functionalized 2-(4-methoxybenzyl)pyrazole 1-oxides could be removed by treatment with TFA and i-Pr(3)SiH in CH(2)Cl(2), providing the corresponding functionalized 1-hydroxypyrazoles.     

First general, direct, and regioselective synthesis of substituted methoxybenzoic acids by ortho metalation

New general methodology of value in aromatic chemistry based on ortho-metalation sites in o-, m-, and p-anisic acids (1-3) (Scheme 1) is described. The metalation can be selectively directed to either of the ortho positions by varying the base, metalation temperature, and exposure times. Metalation of o-anisic acid (1) with s-BuLi/TMEDA in THF at -78 degrees C occurs exclusively in the position adjacente to the carboxylate. On the other hand, a reversal of regioselectivity is observed with n-BuLi/t-BuOK. With LTMP at 0 degrees C, the two directors of m-anisic acid (2) function in concert to direct introduction of the metal between them while n-BuLi/t-BuOK removes preferentially the proton located ortho to the methoxy and para to the carboxylate (H-4). s-BuLi/TMEDA reacts with p-anisic acid (3) exclusively in the vicinity of the carboxylate. According to these methodologies, routes to very simple methoxybenzoic acids with a variety of functionalities that are not easily accessible by other means have been developed (Table 1).     

The first regioselective alpha-deprotonation and functionalization of allenamides. An application in intramolecular Pauson-Khand-type cycloadditions

[rection: see text] The first regioselective alpha-deprotonation and functionalization of electron-deficient allenamines are described here. The acidities of alpha- and gamma-allenic protons of these allenamides are readily differentiated using strong bases, thereby allowing regioselective substitutions at either the alpha- or gamma-allenic position. A specific synthetic application of the novel alpha-substituted allenamides in intramolecular Pauson-Khand-type cycloadditions is also described here.     

In situ anionic shielding for regioselective metalation: directed peri and iterative metalation routes to polyfunctionalized 7-azaindoles

Strolling the ring: a general regioselective directed peri(C4)-metalation route to 1 through an in situ N-anionic protection of C2 is reported. The azaindoles may be elaborated by directed ortho metalation (DoM) and Suzuki coupling to more complex heterocyclic systems. An iterative ring-walk DoM sequence furnishes the exhaustively substituted 2. DMG=directed metalation group, TMEDA=N,N,N',N'-tetramethylethylenediamine, TMS=trimethylsilyl.     

The regioselective deprotonation of conjugated acids

A stereoselective synthesis of E and Z C-4 deuterated 3-methyl-2-butenoic acid from methyl acetoacetate is described; the labelled acids are regioselectively deprotonated s$\text{y}$n to the acid function under kinetic conditions.     

Directing group-controlled hydrosilylation: regioselective functionalization of alkyne

Pt(0)-catalyzed hydrosilylation of unsymmetric alkynes proceeds in a highly regioselective manner with a dimethylvinylsilyl (DMVS) group as the directing group. This hydrosilylation affords a single regioisomer of silylalkenes from propargylic and homopropargylic alcohol derivatives. DMVS also has an accelerating effect that allows group-selective hydrosilylation of the DMVS-attached alkyne prior to that of other alkynes. Combined hydrosilylation and transformation reactions of the resulting silylalkenes afford various tri-substituted alkenes and multi-oxy-functionalized compounds with high regioselectivity from unsymmetric alkynes.     

Highly chemo- and regioselective phosphitylation of unprotected 2′-deoxyribonucleosides

We have developed the chemo- and regioselective phosphitylation of unprotected 2′-deoxyribonucleosides by the use of di-tert-butyl N,N-diethylphosphoramidite, a sterically hindered phosphoramidite. Both N/O- and primary hydroxy group-selectivities were simultaneously achieved, and the selectivity for the 5′-hydroxy groups was up to 97% regardless of the base moiety of the 2′-deoxyribonucleosides. The 3′-O-isomers and the 5′-O-isomers were easily separated by silica gel column chromatography or crystallization to give the pure 2′-deoxyribonucleoside 5′-phosphites in moderate to good yields.     

Steric strain release-directed regioselective functionalization of meso-methyl Bodipy dyes

Starting from some meso-methyl Bodipy dyes, the corresponding meso-styryl derivatives were synthesized by regioselective Knoevenagel-type condensation with different aromatic aldehydes. The reactions were driven by the alleviation of the structural strain of the alkyl substituted Bodipys that could override the differential acidities of the methyl protons at the pyrrole ring of the Bodipy moiety.     

