A very practical and selective method for PMB protection of alcohols

A very simple, practical and efficient one-step heterogeneous protocol for the PMB protection of alcohols using Amberlyst-15 has been developed. The stability and hazard issues regarding PMBCl and PMBBr are totally avoided by directly using anisyl alcohol for the protection. Alcohols are protected in very good yields. The selective mono-PMB protection of diols as well as di-PMB protection of diols was achieved in good yields, along with the demonstration of recyclability of the catalyst.     

Efficient and convenient nonaqueous workup procedure for the preparation of arylboronic esters

An efficient one-pot synthetic protocol for the synthesis of arylboronic esters has been established. The concentrated addition mixture of trimethylborate with aryl Grignard reagents was treated with low molecular weight diols (ethylene glycol, 1,3-propandiol) and toluene, the corresponding arylboronic esters were isolated in a convenient way with high yields. The diols not only serve as water replacement for the workup step, but also as well as the reagent for the preparation of arylboronic esters.     

A convenient synthetic route to furan esters and lactones by palladium-catalyzed carboalkoxylation or cyclocarbonylation of hydroxyl-substituted 3-iodofurans

An effective palladium-catalyzed protocol for the intermolecular carboalkoxylation or intramolecular cyclocarbonylation of hydroxyl-substituted 3-iodofurans under carbon monoxide pressure has been developed. The 3-iodofurans are readily prepared by iodocyclization of 2-(1-alkynyl)-2-alken-1-ones in the presence of various diols.     

Iridium-Catalyzed Oxidant-Free Dehydrogenative CH Bond Functionalization: Selective Preparation of N-Arylpiperidines through Tandem Hydrogen Transfers

Relay to the finish: The atom-economical tandem hydrogen autotransfer catalyzed by iridium(III) has been efficiently applied for the preparation of N-arylpiperidines starting from easily accessible anilines, diols, and aldehydes. This protocol is also compatible with the use of diethyl carbonate as an ecofriendly solvent.     

Indium(III) triflate catalysed transacetalisation reactions of diols and triols under solvent-free conditions

Acyclic acetals and ketals undergo transacetalisation in the presence of catalytic quantities of indium(III) triflate (In(OTf)₃) and diols or triols under solvent-free conditions to generate the corresponding cyclic acetals and ketals in excellent yield. The methodology has been further developed to encompass a tandem acetalisation-acetal exchange protocol, which provides a facile and high yielding route to cyclic ketals from unreactive ketones under very mild reaction conditions.     

Chemoenzymatic synthesis of enantiomerically pure terminal 1,2-diols

A new practical method for the enzymatic synthesis of 1,2-diols has been developed by employing a lipase catalyzed one-pot transesterification protocol. A series of substituted -acetoxyphenylethanones 3a–g have been reduced to the corresponding alcohols under mild conditions employing sodium borohydride and moist neutral alumina, and further subjected for lipase catalyzed irreversible transesterification in the same pot to give mono- and diacetate diols (R)-4 and (S)-5, which on hydrolysis afforded terminal 1,2-diols, (R)- and (S)-6 in high enantiomeric excess.     

CeCl3-mediated addition of acetylenic bis-lithium salts to aldehydes and ketones: An efficient route to bis-substituted alkyne diols

An efficient, chemoselective protocol to access propargylic diols via a CeCl₃-mediated addition reaction is reported. Propargylic alcohols were transformed into the corresponding acetylenic bis-lithium salt intermediates, which react with aldehydes and ketones in the presence of dry CeCl₃ to furnish the corresponding bis-substituted alkyne diols. This protocol does not involve protection-deprotection or transmetallation steps, and allows the use of poorly reactive or highly enolizable substrates.     

Liquid crystalline polybutadiene diols with chiral thiol side-chain units

New chiral thiols with three aromatic rings and the azo group in the molecular core have been synthesized and grafted on polybutadiene diols backbone. The resulting diols possess the OH end groups promising for preparation of the liquid crystal ordered networks. The mesomorphic and physical properties of the synthesized thiols and resulting diols have been studied by polarizing optical microscopy, differential scanning calorimetry, X-ray studies and dielectric spectroscopy. The resulting side-chain diols show the liquid crystalline behaviour. The effects of side-chain structure of the thiols and density of grafting on the polybutadiene diols has been studied and discussed.     

