Electrophilic assistance element

The reactivity of alkynylstannanes toward electrophiles is one element in the oxygen-to-carbon rearrangement of alkynylstannane derivatives of furanyl and pyranyl lactols (e.g., Equation (85)). The cleavage of the anomeric C-O bond is assisted by the Lewis acid to give an oxonium ion, which is trapped in situ by the nucleophilic stannylalkyne. The utility of this process has been demonstrated in the synthesis of the natural product muricatetrocin C, and the drug substance CMI-977.246... 

In contrast to some related reviews, which use reaction class or electrophiles as organizational elements, this chapter is divided into three main sections according to catalyst class (i) Bronsted acid catalysis by phosphoric acid and phosphoramide derivatives, (ii) N—H hydrogen bond catalysis by organic base and ammonium systems, and (iii) combined acid catalysis including Bronsted-acid-assisted Bronsted acid, Lewis-acid-assisted Bronsted acid, and Lewis-acid-assisted Br0nsted acid systems (Figure 5.1). 

The elements of H2O are removed from the amide with the help of POCI3. The amide O is nucleophilic and P is electrophilic, so the first step is likely a simple Sn2 reaction at P with the amide O as nucleophile, with deprotonation of the nucleophile following. The C—O bond is cleaved next with assistance from the lone pair of N. Deprotonation of the nitrilium ion gives the isocyanide. 

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