Edwin Vedejs

Structure-activity relationships among monoterpene inhibitors of protein isoprenylation and cell proliferation

The monoterpene d-limonene inhibits the post-translational isoprenylation of p21ras and other small G proteins, a mechanism that may contribute to its efficacy in the chemoprevention and therapy of chemically induced rodent cancers. In the present study, the relative abilities of 26 limonene-like monoterpenes to inhibit protein isoprenylation and cell proliferation were determined. Many monoterpenes were found to be more potent than limonene as inhibitors of small G protein isoprenylation and cell proliferation. The relative potency of limonene-derived monoterpenes was found to be: monohydroxyl = ester = aldehyde > thiol > acid = diol = epoxide > triol = unsubstituted. All monoterpenes that inhibited protein isoprenylation did so in a selective manner, such that 21-26 kDa proteins were preferentially affected. Perillyl alcohol, one of the most potent terpenes, reduced 21-26 kDa protein isoprenylation to 50% of the control level at a concentration of 1 mM, but had no effect on the isoprenylation of 67, 47 or 17 kDa proteins. In particular, p21ras farnesylation was inhibited 40% by 1 mM perillyl alcohol. At the same concentration, perillyl alcohol completely inhibited the proliferation of human HT-29 colon carcinoma cells. The structure-activity relationships observed among the monoterpene isoprenylation inhibitors support a role for small G proteins in cell proliferation, and suggest that many limonene-derived monoterpenes warrant further investigation as antitumor agents.

Cationic tricoordinate boron intermediates: borenium chemistry from the organic perspective.

The focus of this review is on the reactivity of cationic, tricoordinated boron electrophiles, and on recent applications that rely on intermediates having sextet boron and net positive charge. Older reviews have appeared that address early studies involving a variety of boron cations,1 or that emphasize the structural, bonding, and theoretical aspects of di-, tri-, and tetracoordinate boron cations (borinium, borenium, and boronium, respectively; Fig. 1).2 The latter review by Kolle and Noth is a milestone in the field of boron cation chemistry. It systematizes a number of earlier observations, and defines the currently accepted nomenclature for boron-containing cations. A more recent (2005) review by Piers et al. updates the structural and bonding aspects in depth,3 discusses reactions of boron cations in the gas phase as well as in solution, and explores several other topics of special interest to the organoelement community. We will take a somewhat different approach to follow developments from the organic chemistry perspective, including selected historical aspects of reactivity, some of which have not appeared in prior reviews. Several topics will necessarily overlap with the excellent overview by Piers et al., but detailed coverage will be limited to the solution chemistry of tricoordinate boron cations, the reactive intermediates known as borenium ions according to Noth’s classification.2 This terminology depends on the number of ligands at boron and avoids distinctions based on bond order or resonance contributions. Noth’s treatment also recognizes the iso-electronic relationship between borenium and carbenium ions, both of which feature a formally vacant p-orbital at the central atom (boron and carbon, respectively) and the same overall positive charge. Consistent with the carbenium analogy, borenium ions have long been suspected as intermediates in classical nucleophilic substitution chemistry involving B–X bonds in tetracoordinate boron structures, although we will find that these suspicions were often unfounded. On the other hand, recent studies have revealed fascinating new roles for borenium intermediates, ranging from enantioselective catalysis and memory of chirality applications to hints of C–F activation and C–H insertion chemistry. It would be fair to say that these recent developments were slow to unfold, given the long history of borenium chemistry. Fig. 1 Boron cation nomenclature and structural representations in current use. 2. History of borenium ions: often considered, seldom confirmed 2.1 Suspected intermediates in B-N protonation The first reports to our knowledge where cationic sextet boron was implicated are the two 1933 papers by Wiberg and Schuster that describe the reaction of dichloro(dimethylamino)borane 1 with hydrogen chloride.4 The experiment was said to produce the hydrochloride (chlorhydrat) of 1, drawn using the notation shown as 2a (eq.1). This representation does not specify connectivity, and nothing in the 1933 papers indicates whether 2a is the same or different compared to the borenium salt 2b. Whether the authors intended to distinguish 2a from representations such as 3a or 3b is also uncertain. On the other hand, a 1947 paper by Wiberg and Hertwig shows a similar reaction (R2BNHR + HCl, eq. 2) and provides two generalized drawings of a single ionic structure (5a and 5b) that is clearly identified as the unstable intermediate leading to an isolable covalent adduct, drawn by Wiberg and Hertwig in two unambiguous representations 6a and 6b.5 These drawings and appended comments leave no question about the structure of 5 and the greater stability of 6. However, they may have escaped the notice of subsequent authors, some of whom accepted the borenium structure 2b as a more stable alternative compared to 3b and attributed this conjecture to Wiberg.

