羰基保护基团的新进展

对2000～2006年间文献中报道的一些对羰基化合物进行缩酮(醛)类的保护、去保护的主要方法进行了简要的概述.     

Advances in Protecting Groups for Oligosaccharide Synthesis

Carbohydrates contain numerous hydroxyl groups and sometimes amine functionalities which lead to a variety of complex structures. In order to discriminate each hydroxyl group for the synthesis of complex oligosaccharides, protecting group manipulations are essential. Although the primary role of a protecting group is to temporarily mask a particular hydroxyl/amino group, it plays a greater role in tuning the reactivity of coupling partners as well as regioselectivity and stereoselectivity of glycosylations. Several protecting groups offer anchimeric assistance in glycosylation. They also alter the solubility of substrates and thereby influence the reaction outcome. Since oligosaccharides comprise branched structures, the glycosyl donors and acceptors need to be protected with orthogonal protected groups that can be selectively removed one at a time without affecting other groups. This minireview is therefore intended to provide a discussion on new protecting groups for amino and hydroxyl groups, which have been introduced over last ten years in the field of carbohydrate synthesis. These protecting groups are also useful for synthesizing non‐carbohydrate target molecules as well.     

The Suzuki Reaction Under Solvent-Free Conditions

The coupling reaction of diverse arylhalogenids with phenyl-boronic acid under solvent-free conditions has been performed using a palladium catalyst. The order of reactivity was complementary to the normal Suzuki reaction.     

Selective Protecting Method for the Individual Hydroxyl Groups of Kdn

ABSTRACT

Selective protection for the individual hydroxyl groups of methyl (phenyl 3-deoxy-2-thio-β-D-glycero-D-galacto-2-nonulopyranosid)onate (2) was examined. The 4-, 5-, and 7-hydroxyl groups of methyl (phenyl 3-deoxy-8,9-O-isopropylidene-2-thio-β-D-glycero-D-galacto-2-nonulopyranosid)onate (3) were found selectively to be protected by t-butyldimethylsilyl, methoxymethyl, and benzoyl groups, respectively. In order to obtain the 8- and 9-hydroxyl derivatives selectively, methyl (phenyl 4,5,7-tri-O-acetyl-9-O-t-butyldimethylsilyl-3-deoxy-2-thio-β-D-glycero-D-galacto-2-nonulopyranosid)onate (12) and methyl (phenyl 4,5,7,8-tetra-O-benzyl-9-O-triphenylmethyl-3-deoxy-2-thio-β-D-glycero-D-galacto-2-nonulopyranosid)onate (19) were prepared in moderate yields.     

Electrophilic Vinylation of Thiols under Mild and Transition Metal-Free Conditions

The iodine(III) reagents vinylbenziodoxolones (VBX) were employed to vinylate a series of aliphatic and aromatic thiols, providing E-alkenyl sulfides with complete chemo- and regioselectivity, as well as excellent stereoselectivity. The methodology displays high functional group tolerance and proceeds under mild and transition metal-free conditions without the need for excess substrate or reagents. Mercaptothiazoles could be vinylated under modified conditions, resulting in opposite stereoselectivity compared to previous reactions with vinyliodonium salts. Novel VBX reagents with substituted benziodoxolone cores were prepared, and improved reactivity was discovered with a dimethyl-substituted core.     

Reaction of Thiols and Bisulfite with the Pendant Allyl Groups in Polytriallylamine

The infrared spectrum of polytriallylamine prepared by the free-radical-initiated polymerization of triallylamine indicates the presence of a substantial amount of unsaturation. The usual quantitative methods of determining unsaturation (e. g., bromide/ bromate and mercuric acetate) cannot be used on polytriallylamine because of interference from the amino group. On the other hand, bisulfite was found to react rapidly and quantitatively with the pendant allyl groups of polytriallylamine. The reaction was studied over the pH range 4 to 10 and was fastest at pH 5.2. The reaction is a free-radical reaction which is catalyzed by metal ions and oxygen. The sulfonic acid groups which result from attack of bisulfite on the pendant allyl groups of polytriallylamine form strong zwitterion structures with the amine nitrogens. The addition of thiols is also a free-radical reaction. The reaction of HSCH₂CH₂OH, HSCH₂CO₂H, CH₃SH, H₂S, HSCH₂CH₂N(C₂H₅)₂, CH₅COSH, and C₆H₅SH with polytriallylamine was studied. Of these compounds, only HSCH₂CH₂OH and HSCH₂CO₂H reacted quantitatively. Some reasons for the differences in reactivity are presented.     

