Hydroboration-oxidation Research Articles

Synthesis, molecular, and morphological characterization of initial and modified diblock copolymers with organic acid chloride derivatives

AbstractSix well defined PS‐b‐PB1,2 diblock copolymers (PS: polystyrene and PB: polybutadiene) with almost 100% of 1,2 microstructure for the PB segment were synthesized. Size exclusion chromatography (SEC), membrane osmometry (MO) and proton nuclear magnetic resonance spectroscopy (1H NMR) were used for verification of the molecular characteristics and the 100% ‐1,2 addition for the PB blocks. Modification with heptanoyl or pentadecafluorooctanoyl chloride was accomplished via hydroboration and subsequent oxidation, leading to hydroxylated PB blocks and was verified with 1H NMR and Fourier transform infrared (FTIR) spectroscopy. Only two samples were modified with both organic acid chloride derivatives. Structural characterization was accomplished via transmission electron microscopy (TEM) and small‐angle X‐ray scattering (SAXS) in all cases. The self‐assembly was more evident in the modified copolymers with the corresponding halides due to the increase of the molecular weight of the modified PB block. Taking into consideration the χN values in each case and comparing the results with those of PS‐b‐PI copolymers already reported in the literature the discrepancies with the theoretical predictions are very small or minimal. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011

ChemInform Abstract: Synthetic Studies of Superstolide A: A Carbohydrate Based Synthesis of the C21—C26 Segment.

AbstractA chiral pool approach to the title segment (IX) is disclosed in which the key fragment (VII), obtained from D‐glucose diacetonide, plays a central role serving not only as the source of the C23‐C26 stereocenters but also sets the stage for introducing the C22 stereocenter through regio‐ as well as stereospecific hydroboration oxidation protocol with regard to the exo‐methylenic group in (VII).

Synthetic Studies of Superstolide A: A Carbohydrate Based Synthesis of the C21-C26 Segment

The synthesis of C21-C26 fragment of superstolide A, starting from D-glucose diacetonide, involving regioselective ring opening of epoxide by Grignard reagent and regioselective stereospecefic hydroboration oxidation has been described. Keywords: Superstolide A, Potent cytotoxic, Chiral pool, Regioselective epoxide opening, Stereoselective hydroborationoxidation, Carbohydrate, C21-C26, Grignard reagent, Neosiphonia superstes, polyketide, D-glucose diacetonide, hydroboration oxidation, tosylation, alcohol, epoxidation, benzyl ether, Cleavage, methanolic HCl, hydroxyl group, Oxidative cleavage, CuCN, PDC oxidation, selectively, Debenzylation, BBN, PMB, mesylation, azido, compound

Total synthesis of natural cis-3-hydroxy- l-proline from d-glucose

Synthesis of cis-3-hydroxy- l-proline from d-glucose is reported. The methodology involves conversion of d-glucose into N-benzyloxycarbonyl-γ-alkenyl amine which on 5- endo-trig-aminomercuration gave the pyrrolidine ring skeleton with sugar appendage in 25% yield. Alternatively, N-benzyloxycarbonyl-γ-alkenyl amine on hydroboration–oxidation, mesylation and intramolecular S N2 cyclisation afforded pyrrolidine ring compound in high yield. Hydrolysis of 1,2-acetonide functionality, NaIO 4 cleavage followed by oxidation of an aldehyde into acid and hydrogenolysis afforded cis-3-hydroxy- l-proline in overall 29% yield from d-glucose.

ChemInform Abstract: Organocuprate-Mediated Alkylation of Sugar Epoxides. New Stereospecific Route to 3-C-Alkyl-3-deoxy-α-D-glucose.

The reactivity of the 5,6-anhydroglucofuranose derivatives 1 and 3, readily prepared from D-glucose, toward different cyanocuprates was examined. The former epoxide led to the corresponding C-6 chainextended sugars, e.g., 2. Alkylation of 5,6-anhydro-3-deoxy-1,2-O-isopropylidene-α-D-erythro-hex-3-enofuranose 3 with mixed cyanocuprates RCu(CN)Li [R = Me, Bu] provided a convenient and stereospecific route to 3-C-alkyl (methyl, butyl)-3-deoxy-α-D-glucofuranose derivatives 12 and 14via 1,4-addition followed by hydroboration–oxidation.

Trans–cis isomerization of arylether dendrimers with azobenzene core and terminal hydroxy groups

Azobenzene-cored arylether dendrimers 2a–2c with polar hydroxy groups at the periphery have been prepared from hydroboration–oxidation reaction of azobenzene-cored arylether dendrimers 1a–1c with nonpolar vinyl groups at the periphery. Trans → cis photoisomerization of central azo group has been investigated by UV–vis spectral changes after photolysis. The results show that dendrimers 1a–1c and 2a–2c in methanol/dichloromethane (1/1, v/v) have carried out trans–cis isomerization. Reaction rate for trans → cis photoisomerization of 2a–2c with polar hydroxy groups increases with increasing generation upon irradiation of 350 nm. On the other hand, the reaction rate of cis → trans thermal back isomerization for 2a–2c decreases with increasing the generation. This result is somewhat different from that for 1a–1c with nonpolar vinyl groups. Both the reaction rates of trans → cis photoisomerization and cis → trans thermal back isomerization for 1a–1c decrease in higher generation.

