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Monday, May 31, 2004
Douglass Taber
University of Delaware

Best Synthetic Methods

From time to time, synthetic transformations are reported that appear to be particularly convenient. One such is the procedure described by Paul Hanson of the University of Kansas (Tetrahedron Lett. 2003, 44, 7187. ) for the conversion of an alcohol to the amine. The imide 2, easily prepared by the addition of maleimide to furan, couples under Mitsunobu conditions with 1 to give the imide 3, contaminated with the usual impurities from the condensation. The crude imide 3 smoothly polymerizes under Ru metathesis conditions to give polymeric 3, from which the impurities are easily washed away. Exposure of the washed polymer to hydrazine then liberates the pure free amine 4.

Benzyl and substituted benzyl protecting groups are ubiquitous in organic synthesis. For base-sensitive substrates, the benzyl imidate, e.g. 6, is often used to install this group. Amit Basu of Brown University reports (Tetrahedron Lett. 2003, 44, 2267. *) that in situations such that the usual acidic promoters do not work, metal triflates can be effective. Lanthanum triflate worked particularly well, giving both high yield and high conversion in five minutes at room temperature.

The oxidation of an alcohol to the aldehyde or ketone on large scale would ideally be carried out with an inexpensive, easily handled reagent at ambient temperature. Scott Hoerrner of Merck Process, Rahway, NJ reports (Organic Lett. 2003, 5, 285. ) that stoichiometric iodine in the presence of catalytic TEMPO cleanly converts the sensitive alcohol 8 to the aldehyde 9. While this method was developed particularly for easily oxidized heteroaromatics such as 9, in fact the procedure works well for ordinary alcohol to aldehyde and alcohol to ketone oxidations.

There may be circumstances such that oxidation of an alcohol to the ketone with Cr is warranted. Mark Trudell of the University of New Orleans reports (Tetrahedron Lett. 200344, 2553. ) that such oxidations can be carried out with catalytic Cr and stoichiometric periodic acid, as illustrated by the conversion of 10 to 11.

D. F. Taber, Org. Chem. Highlights 2004, May 31.