Identification of metabolites of the antitumor agentd-limonene capable of inhibiting protein isoprenylation and cell growth

SummaryLimonene has been shown to be an effective, nontoxic chemopreventive and chemotherapeutic agent in chemically induced rat mammary-cancer models. The present study characterized circulating metabolites of limonene in female rats and determined their effects on cell growth. Metabolism of limonene was analyzed in plasma extracts by gas chromatography. Rapid conversion of limonene to two major metabolites was detected. These metabolites comprised more than 80% of the circulating limonene-derived material at 1 h after administration and thereafter, whereas limonene itself accounted for only 15%. The metabolites were characterized by mass spectroscopy and infrared spectroscopy. The probable structures were synthesized, and identities were confirmed by comparison of retention times and mass spectra. The two major circulating metabolites of limonene were found to be perillic acid and dihydroperillic acid. We have previously reported that limonene, perillic acid, and dihydroperillic acid inhibit the posttranslational isoprenylation of p21ras and other 21- to 26-kDa cell-growth-associated proteins in NIH3T3 cells and in mammary epithelial cells. In the present study, perillic acid was found to inhibit cell growth in a dose-dependent manner. Thus, perillic acid and dihydroperillic acid, the two major circulating metabolites of limonene in the rat, are more potent inhibitors of protein isoprenylation than is limonene, and perillic acid is also a more potent inhibitor of cell growth. These data raise the possibility that the antitumor effects of limonene in vivo may be mediated via perillic acid and, perhaps, other metabolites.

AcOLeDMAP and BnOLeDMAP: conformationally restricted nucleophilic catalysts for enantioselective rearrangement of indolyl acetates and carbonates.

The rate of indolyl O- to C-acetyl or carboxyl rearrangement is accelerated by the electron-withdrawing N-diphenylacetyl group (DPA) using the conformationally restricted chiral catalysts AcOLeDMAP (12b) and BnOLeDMAP (13b). Highly enantioselective conversion to quaternary C-acetylated and C-carboxylated oxindoles is observed, even for substrates containing branched substituents. The rearrangement of the carboxylate substrates 19 occurs with complementary enantiofacial selectivity using catalyst 13b compared to the acetyl migrations of 16 catalyzed by 12b. Access to N-unsubstituted oxindoles is demonstrated by DPA cleavage with Et(2)NH.

A Boronium Ion with Exceptional Electrophilicity

During our studies on aromatic borylation,[1] we considered the combination of a highly electrophilic R2BNTf2 reagent with a base that would neutralize the HNTf2 byproduct of borylation without deactivating the electrophile. In principle, these requirements might be satisfied by 1,8-bis(dimethylamino)naphthalene (1), a hindered and exceptionally basic aniline that finds numerous applications as a basic catalyst or reagent due to its legendary lack of nucleophilicity.[2, 3] Strong electrophiles interact weakly, if at all, with the amine nitrogens, and very few examples are known where stable bonds to nitrogen can be formed between 1 and electrophilic groups larger than hydrogen.[2, 4-7] Among these exceptional cases, cyclic boronium structures 2 and 3 are relatively stable because the subunits BH2 and BF2 have minimal steric requirements.[5] However, the more hindered BMe2 derivative 4 has not been detected and no analogous BR2 structures are known.[5a, 8] In view of this long history, we were somewhat surprised to find that an adduct is readily formed simply upon mixing 1 with the 9-BBN bistriflimide reagent 5a despite the transannular steric demands of the 9-BBN core and the need to form adjacent quaternary bonds to boron as well as nitrogen.[9, 10] The remarkable structural features and unusual reactivity of this adduct are the subject of the following communication.

