ok didn't do my usual stuff yesterday. well, I'll do it right now
first, _Organic Synthesis_ roulette. and the word of the day to plug into the orgsyn.org search box is..."powder"
aw jeez it's a strange one
http://orgsyn.org/demo.aspx?prep=v95p0112
another "2+3 dipolar cycloaddition". this is a large class of chemical reactions, of particular use in synthesis of five-membered heterocycles
in this class of reaction, a "dipolarophile", usually an asymmetrical alkene or alkyne, i.e. one with an electron-donating group on one side of the multiple bond and an electron-withdrawing group on the other side, reacts with a "1,3-dipole", a molecule that has a chain of three atoms and a definite difference in positive-negative polarity between the ends of the chain. to pick a very commonplace example, the so-called "click" reaction, the 1,3-dipole is an organic azide, R-N=N⁺=N⁻
well that's neat I guess! I'll try to remember this, if I ever have to make oxazoles...which seems a bit unlikely
in this particular _Organic Syntheses_ reaction, though, the 1,3-dipole is a strange beast indeed, an amide of N-aminopyridine, obtained by amidation of N-aminopyridinium iodide with the methyl ester of N-t-butoxycarbonylglycine. sticking a pyridinium group onto the amide nitrogen unbalances the usual charge distribution in the amide group and converts it to a 1,3-dipole, it would seem!
the "dipolarophile" is an unsymmetrical alkyne here. cycloaddition yields a substituted oxazole