Tandem Z‐Selective Cross‐Metathesis/Dihydroxylation: Synthesis of anti‐1,2‐Diols

A stereoselective synthesis of anti‐1,2‐diols has been developed using a multitasking Ru catalyst in an assisted tandem catalysis protocol. A cyclometalated Ru complex catalyzes first a Z‐selective cross‐metathesis of two terminal olefins, followed by a stereospecific dihydroxylation. Both steps are catalyzed by Ru, as the Ru complex is converted to a dihydroxylation catalyst upon addition of NaIO₄. A variety of olefins were transformed into valuable, highly functionalized, and stereodefined molecules. Mechanistic experiments were performed to probe the nature of the oxidation step and catalyst inhibition pathways. These experiments point the way to more broadly applicable tandem catalytic transformations.     

A general electron transfer reduction of lactones using SmI2-H2O

Herein we describe a strategy for the selective, electron transfer reduction of lactones of all ring sizes and topologies using SmI(2)-H(2)O and a Lewis base to tune the redox properties of the complex. The current protocol permits instantaneous reduction of lactones to the corresponding diols in excellent yields, under mild reaction conditions and with useful chemoselectivity. We demonstrate the broad utility of this transformation through the reduction of complex lactones and sensitive drug-like molecules. Sequential electron transfer reactions and syntheses of deuterated diols are also described.     

Kinetics and mechanism of the oxidation of some vicinal and non-vicinal diols by tetrabutylammonium tribromide

Kinetics of oxidation of five vicinal and four non-vicinal diols, and two of their monoethers, by tetrabutylammonium tribromide (TBATB) has been studied. The vicinal diols yield products arising out of glycol-bond fission, while the non-vicinal diols produce the hydroxycarbonyl compounds. The reaction is first-order with respect to TBATB. Michaelis-Menten type kinetics is observed with respect to diols. The reaction fails to induce the polymerization of acrylonitrile. There is no effect of tetrabutylammonium chloride on the reaction rate. The proposed reactive oxidizing species is the tribromide ion. The effect of solvent composition indicates that the rate increases with increase in the polarity of the solvent. The oxidation of [1,1,2,2-²H₄] ethanediol shows the absence of any primary kinetic isotope effect. Values of solvent isotope effect, k(H₂O)/k(D₂O), at 288 K for the oxidation of ethanediol, propane-1,3-diol and 3-methoxybutan-1-ol are 3.41, 0.98 and 1.02 respectively. A mechanism involving a glycol-bond fission has been proposed for the oxidation of vicinal diols. Non-vicinal diols are oxidised by a hydride-transfer mechanism, as they are monohydric alcohols.     

Kinetics and mechanism of the oxidation of some diols by benzyltrimethylammoniumtribromide

The kinetics of oxidation of five vicinal and four non-vicinal diols, and two of their monoethers by benzyltrimethylammonium tribromide (BTMAB) have been studied in 3:7 (v/v) acetic acid-water mixture. The vicinal diols yield the carbonyl compounds arising out of the glycol bond fission while the other diols give the hydroxycarbonyl compounds. The reaction is first-order with respect to BTMAB. Michaelis-Menten type kinetics is observed with respect to diol. Addition of benzyltrimethylammonium chloride does not affect the rate. Tribromide ion is postulated to be the reactive oxidizing species. Oxidation of [1,1,2,2-²H₄] ethanediol shows the absence of a kinetic isotope effect. The reaction exhibits substantial solvent isotope effect. A mechanism involving a glycol-bond fission has been proposed for the oxidation of the vicinal diols. The other diols are oxidized by a hydride ion transfer to the oxidant, as are the monohydric alcohols.     

Recent advances on nonenzymatic catalytic kinetic resolution of diols

As one of the most efficient and straight-forward strategies for obtaining optically pure compounds, catalytic kinetic resolution has been widely used in laboratory and industry. Although nonenzymatic catalytic kinetic resolution of secondary alcohol has been well developed, it still remains a formidable challenge for kinetic resolution of diols which play a significant role in natural products synthesis. The purpose of this digest is to present the recent progress on nonenzymatic catalytic kinetic resolution of representative diols.     

S,O-acetals as novel "chiral aldehyde" equivalents

Palladium-catalyzed asymmetric allylic alkylations (AAA) to form "chiral aldehyde" equivalents were investigated. Alpha-acetoxysulfones were formed in high enantiomeric excess as single regioisomers in AAA reactions of allylic geminal dicarboxylates with sodium benzenesulfinate. The directing ability of this novel functional group was highlighted by a series of dihydroxylations, affording syn diols exclusively anti to the acetoxy sulfone as single diastereomers in excellent yields. This is the first example of an asymmetric dihydroxylation protocol that gives the equivalent of reaction with a simple enal. The synthetic value of this process was exemplified by subsequent transformations of the diols including the development of a one-pot dihydroxylation-deprotective acyl migration protocol to give differentially protected 1,2-diols.     