Superelectrophilic Intermediates in Nitrogen-Directed Aromatic Borylation

The first examples of borylation under conditions of borenium ion generation from hydrogen-bridged boron cations are described. The observable H-bridged cations are generated by hydride abstraction from N,N-dimethylamine boranes Ar(CH(2))(n)NMe(2)BH(3) using Ph(3)C(+) (C(6)F(5))(4)B(-) (TrTPFPB) as the hydride acceptor. In the presence of excess TrTPFPB, the hydrogen-bridged cations undergo internal borylation to afford cyclic amine borane derivatives with n = 1-3. The products are formed as the corresponding cyclic borenium ions according to reductive quenching experiments and (11)B and (1)H NMR spectroscopy in the case with Ar = C(6)H(5) and n = 1. The same cyclic borenium cation is also formed from the substrate with Ar = o-C(6)H(4)SiMe(3) via desilylation, but the analogous system with Ar = o-C(6)H(4)CMe(3) affords a unique cyclization product that retains the tert-butyl substituent. An ortho-deuterated substrate undergoes cyclization with a product-determining isotope effect of k(H)/k(D) 2.8. Potential cationic intermediates have been evaluated using B3LYP/6-31G* methods. The computations indicate that internal borylation from 14a occurs via a C-H insertion transition state that is accessible from either the borenium pi complex or from a Wheland intermediate having nearly identical energy. The Ar = o-C(6)H(4)SiMe(3) example strongly favors formation of the Wheland intermediate, and desilylation occurs via internal SiMe(3) migration from carbon to one of the hydrides attached to boron.

Borenium ion catalyzed hydroboration of alkenes with N-heterocyclic carbene-boranes

Treatment of alkenes such as 3-hexene, 3-octene, and 1-cyclohexyl-1-butene with the N-heterocyclic carbene (NHC)-derived borane 2 and catalytic HNTf(2) (Tf = trifluoromethanesulfonyl (CF(3)SO(2))) effects hydroboration at room temperature. With 3-hexene, surprisingly facile migration of the boron atom from C(3) of the hexyl group to C(2) was observed over a time scale of minutes to hours. Oxidative workup gave a mixture of alcohols containing 2-hexanol as the major product. A similar preference for the C(2) alcohol was observed after oxidative workup of the 3-octene and 1-cyclohexyl-1-butene hydroborations. NHC-borenium cations (or functional equivalents) are postulated as the species that accomplish the hydroborations, and the C(2) selective migrations are attributed to the four-center interconversion of borenium cations with cationic NHC-borane-olefin π-complexes.

N-Directed Aliphatic C–H Borylation Using Borenium Cation Equivalents

Highly electrophilic boron cations derived from hindered amine borane complexes have been shown to undergo intramolecular aliphatic C-H borylation.

Amine-Directed Hydroboration: Scope and Limitations

Iodine activation induces intramolecular hydroboration of homoallylic and bis-homoallylic amine boranes with good to excellent control of regiochemistry compared to control experiments using excess THF•BH3. Deuterium labeling and other evidence confirm that the iodine-induced hydroboration reaction of homoallylic amine boranes occurs via an intramolecular mechanism equivalent to the classical 4-center process and without competing retro-hydroboration. Longer carbon chain tethers result in lower regioselectivity, whereas the shorter tether in allylic amines results in a switch to dominant intermolecular hydroboration. Regioselectivity in THF•BH3 control experiments is higher for the allylic amine boranes compared to the iodine activation experiments, whereas the reverse is true for homoallylic amine borane activation.

Synthesis of N-methoxy and N-H aziridines from alkenes

Electron-rich alkenes are converted into N-methoxyaziridines by treatment with HN(OCH3)2 and trimethylsilyl triflate. Reduction with Li/ammonia affords the N-H aziridines.