ChemInform Abstract: New Trimethylenemethane Dianion Synthons: Application to the Preparation of Substituted Perhydrofuro[2,3-b]furans.

Abstract The reaction of 3-chloro-2-(chloromethyl)prop-1-ene ( 1 ) with lithium powder and a catalytic amount of naphthalene in the presence of different electrophiles in THF at −78°C yields products 2 . When carbonylic compounds are used as electrophiles the corresponding methylenic diols are obtained, which by tandem hydroboration–oxidation with alkaline hydrogen peroxide and treatment with PCC (for ketone derivatives) or (Ph 3 P) 3 RuCl 2 (for aldehyde derivatives) furnish the expected perhydrofurofurans 3 . Using 2-chloromethyl-3-(2-methoxyethoxy)prop-1-ene ( 4 ) as starting material and under the above conditions, the selective chloro/lithium exchange versus the allylic carbon–oxygen bond cleavage allows the introduction of a first electrophile at −78 to −30°C, and a second electrophile at −30°C to room temperature to give products 5 . The same oxidation protocol mentioned above, when applied to diols 5 , lead to the differently substituted perhydrofurofurans 7 .

Formation of the steroidal 3β-hydroxy-6-oxo-moiety. Synthesis and cytotoxicity of glucolaxogenin

An efficient alternative route to the synthesis of the steroidal 3β-hydroxy-6-oxo moiety starting from diosgenin and cholesterol is described. The sequential tosylation, oxidative hydroboration, and selective reduction steps provided the target 3β-hydroxy-6-oxo moiety, yielding laxogenin 1 in 82% and 3β-hydroxycholestan-6-one 8 in 83% overall yield. The cleavage of the S-O bond of 3β-tosylate-6-oxo intermediates was succeeded by means of sodium naphthalenide at -80 °C; when the reduction was explored at room temperature the cleavage of the C-O bond was favored and the corresponding i-steroids were observed. Glucolaxogenin 15 was synthesized in 68% from 1, and its antiproliferative activity was evaluated in cervical cancer cells HeLa, CaSki and ViBo. The effect on peripheral blood lymphocytes was assessed founding that the cell growth was unaffected showing therefore high selectivity.

Stereocontrolled Synthesis of Enantiopure Polyhydroxylated Azetidines via 1,2-Oxazines

A set of new enantiopure polyhydroxylated azetidine derivatives has been prepared. The key starting materials, 3,6-dihydro-2H-1,2-oxazines, were subjected to a hydroboration―oxidation sequence to introduce the required 5-hydroxy group. Subsequent cleavage of the N―O bond with samarium diiodide, selective protection of the primary hydroxyl group, and ring closure after activation of the secondary hydroxyl group provided the protected azetidine derivatives. The efficacy of each individual step of this sequence depends on the configuration of the starting material. Three representative azetidine derivatives were converted into the deprotected polyhydroxylated azetidines which are interesting candidates as glycosidase inhibitors.

Hyperbranched Polycarbosilanes of Homogeneous Architecture: Regioselective Hydrosilylation of AB2 Monomers and Consecutive Functionalization

A novel AB2 monomer, bis(4-(but-3-enyl)phenyl)methylsilane, 1a, and its analogue AB structure 1b were synthesized and polymerized with Karstedt’s, Speier’s, and a Pt−NHC catalyst to obtain hyperbranched and linear polycarbosilanes. Only the Pt−NHC complex afforded a regioselective hydrosilylation reaction. As a consequence, this led to a uniform polymer microstructure and at the same time to significantly increased molecular weights. Functionalization of the remaining double bonds in the obtained polymer was achieved by two exemplary routes. Hydrosilylation with a functional silane yielded chloromethylsilyl capped polymers, hydroboration and subsequent oxidation led to hydroxy-terminated polymers.

Asymmetric Total Syntheses of Cyclic Nitrone-Containing Phlegmarine-Type Lycopodium Alkaloids, Lycoposerramines-X and -Z

The total syntheses of cyclic nitrone-containing phlegmarine-type Lycopodium alkaloids, lycoposerramines-X and -Z, were accomplished starting from (R)-3-methylcyclohexanone via Pd-catalyzed Suzuki-Miyaura coupling, Johnson-Claisen rearrangement, stereoselective hydroboration-oxidation reaction, and Mitsunobu reaction, thereby establishing the structures including the absolute configuration.

