Oxazolyl triflate

In the model studies toward the total synthesis of dimethyl sulfomycinamate, Kelly et al. successfully carried out the Stille couplings of oxazolyl triflate 18 with an array of organostannanes [19, 20]. Thus, 2-aryl-4-oxalone 17 was transformed into the corresponding triflate 18, which was then coupled with 2-trimethylstannylpyridine under the agency of Pd(Ph3P)4 and LiCl to provide adduct 19. The couplings of triflate 18 with phenyl-, vinyl- and phenylethynyl trimethyltin all proceeded in excellent yields. Unfortunately, application to the more delicate system in the natural product failed and the oxazole moiety was installed from acyclic precursors. 

An efficient and convergent synthesis of the C(l)—C(ll) side chain 83 of leucascandrolide A has been achieved. The key bond connection was made through the use of a Sonogashira cross-coupling [53, 54]. Oxazolyl triflate 82 was treated with alkynamide 80 using the Sonogashira coupling reactions to provide the side chain precursor 83 (part of the synthesis of leucascandrolide A) [53, 54]. 

The bis-alkyne side chain was synthesized as outlined in Scheme 64. Hy-drazone 300 was alkylated, followed by hydrolysis, to give a-hydroxy ketone 301, which was converted to oxazolone by treatment with trichloroacetyl isocyanate. The oxazolyl triflate 302, prepared by reaction of the oxazolone with triflic anhydride, was coupled with alkyne 213 under Sonogashira conditions as reported by Panek [118], followed by deprotection of MPM and oxidation, to furnish the side chain precursor bisalkynyloxazole 299. 

Corey et al. [19] simultaneously reported similar studies using a2,2 -bis(oxazolyl)-6,6 -dimethyl-l,l -biphenyl as copper(I)triflate chelate. This hgand afforded a stable monomeric chiral Cu(I) complex providing a highly... 

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