No abstract

. Two new methods to synthesize macrocyclic P-keto lactones have been developed. The first involves the synthesis of o-halo-P-keto esters and an intran~olecular alkylation of the dianions to these compounds. The reaction is complicated by elimination in the small and medium ring systems and by difficulties in purifying the final products. However, it is possible to obtain modest yields of the desired P-keto lactones. This procedure was used to synthesize the 25-and 27-membered ring P-hydroxy lactones that are the constituents of termite defense compounds. The second method involves the thermolysis of acylated Meldrum's acid derivatives, which leads directly to P-keto lactones. This process gives modest yields of macrocyclic systems and good yield of the unsubstituted 3-oxopentan-5-olide (25). The 14-membered macrocyclic P-keto lactone 9 j has a complex 'H NMR spectrum, which has been interpreted in terms of multiple conformations. The temperature dependence of the NMR spectrum of 9j is consistent with entropic, rather than enthalpic, control of the equilibrium. Quasiharmonic entropy calculations are consistent with this model. On a mis au point deux nouvelles mkthodes de synthkse de P-ckto lactones macrocycliques. La premikre impl~que la synthese des p-cktoesters w-halog6nCs et une alkylation intramolCcula~re des dianions de ces composCs. La reaction est compllqute par une reaction d'elimination dans les systkmes comportant des petits cycles et des cycles moyens et par des difficult& a purifier les produ~ts finals. Toutefois, il est possible d'obtenir des rendements modestes des P-cCto lactones dCsirCes. On a utilisC cette proctdure pour synthktiser les P-hydroxy lactones a 25 et 27 chainons qui font partie des produits de dtfense des termites. La seconde mCthode impllque la thermolyse de derivCs acylCs de I'acide de Meldrum et elle conduit directement aux p-cCto lactones. Cette mCthode donne des rendements modestes des systkmes macrocycliques et un bon rendement du 3-oxopentan-5-olide (25). Le spectre RMN du 'H de la P-ceto lactone macrocyclique ti 14 chainons (9j) est complexe et on I'a interpret6 en fonction de conformations multiples. La relation entre le spectre RMN du composC 9 j et la temperature est en accord avec un contr8le entropique, plut8t qu'enthalpique, de I'tquilibre. Des calculs d'entropie quasiharmonique sont en accord avec ce modkle.[Traduit par la redaction]

A lab practical exam for second-year organic chemistry is presented that tests multiple lab skills and theory that the students have acquired directly from laboratory experiences. This exam motivates students to learn lab skills and is an effective end-of-term test for many of those skills.

Cover: The picture on the cover is a 3D graphic representation of one specific orientation of the prepared azo dye coumarin-containing polymethacrylate. Looking down the centre of the aliphatic methacrylate chain we see the circle of oxygen due to the carbonyl of the methacrylate, and then the circle of the nitrogen due to the azo dye, followed by the circle of the oxygen and the bromine of the coumarin unit. The image shows the folding of the pendant azo dye and the coumarin groups around the methacrylate chain. Next to the 3-D image is the 2-D drawing of the polymer. Further details can be found in the article by