Ethynyl acceptor

The acidity of 4-ethynyl- 1-methylpyrazole is lower than that of phenylacetylene (pK =29.1) [75IZV2351 79JCS(P2)726 84IZV923]. The ethynyl group in other positions of pyrazoles has smaller values i.e., 4-pyrazolyl radicals have a weaker electron-acceptor characteristic than the phenyl ring. 

Di(ethynyl)gold moieties can be used as bridging units between transition metals as demonstrated for square-grid complexes with Pt(dppe) acceptor components.96... 

Thanks to their tt electron clouds and to their small volume, ethynyl groups can sometimes function as bioisos-teres of aromatic rings and give similar donor-acceptor interactions. 

Figure 22. Molecular dimer of 7-ethynyl-6,8-diphenyl-7H-ben2ocyclohepten-7-ol, linked by two cooperative chains of 0-H-"C=C-H -Ph hydrogen bonds (drawn using published atomic coordinates [68]). The red-shifts of v=c h s d of vq h compared to a dilute solution in CCI4 are given. The distances of the H-atoms to the acceptor midpoints are Hc - Mph = 2.58 A and Ho" Mc=c = 2.58 A.

The C=C bond is only a weak n-acceptor, and in consequence, it was the last for which C=C-H—71 hydrogen bonding has been found. This interaction was characterized from a sample of crystalline ethynyl steroids [72], one of which is shown... 

PED. The triple-bond positional isomers of PED, lO-(l-propynyl) estr-4-ene-3,17-dione, and the 10-ethynyl, 10-cyano and 1-methyl cyanide analogs were not suicide inhibi-torsi2l,l3l,l32 10-[[li ]-l-hydroxy-2-propynyl], 10-[[lS]-l-hydroxy-2-propynyl] and 10-(l-oxo-2-propynyl) estr-4-ene-3,17-dione were prepared as potential PED metabolites (Scheme 16) and tested for their aromatase inhibitory activity on human placental enzyme. While the [Ii ]-hydroxy isomer was the only competitive inhibitor of aromatase, the other two oxidized analogs, [I S]-hydroxy and 1-oxo-derivatives, had very weak inhibitory properties compared to PED (Ki 27 jjM and 12 juM vs 23 nM for PED and ty/, of 4 min and 2.16 min vs 10.4 min for PED) . These observations were in contrast to the a priori assumption that 10-(l-oxo-2-propynyl)estr-4-ene-3,17-dione might be the most active derivative in the series, due to its Michael acceptor structure i8,121 similarly, the 19-hydroxylated propargyl derivatives of 3-deoxyandrost-4-ene-17-one and androst-4-ene-3,6,17-trione were very weak mechanism-based inhibitors of aromatase ". ... 

The other simplest combination of the non-equivalent D-A members is a 1. 2-donor/acceptor compound, l,r-bis(l-(anthraquinonyl)ethynyl)ferrocene (1,1 -FcAq2). To this complex, two-step protonation occurs, while only the first protonation causes the intramolecular electron transfer reaction. 

Structural diversity in gew-alkynol compounds, which have hydroxyl and ethynyl groups juxtaposed on the same carbon atom, is now discussed. Both functional groups can behave as hydrogen bond donors (O-H and C=C-H) and acceptors (O-H and... 

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