A peptide-based catalyst approach to regioselective functionalization of carbohydrates

Two small peptide libraries (150 members and 36 members) have been subjected to screening experiments to evaluate their potential for regioselective (i.e. site-selective) acylation of carbohydrate monomers. Two substrates, one diol derived from N-acetyl glucosamine and one tetraol derived from glucose, have served as the test cases. In each case, the inherent regioselection of catalyzed acylation was defined as that derived from the reaction where N-methylimidazole (NMI) is used as the catalyst. With both substrates, peptides were found to perturb the inherent selectivities from those observed with NMI. From the libraries, the catalysts that provide the largest deviation from NMI were subjected to optimization studies. The work sets the groundwork for studies of expanded peptide libraries and development of structure-selectivity relationships to obtain catalysts that can selectively derivatize the unique sites in stereochemically complex polyols.     

Synthesis of diblock methylcellulose derivatives with regioselective functionalization patterns

This article describes a new synthesis strategy to prepare diblock copolymers as model compounds for industrially produced cellulose ethers exemplified with methylcellulose (MC). To elucidate a key structure for thermoreversible gelation of MC, five regioselectively methylated celluloses 1 – 5 (236, 23, 26, 3, and 6 MC), five corresponding methyl‐β‐D‐glucopyranosyl‐(1→4)‐cellulosides 6 – 10 , and five equiv methyl‐β‐D‐glucopyranosyl‐(1→4)‐β‐D‐glucopyranosyl‐(1→4)‐cellulosides 11 – 15 were synthesized for the first time via combination of the glycosyl trichloroacetimidate method and the acid‐catalyzed methanolysis method. The structure of compounds 1 – 15 was confirmed by means of NMR spectroscopy and MALDI‐TOF MS. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Palladium-catalyzed alkenation of thiophenes and furans by regioselective C-H bond functionalization

A palladium-catalyzed direct alkenation of thiophenes and furans has been developed in the presence of AgOAc and pyridine. A variety of olefinic substrates such as acrylates, acrylamides, and acrylonitrile can perform the direct oxidative coupling reactions with various thiophenes and furans to give the mono-alkenylated products in good yields. In most cases, the (E)-isomers were isolated as the major products.     

Ligand-controlled regioselective and chemodivergent defluorinative functionalization of gem-difluorocyclopropanes with simple ketones

Modulating the reaction selectivity is highly attractive and pivotal to the rational design of synthetic regimes. The defluorinative functionalization of gem-difluorocyclopropanes constitutes a promising route to construct β-vinyl fluorine scaffolds, whereas chemo- and regioselective access to α-substitution patterns remains a formidable challenge. Presented herein is a robust Pd/NHC ligand synergistic strategy that could enable the C–F bond functionalization with exclusive α-regioselectivity with simple ketones. The key design adopted enolates as π-conjugated ambident nucleophiles that undergo inner-sphere 3,3′-reductive elimination warranted by the sterically hindered-yet-flexible Pd-PEPPSI complex. The excellent branched mono-defluorinative alkylation was achieved with a sterically highly demanding IHept ligand, while subtly less bulky SIPr acted as a bifunctional ligand that not only facilitated α-selective C(sp³)–F cleavage, but also rendered the newly-formed C(sp²)–F bond as the linchpin for subsequent C–O bond formation. These examples represented an unprecedented ligand-controlled regioselective and chemodivergent approach to various mono-fluorinated terminal alkenes and/or furans from the same readily available starting materials.

A robust Pd/NHC ligand synergistic strategy that enables the exquisite regioselective and chemodivergent C–F bond functionalization of gem-difluorocyclopropanes with simple ketones, is reported.     

Directed ortho-lithiation of unprotected diphenylphosphinic acids

Directed ortho lithiation of diphenylphosphinic acid and subsequent electrophilic trapping provides mono ortho-functionalized derivatives including enantiopure γ-aminophosphinic acids in moderate yields. Copper catalyzed coupling of the ortho anion leads to biphenyl-2,2′-diylbis(phenylphosphinic acid), a phosphorus analogue of biphenyl-2,2′-dicarboxylic acid. Preliminary studies of the metal-binding abilities of this O,O-chelating ligand towards a series of metal cations are included.     

Palladium-catalyzed direct and regioselective C-H bond functionalization/oxidative acetoxylation of indoles

The first general examples of palladium-catalyzed direct and selective oxidative C3-acetoxylation of indoles are presented. The mild reaction conditions (70 °C and with weak base, KOAc) in this indole C-H-acetoxylation are notable.