Toxicity and metabolism of the chloral-derived mammalian alkaloid 1-trichloromethyl-1,2,3,4-tetrahydro-beta-carboline (TaClo) in PC12 cells.

Chloral-derived beta-carbolines, which are structurally similar to the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP, 5), are discussed to contribute to neuronal cell death in idiopathic Parkinson's disease. The cytotoxicity of 1-trichloromethyl-1,2,3,4-tetrahydro-beta-carboline (TaClo, 4) to neuronal-like clonal pheochromocytoma PC12 cells was examined by the determination of lactate dehydrogenase (LDH) release. After incubation for 48 h, 4 showed a strong dose-dependent cytotoxic activity towards PC12 cells with an ED50 value of 230 microM. In PC12 cells reductive dehalogenation of 4 was observed giving rise to the formation of 1-dichloromethyl-1,2,3,4-tetrahydro-beta-carboline (6) as a main TaClo metabolite exhibiting a cytotoxic potential comparable to that of TaClo. An X-ray structure analysis, performed for the trifluoroacetyl derivative of 6, revealed the N-substituent of such a highly chlorinated agent to be dramatically pushed out of the beta-carboline ring 'plane' due to the high steric demand of the huge dichloromethyl group at C(1).

The effects of novel amphipathic block copolymers on stabilization of the rat tear film.

PURPOSE: To determine whether various novel amphipathic polymers could be used to stabilize the tear film of the rat. The rheologic properties of these polymers were examined to investigate whether particular structural or physical characteristics improve the stability of the tear film. METHODS: Amphipathic polymers or particular phospholipids were mixed with a test solution of tears and saline and applied to the clean, dry corneal surface of a rat. The specular reflection of the tear film was observed at high magnification and recorded. For each of the polymers or lipids, the effects on surface regularity and tear break-up time were compared. After the experiments, histologic sections of the tested eyes were prepared and examined for acute cytotoxic effects on the cornea and ocular conjunctiva. RESULTS: Tear film break-up time was markedly affected by differences in polymer structure. Copolymers consisting of separate hydrophobic and hydrophilic regions appeared to be the best stabilizers. No acute cytotoxic effects were observed in histologic sections of corneas to which the polymers had been applied. CONCLUSIONS: Amphipathic polymers can be designed to increase tear film stability. Increased tear film stability occurred more readily with copolymers, possibly through their interaction with both lipid and aqueous tear components.

Short and stereoselective synthesis of C-glycosylated glycine derivatives from glycals by radical addition and reduction.

Only three steps are required for the convenient synthesis of 2-C-branched glyco-amino acids from glycals with good yields and stereoselectivities.

Titanium-Catalyzed Diastereoselective Epoxidations of Ene Diols and Allylic Alcohols with beta-Hydroperoxy Alcohols as Novel Oxygen Donors.

beta-Hydroperoxy alcohols 1-4 serve as effective tridentate oxygen donors for the highly diastereoselective, titanium-catalyzed epoxidation of ene diols 5a-e. Thus, in contrast to the bidentate tert-butyl hydroperoxide, the usual oxygen donor employed in Sharpless-type epoxidations and known to work poorly for polyhydroxy substrates, the tridentate beta-hydroperoxy alcohols efficiently replace the tridentate epoxy diol products 6a-e in the titanium template and thereby the catalytic cycle is sustained by replenishing with efficacy the loadedcomplex necessary for the oxygen transfer. Irrespective of the substitution pattern of the double bond or the configuration (erythro versus threo) of the diol functionalities in the ene diol substrate, high diastereoselectivities are observed for the epoxy diol products. The high stereochemical control is due to the rigid transition state for the oxygen transfer, which is imposed by the multiple titanium-oxygen bonding and coordination in the titanium template. The observed erythro selectivity for the ene diol derives from the additional bonding of its homoallylic hydroxy group to the titanium center, which fixes the substrate conformation in such a way that the oxygen atom to be transferred approaches from the side of the allylic oxygen functionality (cf. loaded complex A). This additional binding of the bidentate ene diol in the titanium template is also manifested in the enhanced reactivity of the ene diol versus the monodentate allylic alcohols. Nevertheless, the less reactive allylic alcohols also display a high erythro selectivity, provided these monodentate substrates possess 1,2-allylic strain. For the first time a direct, diastereoselective, and catalytic epoxidation of ene diols has been made available for synthetic applications, without recourse to protection group methodology.

