Oxazolines thiolysis

A direct conversion of the oxazoline (44) to the thiazoline (45) can be achieved by the thiolysis of the oxazoline (44) with H2S in methanol in the presence of triethylamine followed by cyclodehydration with Burgess reagent. This method is essentially free from racemisation and has been used in the transformation of peptide substrates <95TL6395>. 

Other nucleophiles are known to attack the 2-position of oxazolines. Wipf and co-workers developed a general method to convert oxazolines to thiazolines by thiolysis followed by cyclodehydration (Scheme 8.100). " Initial attack of hydrosulfide at the 2-position of the oxazoline occurs under either acidic or... 

The reaction generally tolerates steric hindrance at the 2-exocyclic position as well as at the 4- and 5-positions. However, the reaction rate was found to be sensitive to steric effects. Thus, thiolysis of a threonine-derived oxazohne 332 was considerably slower than that for the corresponding serine-derived oxazoline 333 (Scheme 8.101). The rate difference could be exploited to selectively convert a serine-derived oxazoline to a thioamide in the presence of a threonine-derived oxazoline. 

Thiolysis with H2S as the nucleophile is employed to open the oxazoline ring in 13.24 Attack on the sp2 carbon in the heterocycle leads to tetrahedral intermediate 63 which decomposes preferentially under C-0 bond cleavage. 

Thiolysis of oxazolines (427) with HjS affords thioamides (428) which by subsequent cyclodehydration with the Burgess reagent (Me02CNS02NEt3) lead to 2-thiazolines (Scheme 104)... 

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See also in sourсe #XX -- [ , ]

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