The 6-aminoquinolone had previously been identified as a new class of quinolone antibacterial agents. To continue our structure-activity relationship (SAR) study in this series, novel 6-amino-8-methylquinolone derivatives have now been synthesized and evaluated for in vitro antibacterial activity. We have shown that the coupled presence of a methyl group at the C-8 position with an amino group at C-6 is effective for enhancing antibacterial activity, particularly against Gram-positive bacteria. The SARs associated with the N-1, C-6, and C-7 are discussed. The 1,2,3,4-tetrahydroisoquinolinyl derivative 19v showed the highest antibacterial activity with MIC values on Gram-positive bacteria superior to that of ciprofloxacin, especially against Staphylococcus aureus strains, including those strains which are methicillin-and ciprofloxacin-resistant.

Triplet dioxygen was reduced by TEMPO or trityl radicals in the presence of two molar equivalents of the strong B(p-C F X) (X: F or H) boron Lewis acids under mild conditions to give the bis(borane)superoxide systems 2. The sensitive radical anion species were isolated and characterized by methods including X-ray crystal structure analysis and EPR spectroscopy.

The β-borylborole 3 adds phenylacetylene in a [4+2]-cycloaddition to give the boryl-substituted 7-boranorbornadiene derivative 4 b. Thermolysis (60 °C) results in a rearrangement with B-B coupling to give the tetraaryldiborane(4) 6 b. It splits dihydrogen with cleavage of the B-B bond. With the terminal acetylene 1-pentyne the diborane(4) compound 6 b undergoes a rare 1,1-diboration reaction to yield the respective geminal diborylalkene product 14.

Persistent radicals undergo hydrogen atom abstraction reactions with a great variety of substrates, but not with dihydrogen. It has now been found that the TEMPO radical splits dihydrogen under mild conditions in the presence of the strong bulky B(C6 F5 )3 boron Lewis acid. The reaction is thought to proceed by a typical frustrated Lewis pair mechanism with the TEMPO radical acting as the active Lewis base. The reaction was analyzed by DFT, which indicates that no significant spin density on the hydrogen atoms is accumulated along the H2 splitting reaction path.

In a furtherance of our SAR study on the C-6 position of quinolone antibacterials, a series of 6-desfluoro-8-methylquinolones were synthesized and evaluated for their in vitro antimicrobial activity. As a result of this study, compounds with strong activity against Gram-positive bacteria, including ciprofloxacin-resistant and methicillin-resistant Staphylococcus aureus, were identified. The best Gram-positive antibacterial activity was exhibited by piperidinyl derivative 6c, which was 17 times more potent than ciprofloxacin and displayed extremely high activity against Streptococcus pneumoniae with an MIC value of <0.016 microg/mL. Thus, we have shown that substituent combinations in the quinolone ring, excluding the C-6 fluorine atom, might produce powerful antibacterial agents.