Acyl-sulfur linkage

Acyl and aroyl disulfides react by a somewhat different route. Both acetyl disulfide (225) and benzoyl disulfide (125) react exothermically with triethyl phosphite at room temperature to produce 0,0,0-triethyl phosphorothioate and the acyl or aroyl sulfide. This result suggests that the acyl-sulfur linkage in the phosphonium intermediate is more susceptible to cleavage by mercaptide ion than is the alkyl-oxygen bond. 

Just as a multiplicity of hydroxyl groups is normally related to sweetness, multiple nitro groups and the sulfur atom in the —S—S— or —C=S— linkage have been associated with bitter taste. Thus, it was obseiwed that a compound containing three nitro groups, such as picric acid, is usually bitter, and that those with two nitro group may be bitter. Compounds having structure A are also frequently bitter, and it was deduced that the bitterness of the acyl-thiocarbamide class of compounds is due to structure B. 

Oxidation of deferrialbomycin S2 with performic acid cleaves the hydroxamic acid linkages and converts the bound NS-hydroxyomithine to glutamic acid, the latter still in peptide combination. Partial acid hydrolysis and identification of fragments affords the structure cyclo-(seryl)s-(glutamyl)3 (135). The oxygen of the side chain of the first serine is linked via sulfur to N1 of 3-methylcytosine, which in turn is acylated with an unknown fragment. 

In order to obtain alkyl-, aryl- and acyl- guanidines Wang linker is treated with carbonylimida-zole and thiourea consecutively. The displacement of sulfur by primary or secondary amines leads to acid-labile carbamate linkage [119]. 

Figure 16 Proposed mechanism of the LipA reaction. Cataiysis occurs on an octanoyi substrate attached in an amide linkage to a lipoyl carrier protein (LCP). In the first half-reaction, reductive cleavage of SAM to generate a 5 -deoxyadenosyl 5 -radical allows for hydrogen atom abstraction at C6 of the fatty acyl chain. The proposed source of sulfur is a second [4Fe-4S] cluster, housed in the CX4CX5C motif conserved in lipoyl synthases. In the second half-reaction, the second 5 -deoxyadenosyl 5 -radical abstracts a hydrogen atom from C8 of the fatty acyl chain, allowing for sulfur insertion at this position. The addition of two protons affords lipoic acid in its reduced form.

---