Chameleonic reactivity of vicinal diazonium salt of acetylenyl-9,10-anthraquinones: synthetic application toward two heterocyclic targets.

The nature of products in the diazotization of 1-amino-2-acetylenyl-9,10-anthraquinones strongly depends on the nature of substituents at both the alkyne and at the anthraquinone core. Donor substitution (NHAr, OH) at the fourth position stabilizes the diazonium salt at C1, decelerating electrophilic cyclization at the arylethynyl substituent at C2. This effect allows the replacement of the diazonium with azide group and subsequent closure into isoxazole ring with preservation of the alkyne. In contrast, electrophilic 5-exo-dig cyclizations to condensed pyrazoles is observed for the combination of donor substituents at the aryl alkyne moiety and an OAc substituent at C4. The latter process provides a new synthetic route to 3-ethynyl-[1,9-cd]isoxazol-6-ones that are difficult to access otherwise. DFT calculations suggest that donor substituents have only a minor effect on alkyne and diazonium polarization in the reactant but provide specific transition state stabilization by stabilizing the incipient vinyl cation. This analysis provides the first computational data on electrophilic 5-exo-dig cyclization in its parent form and the nucleophile-promoted version. This cyclization is a relatively fast but endothermic process that is rendered thermodynamically feasible by the enol-keto tautomerization with concomitant aromatization in the five-membered heteroaromatic ring. Computations suggest that the importance of nucleophilic assistance in the transition state for a relatively weak nucleophile such as water is minor because the energy gain due to the Lewis base coordination to the carbocationic center is more than compensated for by the unfavorable entropic term for the bimolecular proces.

Radical dinitroalkane dianions from the nitration of nitroalkanes by peroxynitrite.

In alkaline solutions at pH >10, peroxynitrite (ONOO(-)) rapidly and efficiently nitrates aci-nitroalkane anions, RCH=NO(2)(-) (R = H, CH(3), or CH(3)CH(2)), to give the radical dinitrodianions, RC(NO(2))(2)(2-). These anions have been characterized by EPR and have multiplets based on 1:2:3:2:1 pentets consistent with a hyperfine coupling with two equivalent (14)N nuclei of the nitro groups, and the following hyperfine coupling constants, in gauss: R = H, a(N) = 9.96 and a(H) = 1.85; R = CH(3), a(N) = 9.90 and a(H) = 3.22; and R = CH(3)CH(2), a(N) = 9.57 and a(H) = 4.01. Nitration is attributed to the trapping of nitrogen dioxide, formed by ONO-OCO(2)(-) homolysis for the carbon dioxide adduct of peroxynitrite, by the aci-nitroalkane anion. In air, the radical dinitrodianions are oxidized to the monoanions, and the kinetics of its formation is readily followed by UV/vis spectroscopy of the rise in absorption at 380 nm. This report represents the first successful spin trapping of nitrogen dioxide formed from peroxynitrite, and the method may be a useful one for preparing geminal dinitroalkanes.

Reaction of nitric oxide at the beta-carbon of enamines. A new method of preparing compounds containing the diazeniumdiolate functional group.

The reaction of nitric oxide (NO) with enamines has been investigated. Unlike previously reported reactions of NO as a free radical with alkenes, the electrophilic addition of NO to the beta-carbon of enamines results in the formation of compounds containing the diazeniumdiolate functional group (-[N(O)NO](-)). This reaction between NO and enamines has been shown to be quite general and a variety of enamine-derived diazeniumdiolates have been isolated and characterized. While enamines derived from aldehydes and ketones whose structures allow for sequential multiple electrophilic additions tended to undergo overreaction leading to unstable products, it has been shown that this complication may be overcome by suitable choice of reaction solvent. The products obtained may exist as zwitterionic iminium salts or as neutral species depending upon the structure of the parent enamine. The diazeniumdiolate derived from 1-(N-morpholino)cyclohexene is unique among the new compounds in that it spontaneously releases NO upon dissolution in buffered aqueous solution at pH 7.4 and 37 degrees C. While the total quantity of NO released by this material (ca. 7% of the theoretical 2 moles) is apparently limited by a competing reaction in which it hydrolyzes to an alpha-diazeniumdiolated carbonyl compound and the parent amine, this feature may prove to be of great value in the development of multiaction pharmaceuticals based upon this new type of NO-releasing compound. Reports of enzymatic (oxidative) release of NO from previously known carbon-bound diazeniumdiolates also suggest that analogues of these compounds may be useful as pharmaceutical agents. This new method of introducing the relatively rarely studied diazeniumdiolate functional group into organic compounds should lead to further research into its chemical and biological properties.

