Model studies on the mechanism of biotin dependent carboxylations. Site of protonation versus CO2-transfer

The syntheses of two potential N1-carboxy-biotin model compounds in which a formal positive charge has been placed at the ureido carbonyl oxygen atom have been described. The reactivity of these substrates, N-carbomethoxy-N'-methyl-2-ethoxyimidazolinium fluoroborate, 19, and N-carbomethoxy-N'- methyl-2-(2',6'-dimethylphenoxy)-imidazolinium fluoroborate, 20, toward various oxygen, nitrogen and carbon nucleophiles was examined. A third model compound, N-carbomethoxy-N,N'-dimethyl-2-oxoimidazolinium fluoroborate, 21, has also been prepared and its reactivity subsequently examined. This compound is an "irreversibly acidified" prototype of biotin protonated at the N'-nitrogen atom. Treatment of compounds 19 and 20 with nucleophiles led to either open-chain products via attack at the C-2 carbon (A[lowered AC]2), or to N-carbomethoxy-N'-methylimidazolidone, 29, by attack at the O-ethyl side chain of compound 19 (A[lowered AL]2 or E2). The A[lowered AC]2 pathway was the exclusive mechanism observed for compound 20. The emergence of the A[lowered AC]2 route for compound 19 can be explained in terms of the reversible formation of the tetrahedral intermediate via attack at the C-2 position. In these cases the reaction can conceivably channel out through the irreversible S[lowered N]2 process (A[lowered AL]2). The reactions of compound 21 are best explained by attack at the C-2 carbon atom, and by the desired attack at the carbomethoxy carbonyl group with subsequent transfer of this moiety to the nucleophile. Protonated intermediates are often postulated as key intermediates in enzyme-induced reactions. Model compounds 19. 20 and 21 are "irreversibly acidified" equivalents of two potential intermediates for N'-carboxybiotin. Of these three compounds, only 21 leads to the transfer of the carboxy group upon treatment with nucleophiles. Although the carbonyl oxygen atom has been demonstrated to be the most basic site m the imidazolidone ring,[raised 53] this fact does not rule out the possibility that the enzyme specifically delivers a proton to the N'-position or that N'-protonation leads to a kinetically active species which is product determining.