Domino Heck/Lactonization-Catalyzed Synthesis of 3-C-Linked Mannopyranosyl Coumarins

Selective syntheses of methyl alpha- (8) and beta-C-mannopyranosyl acrylates (9) were obtained from alpha-C-allyl mannopyranoside (3) by ozonolysis to 4 followed by alpha-methylenation to provide intermediate aldehydes 5 and 6. The beta-anomer 6 was obtained by in situ anomeric epimerization. The acrylates and the homologous alpha-anomer 16, obtained by oxidative hydroboration, oxidation, and alpha-methylenation, were converted into 3-C-linked mannopyranosyl coumarins 11, 12, and 19 in good yields under one-pot Heck/lactonization conditions.

Introduction of Vinyl and Hydroxymethyl Functionalities at C-4 of Glucose-Derived Substrates: Synthesis of Spirocyclic, Bicyclic, and Tricyclic Nucleosides

Installing hydroxymethyl and hydroxyethyl substitutions at C-4 through vinylation and hydroboration-oxidation reactions of the C-4 bis-hydroxymethyl derivative of d-glucose based substrate, and inserting heteroatoms thereafter permitted formation of N-, O-, or S-heterocycles leading to [4,5]- or [5,5]-spirocycles and a bicyclo[3.3.0]octane product. Some of the spirocycles were converted to spironucleosides under Vorbruggen glycosidation reaction conditions. Similarly, the bicyclic product was elaborated to the corresponding bicyclic nucleoside as well as an unexpected tricyclic nucleoside.

Brown Hydroboration Reaction

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

CP-225,917 synthetic studies: unusual hydroboration regioselectivity influenced by remote functional groups

Hydroboration–oxidation of 1,1-disubstituted alkenes with borane–methyl sulfide complex in bridged tricyclic intermediates of the CP-225,917 ring system were observed to produce significant quantities of tertiary alcohol products. This net Markovnikov addition of water across an alkene is influenced by a combination of remote functional groups. Computations at the B3LYP/6-31G ∗ level of theory correctly predicted this reversal in selectivity and directed the selective removal of functional groups to restore selectivity for the primary alcohol.

An unexpected access to 5- epi-cyclophellitol: a new cyclitol member

A thorough reinvestigation of the hydroboration–oxidation of methylene epoxycyclohexane 3 has revealed the formation of a direct precursor of the hitherto unreported 5- epi-cyclophellitol. Hydroboration of the starting precursor 3 takes place with total and opposite stereocontrol to that previously described in the literature. The stereochemistry of this new cyclophellitol isomer has been unambiguously confirmed by comparison with reference compounds obtained by independent methods.

A simple and convenient method for the preparation of diborane from tetrabutylammonium borohydride and benzyl chloride for application in organic synthesis

Diborane is readily generated in situ at 25 °C in toluene using the Bu 4NBH 4/PhCH 2Cl and Bu 4NBH 4/I 2 reagent systems. The reagent prepared in this way is used for the reduction of carbonyl compounds and hydroboration–oxidation of olefins to obtain the corresponding alcohols in good yields.

Asymmetric hydroboration and Matteson homologation for the preparation of fluorinated α-phenethanols

The asymmetric catalytic hydroboration–oxidation of ring-fluorinated styrenes (F-PhCH CH 2) was achieved with catecholborane along with a combination of [Rh(COD) 2] +BF 4 − and ( R)-BINAP providing 81–96% enantioselectivities for the product alcohols for ortho-unhindered styrenes. A deleterious effect of a 2,6-disubstitution on the enantioselectivity of the product alcohol was observed. 2-Trifluoromethylstyrene also provides only 53% ee, probably due to the steric bulk of the CF 3 group at the ortho-position of styrene. Asymmetric homologation of fluorophenylmetals (magnesium bromide or lithium) with pinanediol α-chloroethylboronate, followed by oxidation readily furnished the desired 1-(2,6-difluorophenyl)- and 1-(perfluorophenyl)ethanols in 94–95% ee.

Highly Enantioenriched 2‐Azabenzonorbornenes from 7‐Azabenzonorbornadienes by Asymmetric Hydroboration—Oxidation and Tandem Deoxygenation—Rearrangement—Electrophile Trapping.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Polar Addition to C=C Group: Why Is Anti-Markovnikov Hydroboration–Oxidation of Alkenes Not "Anti-"?

For 137 years Markovnikov's rule has been extensively used in organic chemical education and research to describe the regioselectivity in electrophilic addition reactions to alkenes and alkynes. When the structures of the final reaction products are used as reference, the rule requests that certain polar addition reactions be termed anti-Markovnikov. We use first principles thermodynamic calculations to analyze six polar addition reactions to alkenes, expected to be of either Markovnikov or anti-Markovnikov regioselectivity. Our results show no difference between the two reaction types: in each case the alkene atomic charges and the form of the intermediate completely determine the course of the overall reaction and the form of the predominant final product. We redefine this type of regioselectivity and propose that teaching of organic chemistry should be based more on robust and portable concepts such as energy difference and atomic charge than on historical labels.