3,7-disubstituted bicyclo[3.3.1]nonanes—III

Synthesis of 1,2-Disubstituted Adamantane Derivatives by Construction of the Adamantane Framework

This review summarizes achievements in the synthesis of 1,2-disubstituted adamantane derivatives by the construction of the tricyclic framework either by total synthesis or by ring expansion/contraction reactions of corresponding adamantane homologues. It is intended to complement reviews focusing on the preparation of 1,2-disubstituted derivatives by C-H functionalization methods.

Hydrodealkenylative C(sp3)-C(sp2) bond fragmentation

Chemical synthesis typically relies on reactions that generate complexity through elaboration of simple starting materials. Less common are deconstructive strategies toward complexity-particularly those involving carbon-carbon bond scission. Here, we introduce one such transformation: the hydrodealkenylative cleavage of C(sp³)-C(sp²) bonds, conducted below room temperature, using ozone, an iron salt, and a hydrogen atom donor. These reactions are performed in nonanhydrous solvents and open to the air; reach completion within 30 minutes; and deliver their products in high yields, even on decagram scales. We have used this broadly functionality tolerant transformation to produce desirable synthetic intermediates, many of which are optically active, from abundantly available terpenes and terpenoid-derived precursors. We have also applied it in the formal total syntheses of complex molecules.

Fast Amide Bond Cleavage Assisted by a Secondary Amino and a Carboxyl Group-A Model for yet Unknown Peptidases?

Unconstrained amides that undergo fast hydrolysis under mild conditions are valuable sources of information about how amide bonds may be activated in enzymatic transformations. We report a compound possessing an unconstrained amide bond surrounded by an amino and a carboxyl group, each mounted in close proximity on a bicyclic scaffold. Fast amide hydrolysis of this model compound was found to depend on the presence of both the amino and carboxyl functions, and to involve a proton transfer in the rate-limiting step. Possible mechanisms for the hydrolytic cleavage and their relevance to peptide bond cleavage catalyzed by natural enzymes are discussed. Experimental observations suggest that the most probable mechanisms of the model compound hydrolysis might include a twisted amide intermediate and a rate-determining proton transfer.

Anti-allodynic effect of 2-(aminomethyl)adamantane-1-carboxylic acid in a rat model of neuropathic pain: a mechanism dependent on CaV2.2 channel inhibition

Neuropathic pain is a serious physical disabling condition resulting from lesion or dysfunction of the peripheral sensory nervous system. Despite the fact that the mechanisms underlying neuropathic pain are poorly understood, the involvement of voltage-gated calcium (Ca(V)) channels in its pathophysiology has justified the use of drugs that bind the Ca(V) channel α₂δ auxiliary subunit, such as gabapentin (GBP), to attain analgesic and anti-allodynic effects in models involving neuronal sensitization and nerve injury. GBP binding to α₂δ inhibits nerve injury-induced trafficking of the α₁ pore forming subunits of Ca(V) channels, particularly of the N-type, from the cytoplasm to the plasma membrane of pre-synaptic terminals in dorsal root ganglion neurons and dorsal horn spinal neurons. In the search for alternative forms of treatment, in this study we describe the synthesis and pharmacological profile of a GABA derivative, 2-aminoadamantane-1-carboxylic acid (GZ4), which displays a close structure-activity relationship with GBP. Behavioral assessment using von Frey filament stimuli showed that GZ4 treatment reverted mechanical allodynia/hyperalgesia in an animal model of spinal nerve ligation-induced neuropathic pain. In addition, using the patch clamp technique we show that GZ4 treatment significantly decreased whole-cell currents through N-type Ca(V) channels heterologously expressed in HEK-293 cells. Interestingly, the behavioral and electrophysiological time course of GZ4 actions reflects that its mechanism of action is similar but not identical to that of GBP. While GBP actions require at least 24 h and imply uptake of the drug, which suggests that the drug acts mainly intracellularly affecting channels trafficking to the plasma membrane, the faster time course (1-3 h) of GZ4 effects suggests also a direct inhibition of Ca(2+) currents acting on cell surface channels.

Identification of individual tetra- and pentacyclic naphthenic acids in oil sands process water by comprehensive two-dimensional gas chromatography/mass spectrometry

The oils sands industry of Canada produces large volumes of process water (OSPW) which is stored in large lagoons. The OSPW contains complex mixtures of somewhat toxic, water-soluble, acid-extractable organic matter sometimes called 'naphthenic acids' (NA). Concerns have been raised over the possible environmental impacts of leakage of OSPW and a need has therefore arisen for better characterisation of the NA. Recently, we reported the first identification of numerous individual tricyclic NA in OSPW by comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GCxGC/ToF-MS) of the methyl esters. The acids were diamondoid adamantane acids, resulting, it was proposed, from biotransformation of the corresponding alkyladamantane hydrocarbons, which is a known process. Biotransformation of higher alkylated diamondoid hydrocarbons was, until now, unknown but here we describe the identification of numerous pentacyclic NA as diamantane and alkyldiamantane acids, using the same methods. Further, we suggest tentative structures for some of the tetracyclic acids formed, we propose, by ring-opening of alkyldiamantanes. We suggest that this is further evidence that some of the acid-extractable organic matter in the OSPW originates from extensive biodegradation of the oil, whether in-reservoir or environmental, although other oxidative routes (e.g. processing) may also be possible. The results may be important for helping to better focus reclamation and remediation strategies for NA and for facilitating the identification of the sources of NA in contaminated environmental samples.

Dispiro-1,2,4-trioxane analogues of a prototype dispiro-1,2,4-trioxolane: mechanistic comparators for artemisinin in the context of reaction pathways with iron(II)

Single electron reduction of the 1,2,4-trioxane heterocycle of artemisinin (1) forms primary and secondary carbon-centered radicals. The complex structure of 1 does not lend itself to a satisfactory dissection of the electronic and steric effects that influence the formation and subsequent reaction of these carbon-centered free radicals. To help demarcate these effects, we characterized the reactions of achiral dispiro-1,2,4-trioxolane 4 and dispiro-1,2,4-trioxanes 5-7 with ferrous bromide and 4-oxo-TEMPO. Our results suggest a small preference for attack of Fe(II) on the nonketal peroxide oxygen atom of 1. For 4, but not for 5 and 6, there was a strong preference for attack of Fe(II) on the less hindered peroxide bond oxygen atom. The steric hindrance afforded by a spiroadamantane in a five-membered trioxolane is evidently much greater than that for a corresponding six-membered trioxane. Unlike 1, 5-7 fragment by entropically favored beta-scission pathways forming relatively stable alpha-oxa carbon-centered radicals. These data suggest that formation of either primary or secondary carbon-centered radicals is a necessary but insufficient criterion for antimalarial activity of 1 and synthetic peroxides.