Dipolar Cycloaddition Reactions of Dihydropyrimidine-Fused Mesomeric Betaines. An Approach toward Conformationally Restricted Dihydropyrimidine Derivatives(1).

The bimolecular and intramolecular cycloaddition potential of various 4-aryldihydropyrimidine-fused mesomeric betaines was investigated. Dihydropyrimidine-fused isothiomünchnones and isomünchnones were found to undergo 1,3-dipolar cycloaddition reactions with electron-deficient dipolarophiles such as DMAD, methyl propiolate, methyl vinyl ketone, or N-methylmaleimide. In contrast, cross-conjugated mesomeric thiazinium betaines underwent 1,4-dipolar cycloaddition reaction with electron-rich dipolarophiles such as ynamines or ketene acetals. In general, these cycloadditions show a high degree of regioselectivity, facial selectivity, and exo/endo diastereoselectivity. Intramolecular variations of the above processes involving o-alkenylaryl-tethered dihydropyrimidine-fused isomünchnones lead to polycyclic dihydropyrimidine analogs that closely mimic the proposed receptor-bound conformation of dihydropyridine calcium channel modulators. These cycloadducts are the result of an endo-addition of the pi-bond to the carbonyl ylide dipole embedded in the isomünchnone system. The relative stereochemistry of these cycloadducts was established by single-crystal X-ray analysis.

Addition of Bromine to Ketenes and Bisketenes: Electrophilic Attack at Carbonyl Carbon and Neighboring Group Participation.

Addition of bromine to bisketene (Me(3)SiC=C=O)(2) (1) gave the fumaryl dibromide E-7, whose stereochemistry was proven by X-ray structure determination. Upon warming, E-7 rearranged to the furanone 8, and this process was faster in the more polar CD(3)CN compared to CDCl(3), consistent with an ionization pathway for the rearrangement. The bromination of 1 in CH(2)ClCH(2)Cl followed second-order kinetics with a rate constant (2.1 +/- 0.1) x 10(4) M(-)(1) s(-)(1) at 25 degrees C. The first-order dependence of bromine addition to 1 on [Br(2)] is attributed to intramolecular nucleophilic assistance by the second ketenyl moiety in an initial complex of 1 and Br(2) to give E-7. A transition structure for this process has been calculated by ab initio methods. By contrast PhMe(2)SiCH=C=O 11 and gamma-oxoketene 16 underwent bromination by third-order kinetics, second order in [Br(2)], indicating the absence of neighboring group participation in the rate-limiting step. The bisketene Me(2)Si(CH=C=O)(2) (13) underwent bromination by mixed kinetics with both first- and second-order terms in [Br(2)].

The role of different phospholipids on tear break-up time using a model eye.

PURPOSE: The effect of different phospholipids in stabilising the tear film was investigated to determine if particular polar head groups gave greater stability than others. METHODS: Purified phosphatidylcholine (PC), lysophosphatidylcholine (LPC), phosphatidylethanolamine (PE), phosphatidylglycerol (PG), phosphatidylserine (PS) and cardiolipin (CL) were used. These were applied to a model eye loaded with an artificial tear fluid and the tear break-up time (TBUT) was measured. Three variants of the artificial tear fluid were utilised: buffered saline alone; one with proteins and mucins; and one containing proteins, mucins and lipids. RESULTS: TBUT was improved by the presence of phospholipids. In particular, the best performance was with PI applied to artificial tear fluid containing proteins mucins and lipids. Use of buffered saline as the artificial tear fluid gave very short break-up times. CONCLUSION: Increase in tear film stability by phospholipids is probably not due to the charge carried by the polar head group, but more likely due to the charge distribution, and the presence of hydroxyl groups in the head group also tends to increase stability, possibly through specific interactions with proteins and mucins in the subphase.