Electro-Deprotection—Electrochemical Removal of Protecting Groups

The reactions of electrochemical splitting may be successfully used for removal of protecting groups. Theoretical and preparative aspects of the method of electro-deprotection are discussed, and examples of its use are given. The removal often requires high potentials. The use of modified protecting groups (“inner activation”) or of catalysts (electron carriers) which facilitate electron transfer against the standard potential gradient (“external activation”), can greatly increase the scope of the electrochemical method.     

Quantum Dot-Catalyzed Photoreductive Removal of Sulfonyl-Based Protecting Groups

This Communication describes the use of CuInS₂/ZnS quantum dots (QDs) as photocatalysts for the reductive deprotection of aryl sulfonyl-protected phenols. For a series of aryl sulfonates with electron-withdrawing substituents, the rate of deprotection for the corresponding phenyl aryl sulfonates increases with decreasing electrochemical potential for the two electron transfers within the catalytic cycle. The rate of deprotection for a substrate that contains a carboxylic acid, a known QD-binding group, is accelerated by more than a factor of ten from that expected from the electrochemical potential for the transformation, a result that suggests that formation of metastable electron donor–acceptor complexes provides a significant kinetic advantage. This deprotection method does not perturb the common NHBoc or toluenesulfonyl protecting groups and, as demonstrated with an estrone substrate, does not perturb proximate ketones, which are generally vulnerable to many chemical reduction methods used for this class of reactions.     

The Effect of Protecting Groups on Chelation Control

Relative chelating abilities of alcohols, ethers and silyl ethers are rationalized in terms of the π accepting character of the group attached to oxygen. This in turn may be assessed by examination of the bond angle about oxygen.     

Synthesis of Fluorescein Phosphorotriesters Using Photolabile Protecting Groups

Fluorescein phosphorotriesters 5, each having four identical photoactivatable adducts have been synthesized to investigate their applications as intracellular fluorescence indicators.     

聚合物保护基团在有机合成中的应用

本文论述了聚合物保护基团在对称双功能基单保护合成不对称化合物,作为助剂合成手性化合物和作为载体合成昆虫性信息素和不对称卟啉,酞菁类化合物方面的应用。     

Synthesis of Thiocarbamates from Thiols and Isocyanates Under Catalyst- and Solvent-Free Conditions

Summary. A simple and efficient procedure was developed for the synthesis of S-alkyl (aryl) thiocarbamates under solvent-free conditions without the use of a catalyst. The significant features of this protocol are (a) operational simplicity, (b) mild reaction conditions, (c) short reaction times, (d) solvent-free conditions, and (e) high product yields.     

The Concept of Docking/Protecting Groups in Biohydroxylation

A general principle for biohydroxylation , in which time-consuming screening and enrichment techniques are avoided, is demonstrated by the introduction of a docking/protecting group into the substrate. This facilitates acceptance by the microorganism and allows the use of a narrow range of microorganisms, for example Beauveria bassiana ATTC 7159 (B. b.), for the hydroxylation of compounds with diverse structures. After the biohydroxylation, the docking/protecting group is removed (see scheme).     

2-(Glucosylthio)ethyl Groups as Potential Biolabile Phosphate- Protecting Groups of Mononucleotides

The in vitro anti-HIV effects and the stability studies of mononucleoside phosphotriester derivatives 1 – 3 of 3′-azido-3′-deoxythymidine (AZT) containing 2-(glucosylthio)ethyl moieties as potential biolabile phosphate-protecting groups are reported. The results of the anti-HIV evaluation demonstrate that the described compounds act via the release of the free nucleoside analogue and cannot be considered as mononucleotide prodrugs (pronucleotides). These data can be related to the lack of substrate affinity of these derivatives towards target-enzymes as corroborated by decomposition studies in various media and experiments with a purified β-D glucosidase.     

Electrophilic Vinylation of Thiols under Mild and Transition Metal‐Free Conditions

The iodine(III) reagents vinylbenziodoxolones (VBX) were employed to vinylate a series of aliphatic and aromatic thiols, providing E‐alkenyl sulfides with complete chemo‐ and regioselectivity, as well as excellent stereoselectivity. The methodology displays high functional group tolerance and proceeds under mild and transition metal‐free conditions without the need for excess substrate or reagents. Mercaptothiazoles could be vinylated under modified conditions, resulting in opposite stereoselectivity compared to previous reactions with vinyliodonium salts. Novel VBX reagents with substituted benziodoxolone cores were prepared, and improved reactivity was discovered with a dimethyl‐substituted core.