Micro reactors: a new tool for the synthetic chemist

This review focuses on the use of micro reactors as tools in synthetic organic chemistry, aiming to highlight the many advantages associated with their use, in particular their ability to synthesise products in high yield, purity and, where relevant, selectivity.     

Diethyl chlorophosphite: a versatile reagent

Diethyl chlorophosphite (DECP) was previously described as a reducing agent for nitro compounds to the corresponding amines (Fischer, B.; Sheihet, L. J. Org. Chem. 1998, 63, 393). Here, the utility of this reagent was extended to chemical conversions of other oxygenated functional groups. In this paper we report on the scope of the reaction of DECP with N-oxides, epoxides, sulfones, sulfoxides, hydroxylamines, ketoximes, and aldoximes. The chemoselectivity of DECP is described, and conditions for a stepwise multiple conversion of functional groups on the same molecule with this reagent are provided.     

The Vilsmeier reagent: a useful and versatile reagent for the synthesis of 2-azetidinones

(Chloromethylene)dimethylammonium chloride (Vilsmeier reagent), prepared easily from N,N-dimethylformamide and oxalyl chloride or thionyl chloride, works as a versatile acid activator reagent for the direct [2+2] ketene-imine cycloaddition of substituted acetic acid and imines in one-pot synthesis under mild conditions. Monocyclic, spirocyclic and 3-electron-withdrawing group β-lactams were synthesized by this method and optimization of conditions were performed.     

Ethyl dibromofluoroacetate: a versatile reagent for the synthesis of fluorinated molecules

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Automated high resolution mass spectrometry for the synthetic chemist

Exact mass measurement at high resolution is an important tool alongside other spectroscopic methods to help confirm the structure of a novel compound prepared by the synthetic chemist. Exact mass measurement is used in the pharmaceutical industry to confirm the expected empirical formula of a product when problems have been experienced using elemental analysis. Because of the amount of manual intervention necessary when acquiring exact mass measurements, especially when using probe ionization techniques such as fast atom bombardment ionization or electron ionization, this method has been seen to be time consuming and labor intensive for the mass spectrometrist. An automated high resolution mass spectrometric method has been developed at Pfizer Central Research which has streamlined exact mass measurement. The method, which uses electrospray ionization on a double focusing mass spectrometer, is described. The samples are analyzed using a flow injection technique, with sodiated polyethylene glycol present in the mobile phase to provide mass reference peaks. The data are acquired and processed using a macro developed “in house.” This automated technique can process 15–20 samples an hour including data processing and report generation, using very small amounts of compound (∼25 μg), but more importantly it can be left to run unattended overnight. This allows the instrument to be used for more complex experiments during the day when it is important to have a mass spectrometrist present. The results presented here demonstrate that this method gives exact mass measurements within an acceptable limit of 5 ppm, and the variation on one sample, injected 10 times, is not excessively high (−1.8 to +1.6 mDa).     

Di-tert-butyl dicarbonate: a versatile carboxylating reagent

Carbon nucleophiles generated by a non-nucleophilic base (LDA) were effectively trapped with di-tert-butyl dicarbonate (Boc-anhydride) to provide the corresponding tert-butyl aryl acetates, di-tert-butyl aryl malonates, unsymmetrical aryl malonates and tert-butyl benzoates in high yields. This reaction represents another useful way to prepare a variety of tert-butyl carboxylates and highlights the synthetic utility of di-tert-butyl dicarbonate as a versatile carboxylating reagent.     

Ns strategies: a highly versatile synthetic method for amines

A highly efficient and versatile synthetic method for amines was established using nitrobenzenesulfonamides (Ns-amides) as both a protecting and activating group. The alkylation of N-monosubstituted Ns-amides either proceeded conventionally or under Mitsunobu conditions to provide the N,N-disubstituted sulfonamides, and the Ns group was removed easily with soft nucleophiles via Meisenheimer complexes to give the corresponding secondary amines. The major advantage of this protocol is that both alkylation and deprotection proceed under mild conditions. Thus, with this methodology, the total synthesis of linear and/or macrocyclic natural polyamines can be accomplished efficiently.     

