3-Elimination reaction, ester-bound residue

Hydroxylaminolysis, treatment with stronger alkali (0.5 m NaOH, 2 h, 100°C) and alkaline methanolysis (0.25 m NaOMe, 1 h, 50°C) lead to complete O-deacylation of LPS and lipid A (176). Particularly in the case of alkaline methanolysis, ester-linked 3-acyloxyacyl residues undergo, in addition to transmethylation, a -elimination reaction, whereby the (R)-3-hy-droxy fatty acid ester is first transformed into the a,/ -unsaturated and then into the (S.-R -methoxy fatty acid methyl ester. The acyl substituent, on the other hand, is eliminated in the form of the free fatty acid (176). In fact, the presence of a 3-methoxyacyl derivative in the fatty acid spectrum of a given LPS is a strong indication for the presence of an ester-bound 3-acyloxyacyl... 

Resin-bound diazoimides 190 were subsequently reacted with different electron deficient acetylenes 191 in benzene at 80°C for 2 hours in the presence of Rh2(OAc)4 as a catalyst. Analysis of the crude products showed exclusively the presence of the desired furans 192 and excess unreacted acetylene, which, when sufficiently volatile (e.g propiolate esters), were eliminated in vacuo to provide furans of high purity. To avoid contamination of the desired furan product with residual, non-volatile acetylene, a two step sequence was implemented for the cycloaddition reaction. Thus, C-labelled diazoimide 193 was allowed to react with a large excess (10 eq) of dimethyl acetylenedicarboxylate (DMAD) 194 in the presence of Rhj(OAc)4 at room temperature in anticipation of trapping the bicyclic intermediate 196 on the polymeric support. After washing the... 

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