Unprecedented cyclizations of calix[4]arenes with glycols under the Mitsunobu protocol, part 2.(1) O,O-and O,S-bridged calixarenes

[reaction: see text] Cycloalkylations of p-tert-butylcalix[4]arene (CA) and p-tert-butylthiacalix[4]arene (TCA) with various aliphatic glycols were performed under the Mitsunobu protocol using the DEAD/TPP system. CA gave 1,3-dialkylated diols, while C(2)-C(10) glycols gave 1,2- and 1,3-bridged calixarenes. The reaction of TCA with C(2) diols afforded sulfonium phenoxide betaines via O,S-cyclization, which is the first example for the alkylation of the sulfide bridge.     

Synthesis and Mechanism of a Main‐Chain Chiral Polymer Based on Asymmetric Cyclopolymerization

A systematic study of the asymmetric cyclocopolymerization of bis(4‐vinylbenzoate)s, derived from chiral diols, with styrene has been made from the viewpoint of synthesizing the main‐chain chiral polymer. On the basis of using over 30 chiral diols as templates, we summarize the relationship between the structure of the chiral template and the chiroptical properties of the template‐free polymer. For simple chiral diols, the chirality induction efficiency increased in the order 1,2‐diol < 1,4‐diol < 1,3‐diol. Chiral diols with two chiral centers exhibited higher chirality induction efficiency than those having one chiral center only. The chirality induction efficiency for cyclic 1,2‐diols increased with the ring size in the order 5‐ < 6‐ < 7‐ < 8‐membered rings, and that for acyclic 1,2‐diols increased with the bulkiness of the substituent at the chiral center. In addition, a chirality induction mechanism has been proposed on the basis of model radical cyclization experiments and computational studies. Chirality induction could be caused by the inhibition of the formation of one racemo unit among the four stereoisomers due to the strong dependence of the stereoselectivity in intermolecular additions on the absolute configuration of the cyclized radical. The mechanism was examined using the Lewis‐acid and monomer‐concentration effects.     

Double Catalytic Kinetic Resolution (DoCKR) of Acyclic anti‐1,3‐Diols: The Additive Horeau Amplification

The concept of a synergistic double catalytic kinetic resolution (DoCKR) as described in this article was successfully applied to racemic acyclic anti ‐1,3‐diols, a common motif in natural products. This process takes advantage of an additive Horeau amplification involving two successive enantioselective organocatalytic acylation reactions, and leads to diesters and recovered diols with high enantiopurities. It was first developed with C ₂‐symmetrical diols and then further extended to non‐C ₂‐symmetrical anti diols to prepare useful chiral building blocks. The protocol is highly practical as it only requires 1 mol % of a commercially available organocatalyst and leads to easily separable products. This procedure was applied to the shortest reported total synthesis of (+)‐cryptocaryalactone, a natural product with anti‐germinative activity.     

The phase behavior of fluorinated diols,divinyl adipate and a fluorinated polyester in supercritical carbon dioxide

The use of supercritical carbon dioxide as a reaction medium for polyester synthesis is hindered by the low solubility of diols in CO₂. However, it has been previously demonstrated that fluorinated compounds can exhibit greater miscibility with carbon dioxide than their hydrocarbon analogs. Therefore, the phase behavior of fluorinated diols and divinyl adipate (DVA), an activated diester, in supercritical carbon dioxide has been investigated at 323 K. The phase behavior of equimolar mixtures of DVA with the most carbon dioxide-soluble diol, 3,3,4,4,5,5,6,6-octafluorooctan-1,8-diol (OFOD), was also determined. The solubility of a polyester synthesized from DVA and 2,2,3,3-tetrafluoro-1,4-butanediol (TFBD) was found to be less CO₂-soluble than its monomers. DVA was much more soluble in CO₂ than any of the fluorinated diols, therefore, no attempt was made to fluorinate the DVA structure. Because both substrates and polyester product were soluble in carbon dioxide, the enzymatic synthesis of a fluorinated polyester from DVA and octafluorooctandiol was performed in supercritical carbon dioxide, resulting in a polymer with a weight average molecular weight of 8232 Da.     

Expeditious synthesis of β-cycloacetalic sulfoxides. Introducing 1-phenylsulfinyl-2-phenylsulfanylethylene (SOSE), a promising new alkenylsulfur reagent

In similarity with the strongly electrophilic BPSEs and despite its more electron-rich character, 1-phenylsulfinyl-2-phenylsulfanylethylene (SOSE) reacts with nucleophiles with displacement of the phenylthio moiety. Specifically it reacts with diols under basic conditions to produce β-cycloacetalic sulfoxides. The reaction has been amply developed in carbohydrate chemistry.