A versatile synthetic approach to peptidyl privileged structures using a "safety-catch" linker

Peptidyl privileged structures have been widely used by many groups to discover biologically active molecules. In this context, privileged substructures are used as "hydrophobic anchors", to which peptide functionality is appended to gain specificity. Utilization of this concept has led to the discovery of many different active compounds at a wide range of biological receptors. A synthetic approach to these compounds has been developed on a "safety-catch" linker that allows rapid preparation of large libraries of these molecules. Importantly, amide bond formation/cleavage through treatment with amines is the final step; it is a linker strategy that allows significant diversification to be easily incorporated, and it only requires the inclusion of an amide bond. In addition, chemistry has been developed that permits the urea moiety to be inserted at the N-terminus of the peptide, allowing the same set of amines (either privileged substructures or amino acid analogues) to be used at both the N- and C-termini of the molecule. To show the robustness of this approach, a small library of peptidyl privileged structures were synthesized, illustrating that large combinatorial libraries can be synthesized using these technologies.     

Ferric chloride: a mild and versatile reagent for the formation of 1,6-anhydro glucopyranoses

A novel method for the preparation of 1,6-anhydro glucopyranoses (mono- and disaccharides) utilizing anhydrous FeCl₃ as Lewis acid is described. Treatment of methyl 6-O-benzyl and 6-O-p-methoxybenzyl-α/β d-glucopyranosides derivatives with FeCl₃ in CH₂Cl₂ at room temperature and 40°C afforded 1,6-anhydro glucopyranosides in moderate to good yields, through a debenzylation and intramolecular glycosidation in one step. A plausible reaction pathway is proposed.     

An insight into hexamethylenetetramine: a versatile reagent in organic synthesis

Hexamethylenetetramine is a versatile reagent in organic synthesis. It plays a major role in modern organic synthesis. This review focuses on hexamine reagent for its significant role in organic synthesis during the past decades and is able to provide a valuable perspective from synthetic point of view.     

Allyl tetrahydropyranyl ether: a versatile alcohol/thiol protecting reagent

Allyl tetrahydropyranyl ether (ATHPE) can be used as a versatile protecting reagent. In combination with NBS/I₂, O-allyl group can easily be replaced by hydroxyls (including tertiary-OH) or thiols, in the molecules comprising other reactive functional groups such as halogen, nitro, acetonide and alkene under mild reaction conditions (near neutral pH and ambient temperature).     

High-load, oligomeric dichlorotriazine: a versatile ROMP-derived reagent and scavenger

A new high-load, soluble oligomeric dichlorotriazine (ODCT) reagent derived from ring-opening metathesis polymerization (ROMP) is reported as an effective coupling reagent, scavenger of nucleophilic species, and activator of DMSO for the classic Swern oxidations. Two variants of this reagent (2G)ODCT 4 and (1G)ODCT 16, possessing theoretical loads of 5.3 and 7.3 mmol/g, respectively, have been synthesized. Preparation was accomplished via simple synthetic protocols affording free flowing powders, amenable for large-scale production. Removal of the spent oligomeric reagent was achieved via either precipitation of the spent reagent or simple filtration utilizing a silica SPE, followed by solvent removal, to deliver products in excellent yield and purity. In addition, the corresponding norbornenyl monomer 3 was successfully demonstrated in a couple-ROMP-filter protocol utilizing in situ polymerization, achieving comparable results versus the corresponding oligomeric variant.     

Acetic anhydride as a synthetic reagent

Acetic anhydride has been used widely in synthetic organic chemistry, especially in syntheses and transformations of heterocyclic compounds. These utilities are reviewed under the following classification.     

Iodine as a versatile reagent for the Prins-cyclization: an expeditious synthesis of 4-iodotetrahydropyran derivatives

Iodine is found to be an efficient reagent for the coupling of homoallylic alcohols with aldehydes under mild conditions to produce 4-iodotetrahydropyran derivatives in excellent yields in a short reaction time with high selectivity. The use of iodine makes this procedure simple, convenient and cost-effective.     

3-Methoxalylchromone--a novel versatile reagent for the regioselective purine isostere synthesis

The first synthesis of 3-methoxalylchromone was described. The reaction of the latter with electron-rich aminoheterocycles afforded a set of heteroannelated pyridines bearing a CO(2)Me substituent located at the α-position of the pyridine core.     

O-(2,3,4,5,6-pentafluorophenyl)methylhydroxylamine hydrochloride: a versatile reagent for the determination of carbonyl-containing compounds.

A review on the use of O-(2,3,4,5,6-pentafluorophenyl)methylhydroxylamine hydrochloride (PFBHA) for the determination of carbonyl-containing compounds is presented. PFBHA has been used in the determination of such diverse compounds as thromboxane B2, prostaglandins, amygdalin and a variety of other aldehydes, ketones and acids. PFBHA has been used for the determination of these compounds found in water, blood, urine, air and even clothing. The review covers literature referenced in Chemical Abstracts from 1975, when PFBHA was first synthesized, through March 1992.