Thiolactones nucleophilic attack

Nucleophilic attack by an amino group on the carbonyl function of a /3-thiolactone forms the basis for a novel peptide synthesis, as illustrated by the preparation of (63) (64RCR493). Amines undergo Michael addition to the carbon-carbon double bond of thiete 1,1-dioxide (61) (62JA399). 

This reaction is not quenched by 2,5-dimethyl-2,4-hexadiene or naphthalene. C(3)- and C(5)-alkylsubstituted 2(5//)-thiophenones behave similarly, but alkyl groups on C(4) prevent this photosolvolysis [18]. The rates for these reactions are similar in water, methanol, ethanol, 2-propanol or t. butanol but about ten times slower in 2,2,2-trifluoroethanol. All this suggests that the (singlet) excited thiolactone is attacked at C(4) by nucleophilic reagents to give an intermediate ketene which then affords the -configured ester (or acid) selectively (Scheme 19). 

Just as thioesters RCOSR are known to be much more easily attacked by nucleophiles than esters RCO2R, so too are / -thiolactones vis-a-vis ) -lactones. This has been demonstrated by competition studies using the 2-benzyl derivatives (105 X = 0, S) and the nucleophiles butanethiol BuSH and isobutylamine Bu NH2. Indeed, when an equimolar mixture of the two compounds was treated with BuSH in MeCN at 348 K, the -lactone... 

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