Reversible and irreversible hemichrome generation by the oxygenation of nitrosylmyoglobin.

The repeated oxygenation/reduction/nitrosylation of nitrosylmyoglobin produces low-spin ferric heme hemichromes which have been characterized by electron spin resonance spectroscopy. The predominant myoglobin hemichrome is a chemically reversible dihistidyl complex identified by the g values 1.53, 2.21, and 2.97. Also present is a low-spin ferric hydroxide derivative which is represented by the g values 1.83, 2.18, and 2.59. The formation of these species goes undetected by UV-vis spectroscopy, but the oxygenation of myoglobin to metmyoglobin is correlated with complete conversion of nitric oxide to nitrate which is released following a clear induction period. These results are interpreted in terms of the intermediates generated during the MbNO oxygenation reaction.

Sperm motility enhancement by nitric oxide produced by the oocytes of fathead minnows, Pimephelas promelas.

The effects of nitric oxide (NO) on sperm motility were examined in the fathead minnow, Pimephelas promelas, using computer-assisted sperm analysis (CASA). The observed effects underscore the dual nature of NO as both a low-concentration regulatory agent and, at higher doses, a cytotoxic agent. At 1 x 10(-6) M concentration, NO donor sodium nitroprusside (SNP) enhanced sperm motility percentages and increased CASA velocity parameters curvilinear velocity, straight-line velocity, and average path velocity, whereas 1 x 10(-2) M concentration inhibited percent motility and decreased velocities. Fathead minnow ova-produced NO was subsequently trapped as a paramagnetic ferrous iron complex and detected by electron spin resonance spectroscopy. The distinctive triplet spectrum, with a(N) = 12.5G and g(iso) = 2.04, was recorded during a critical 5-minute period following laying. Nitric oxide synthase (NOS) was histochemically localized at the micropyle of mature unfertilized fathead eggs, and an inhibitor of NOS blocked histochemical staining. CASA analysis of sperm motility in the presence of ovaproduced NO over an 8-minute time course reveals an optimum motility enhancement at 4 minutes that is similar to the effect of 1 x 10(-6) M SNP. This transient NO production by freshly laid ova and the localization of NOS near the site of sperm entry, together with the motility-enhancing effect of 1 x 10(-6) M SNP on sperm, indicates an active role for low-concentration NO in fertilization.

Heme compounds in dinosaur trabecular bone.

Six independent lines of evidence point to the existence of heme-containing compounds and/or hemoglobin breakdown products in extracts of trabecular tissues of the large theropod dinosaur Tyrannosaurus rex. These include signatures from nuclear magnetic resonance and electron spin resonance that indicate the presence of a paramagnetic compound consistent with heme. In addition, UV/visible spectroscopy and high performance liquid chromatography data are consistent with the Soret absorbance characteristic of this molecule. Resonance Raman profiles are also consistent with a modified heme structure. Finally, when dinosaurian tissues were extracted for protein fragments and were used to immunize rats, the resulting antisera reacted positively with purified avian and mammalian hemoglobins. The most parsimonious explanation of this evidence is the presence of blood-derived hemoglobin compounds preserved in the dinosaurian tissues.