Thiolation thiolactone

The conversion of a thiolactone to a cyclic ether can also be used as a key step in the synthesis of functionalized, stereochemically complex oxacycles (see 64—>66, Scheme 13). Nucleophilic addition of the indicated higher order cuprate reagent to the C-S double bond in thiolactone 64 furnishes a tetrahedral thiolate ion which undergoes smooth conversion to didehydrooxepane 65 upon treatment with 1,4-diiodobutane and the non-nucleophilic base 1,2,2,6,6-pentamethylpiperidine (pempidine).27 Regio- and diastereoselective hydroboration of 65 then gives alcohol 66 in 89 % yield after oxidative workup. Versatile vinylstannanes can also be accessed from thiolactones.28 For example, treatment of bis(thiolactone) 67 with... 

Thiolation of peptides and other small molecules containing amines proceeds easily with N-acetyl homocysteine thiolactone. However, protein modification often results in much lower yields unless the reaction is done for extended periods at pH 10-11. 

It has been found that silver ions catalyze the thiolation process with proteins, allowing the reaction to be completed rapidly at physiological pH (Benesch and Benesch, 1958). The addition of an equal molar concentration of AgNO j forms an insoluble complex with the thiolactone, and this in turn reacts with protein amines. 

Add N-acetyl homocysteine thiolactone (Aldrich) to the bicarbonate reaction mixture to obtain a concentration representing a 10- to 20-fold excess over the amount of amines present. For protein thiolation, add the same molar excess of thiolactone reagent to the water reaction medium, and then slowly add an equivalent molar quantity of silver nitrate (AgNO j). Maintain the pH at 7.0-7.5 with periodic addition of NaOH. 

A plausible mechanism for the formation of 4 is rationalized on the basis that photolysis of 3 results in [2-1-2] cyclization to thietane 4 and is subsequently followed by rearrangement to thiolactone 5 (Scheme 6). Ring opening of the initially formed thietane 4 leads to a zwitterion, which is facilitated by lone pair electrons of nitrogen and oxygen atoms, and nucleophilic reaction of the thiolate anion to carbonyl carbon gives 5. For the tricyclic thietane 4a, nucleophilic addition of the thiolate anion is difficult, and results in the formation of stable thietane 4a. 

It may seem odd that the technique of spectrophotometric titration, so extensively applied to the tyrosyl residues of proteins, has not been used in the study of protein sulfhydryl groups. The only study of this sort known to the author is the determination of the ionization constant of —SH grouj of thiolated gelatin (introduced by reaction with A -acetylhomocysteine thiolactone). By measuring the change in absorptivity as a function of pH at 2380 A, Benesch and Benesch (1958) found the apparent acidity of the thiol groups of the modified gelatin (pK = 9.8) to be nearly the same as that of an appropriate model compound, A -acetylhomocysteine (pK = 10.0). 

Stannyl thiolates were recently shown to be applicable to the formation of the macrocyclic poly-thiolactones (23) and (24), according to equation (16). ... 

However, one of the first matrices used for insolubilization of the aflBnant was cellulose that had been partially etherified with 3-hydroxy-phenoxy or 4-diazobenzyl groups (which react with diazotized protein or protein, respectively). Amino compounds may also be attached to the acid chlorides of carboxy polysaccharides.0-(3-Allyloxy-2-hydroxy-propyl)-cellulose and -Sephadex can be mercuriated and, in this form, used for isolation of thiolated antibodies and fractionation of mononucleotides according to their aflBnity for an organomercurial. Another mercurial, prepared by treatment of 0-(2-aminoethyl)Sephadex with IV-acetylhomocysteine thiolactone to give a thiol (49) which, on treatment with 2,5-bis[(acetoxymercuri) methyl]-1,4-dioxane, gives 50, can also be used in aflSnity chromatography of thiolated proteins. 

Several successful attempts have been made at introducing mercapto groups into proteins without degrading the polypeptide chain. N-acetylhomocysteine thiolactone (LIV) in the presence of silver ion reacted readily with the primary amino group of lysine residues to produce thiolated proteins (LV) (77). 

An important homocysteme-y-thiolactone derivative is iV-acetyUiomocystei-nethiolactone or citiolone 2 (Scheme 2), a commercial compound that was introduced as a thiolating agent for proteins. This thiolation consists of aminolysis of the water-soluble A -acetyUiomocysteinethiolactone 2 by the e-NH2 groups of lysine residues [92-94]. Thiolation of a large variety of macromolecular biochemical systems has been reported [80, 94—106]. 

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