2,2 -dipyridyl disulfide, and

Additional acceleration of acylation can be obtained by inclusion of cupric salts, which coordinate at the pyridine nitrogen. This modification is useful for the preparation of highly hindered esters.122 Pyridine-2-thiol esters can be prepared by reaction of the carboxylic acid with 2,2 -dipyridyl disulfide and triphenylphosphine123 or directly from the acid and 2-pyridyl thiochloroformate.124... 

Dipyridyl disulfide and bis(2-aminophenyl) disulfide do not undergo hydroxysulfenylation using Pb(OAc)4 or in the absence of a metal salt, but each reacts satisfactorily in the presence of catalytic amounts of copper(II) acetate.2... 

Pyridinethiol esters. In combination with triethylamine 1 converts acids into 2-pyridinethioi esters in over 95% yieid. The method is superior to the usual eariier route invoiving treatment of an acid with 2,2 -dipyridyl disulfide and tri-phenylphosphine (5, 286 6, 246). ... 

Another efficient method seems to be Mukaiyama s redox condensation, which has found widespread application in total syntheses of natural products. Originally 5-(2-pyridyl)carbothioates (18), obtained from carboxylic acids, 2,2 -dipyridyl disulfide and triphenylphosphine (equation 8), were used in follow-up reactions. The use of (18) has been reviewed in the literature. - ... 

Corey et al. [36] developed an efficient and mild lactonization method using 2-pyridinethiol ester. Slow addition of 2-pyridinethiol esters, prepared from to-hydroxy acids by reaction with 2,2 -dipyridyl disulfide and Ph3p or 2-fhiopyridyl chloroformate and EljN, to refluxing xylene under dilution conditions yielded... 

Macrocyclic lactones. Corey et al. have published several more examples of the conversion of co-hydroxyalkanoic acids to macrocyclic lactones by use of 2,2 -dipyridyl disulfide and triphenylphosphine. Several lactones in the prostaglandin series have been prepared, for example, (1) and (2). The process has been applied to even more complex natural products and has also been used... 

Thiosalicylamide was alkylated with ethyl chloroformate in CH2Ci2 to give the thioi add (61), which underwent smooth cyclization to give the l,4-benzothiazepine-2,5-dione (62) in 30-40% yieid. This type of cyclization also occurred with PPA, with 2,2 -dipyridyl disulfide, and triphenyl-phosphine (Equation (14)) <86JMC784>. 

The Chi synthesis modulators that Rebeiz et al. (13, 14) used in conjunction with ALA could be divided into three categories A) enhancers of ALA conversion to porphyrins (2-pyridine aldoxime, 2-pyridine aldehyde, picolinic acid, 2,2 dipyridyl disulfide, 2,2 -dipyridyl amine, 4,4 dipyridyl, and phenanthridine), B) inducers of ALA biosynthesis and porphyrin accumulation (2,2 -dipyridyl and 1,10-phenanthroline), and C) inhibitors of MV PChlide synthesis (2,3-dipyridyl, 2,4-dipyridyl, 1,7-phenanthroline, and 4,7-phenanthroline). Compounds in group A did not cause significant porphyrin accumulation alone however, they enhanced dark conversion of exogenous ALA to porphyrins. This group was further subdivided into compounds that enhanced conversion of ALA to MV PChlide (2-pyridine aldoxime, 2-pyridine aldehyde, picolinic acid, and 2,2 -dipyridyl disulfide) and those that stimulated conversion to DV PChlide (4,4 dipyridyl, 2,2 dipyridyl amine, and phenanthridine). To qualify as an ALA biosynthesis and porphyrin accumulation inducer (category B), the compound had to cause these effects in the absence of ALA. Compounds in category C had to inhibit accumulation of MV PChlide with or without ALA. In most cases, in conjunction with ALA, the compounds stimulated DV PChlide accumulation compared to the ALA-treated control. 

Dipyridyl disulfide and related compounds have also been used as a method for macrolide formation the addition of thio-philic metal cations and/or pyridine derivatives has been found to assist this process. This oxidation-reduction condensation, using 2,2 -dipyridyl disulfide, constitutes an excellent strategy for the solid-phase synthesis of peptides. This method does not affect amino acids sensitive to oxidation, proceeds under mild conditions without the requirement of basic or acid catalysts, and has the advantages of minimizing both racemization of carboxyl component and side-reactions of certain amino acids. Furthermore, it has been successfully applied to phosphorylation reactions, such as the synthesis of coenzyme A, oligothymidilates, and nucleotide cyclic phosphates, and nucleotides from G ,2 -cyclouridine. ... 

Corey and Nicolaou used the S-pyridyl ester [19], which was generated by Mukaiyama thioester formation using 2,2 -dipyridyl disulfide and PPhj [20], as an activated precursor for lactonization. In 1976, they first accomphshed the total synthesis of some macrolide molecules including recifeioUde (15), as shown in Scheme 5.1 [21]. 

Macrocyclic lactones have been prepared by double activation of both carboxyl and hydroxyl functions by internaJ proton transfer without the need for the usual basic or acidic catalysts 2,2 -dipyridyl disulfide and triphenylphosphine are used as reagents . Various heterocycles can be prepared via extrusion of methylenetriphenylphospho-rane . High yields of thiasilaazoles could be obtained by using 1,8-bis(dimethylamino)naphthalene, a strong, non-nucleophilic base ... 

These esters can be prepared by reaction of the carboxylic acid with 2,2 -dipyridyl disulfide and triphenylphosphine " or directly from the acid and 2-pyridyl thior... 

Disaccharide Formation. Treatment of glycoside (3) with 2,2 -dipyridyl disulfide and Tri-n-butylphosphine in CH2CI2 rapidly yields thiopyridyl derivative (4) as a mixture of anomers. Activation of (4) with iodomethane, followed by treatment with glycoside acceptor (5), affords the disaccharide fragment of aver-mectin (6), exclusively a-linked in 78% yield (eq 4). Although other methods are available, this protocol offers advantages in practicality and stereoselectivity. 

Other Reactions. Treatment of an active hydroxy compound with 2,2 -dipyridyl disulfide and n-BuyP yields the corresponding thiopyridyl derivative. This methodology has been applied to the preparation of 5-arylthio-5 -deoxyribonucleosides (eq 8). Monophosphate esters [R0P(0)(0H)2] will react similarly to form the activated triphenylphosphonium adduct, which, in the absence of an added external nucleophile, dimerizes yielding a pyrophosphate. A-Methylimidazole has been found to catalyze this transformation. The addition of alcohols or amines, however, traps the phosphoryloxyphosphonium salt as the mixed diphosphate ester or mixed ester/amide, respectively (eq 9). Chlorotrimethylsilane and (pyS)2 have also been reported to facilitate the oxidation of phosphites to phosphates. ... 

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