1-Trichloromethyl-1,2,3,4-tetrahydro-beta-carboline (TaClo) and related derivatives: chemistry and biochemical effects on catecholamine biosynthesis.

1-Trichloromethyl-1,2,3,4-tetrahydro-beta-carboline (TaClo, 2) is a mammalian alkaloid that readily originates in the human organism, by Pictet-Spengler condensation of endogenously present tryptamine (Ta) and the non-natural hypnotic agent trichloroacetaldehyde (chloral, Clo). Due to its structural analogy to the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP, 1), TaClo is discussed to possibly contribute to the pathogenesis of Parkinson's disease acting as an environmental toxin. Previous investigations on rats and neuronal cell cultures revealed 2 to be capable of inducing severe disturbances on the dopamine metabolism. In this paper, we report on the effects of 2 on the activity of tyrosine hydroxylase [L-tyrosine, tetrayhydropteridine/oxygen oxidoreductase (3-hydroxylating), EC 1.14,16.2; TH] in vitro using rat brain homogenates prepared from the TH-rich nucleus accumbens. TaClo (2) dose-dependently inhibited basal TH activity (IC(50)=3 microM), and after enzyme activation by pituitary adenylate cyclase-activating polypeptide (PACAP-27), it also reduced L-DOPA formation (IC(50)=15 microM). Moreover, two presumable TaClo metabolites, 2-methyl-TaClo (N-Me-TaClo, 3) and 1-dichloromethylene-1,2,3,4-tetrahydro-beta-carboline (1-CCl(2)-TH beta C, 4), which were synthesized in good yields, also proved to be potent inhibitors of TH, with the strongest effect on basal activity (similar to TaClo) being observed for 3 (IC(50)=3 microM). In contrast to TaClo, however, 3 and 4 showed biphasic effects after TH activation with PACAP-27, inducing a marked increase of enzyme activity in the nanomolar range (<0.1 microM), while TH activity was nearly completely blocked at high concentrations (IC(100)=0.1mM). An X-ray diffraction investigation on the 3-dimensional structure of the 1-CCl(2)-TH beta C-derived trifluoroacetamide 7 revealed the voluminous and quite rigid dichloromethylene substituent to be only moderately twisted out of the beta-carboline ring 'plane', thus resulting in an increased ring strain of the partially hydrogenated pyrido moiety accompanied by a strong steric hindrance of Cl(1), Cl(2), C(13), and N(2), which pushes the N-trifluoroacetyl group upwards to an even higher extent than for the TaClo-related trifluoroacetamide 8.

5'-O-demethyldioncophylline A, a new antimalarial alkaloid from triphyophyllum peltatum.

From the root bark of Triphyophyllum peltatum, the new naphthylisoquinoline alkaloid 5'-O-demethyldioncophylline A has been isolated. Its otherwise difficult separation from the largely dominating main alkaloid, dioncophylline A, was greatly facilitated by a bromination-benzylation procedure. From the resulting derivative, a crystal structure analysis with an anomalous X-ray diffraction was achieved, allowing confirmation of the full absolute stereostructure of the new alkaloid. The structure was further corroborated by a partial synthesis from dioncopeltine A. The natural product was shown to exhibit antimalarial activity against erythrocytic forms of Plasmodium falciparum.

Atropo-enantioselective total synthesis of knipholone and related antiplasmodial phenylanthraquinones.

The "lactone concept" has been efficiently employed for the first atropo-enantioselective synthesis of knipholone and related natural phenylanthraquinones. Besides the regio- and stereoselective construction of the biaryl axis, another important step was the "synthetically late" introduction of the C-acetyl group, either by a Friedel-Crafts type acetylation or by an ortho-selective Fries rearrangement first tested on simplified model systems and subsequently applied to the highly atroposelective preparation of the natural products and of simplified analogs thereof for biotesting. The synthetic availability of these natural biaryls, their precursors, and their unnatural analogs permitted a broader investigation of the antiplasmodial activities of these interesting biaryls.