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C A Dvorak

Publications and source records attributed to C A Dvorak.

8 recordsLinked to original sources
PubMed2001

An asymmetric approach to spirocylic systems: a formal synthesis of zizaene.

A general route to enantiopure spirocarbocycles is described. The use of various chiral bicyclic lactams 1 that have been doubly alkylated with olefinic halides gives good yields of alpha,alpha-disubstituted chiral lactams 2 which were cyclized to spiro-olefins using ring closure metathesis methodology (Grubbs' catalyst). These spirolactams 3, formed in generally excellent yields, were shown to be smoothly transformed into spirocyclopentenone 6, spirocyclohexenone, 7, and spirolactams 8. Further demonstration of this spirocyclization methodology was featured in a formal synthesis of zizaene, by preparing in enantiomeric form the Coates' intermediate 21. This synthetic effort provided additional examples of the synthetic versatility of chiral bicyclic lactams 2a,b.

Journal Article↗
PubMed1990

Wavelength-dependent temporal properties of retinal horizontal cells in turtles.

Electrical responses of luminosity horizontal cells (L cells) to monochromatic stimuli were analyzed by intracellular recordings in the retinas of the freshwater turtle (Pseudemys scripta elegans) and of the sea turtle (Chelonia mydas mydas). Light intensity, duration, and wavelength were varied to assess temporal effects. For a given intensity of monochromatic light, response amplitude increased with stimulus duration until maximum amplitude occurred at a specific duration. This suprathreshold metric of temporal integration is called here summation time, and it is wavelength-dependent. L cells always had some level of red-sensitive cone input, although in some cells inputs from green- and blue-sensitive cones were also observed. For these latter cells, summation times were shorter for 640-nm than for 540-nm or 450-nm lights. These results were most evident in cells that received dominant inputs from blue- or green-sensitive cones. Responses of some other L cells were almost completely dominated by inputs from red-sensitive cones. Summation times of these cells were not wavelength-dependent. But when these inputs also included green-sensitive cones, shorter summation times were obtained to 640-nm light than to 540-nm light, even though dominant inputs were still from red-sensitive cones. These results, obtained from both retinal and 3,4-dehydroretinal photopigment systems, are consistent with reported observations in Pseudemys scripta elegans that show linear responses of red-sensitive cones to have shorter integration times and times-to-peak than green-sensitive cones. Responses from horizontal cells dominated by blue-sensitive cone inputs were the most sensitive of all; they also had the longest summation times.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1983

9-[(1,3-Dihydroxy-2-propoxy)methyl]guanine: a new potent and selective antiherpes agent.

The synthesis of a new acyclic analogue of deoxyguanosine, 9-[(1,3-dihydroxy-2-propoxy)methyl]guanine (DHPG, 1), is described starting from epichlorohydrin via condensation of 2-O-(acetoxymethyl)-1,3-di-O-benzylglycerol (5) with N2,9-diacetylguanine (6). In vitro studies indicate that DHPG is a potent and broad-acting (herpes simplex virus types 1 and 2, cytomegalovirus, and Epstein-Barr virus) antiherpetic agent. In vivo studies indicate its lack of toxicity [LD50 (mice) = 1-2 g/kg, ip] and its superiority over acyclovir [oral ED50 = 7 (mg/kg)/day vs. 550 (mg/kg)/day in HSV-2 infected mice].

Acyclovir↗
PubMed1982

Quantitative electron microscopic analysis of the optic nerve of the turtle, Pseudemys.

It is estimated by means of electron microscopy that the optic nerve of the turtle Pseudemys scripta elegans contains 394,900 fibers of which approximately 80% are myelinated. The total fiber count agrees well with counts obtained from electron microscopic studies on other turtle species. There are, however, differences among these species in the percentage of myelinated fibers in the optic nerve. The axon diameter distribution of the myelinated fibers (excluding myelin) has a mode at 0.87 micrometer while that of the unmyelinated fibers has a mode at 0.42 micrometer. Both distributions are unimodal and are of a similar form in all areas of the nerve sampled. The total fiber count reported here agrees well with previous reports of ganglion cell counts in Pseudemys and the characteristics of the fiber distributions are comparable to those reported for nonreptilian vertebrates.

Animals↗
PubMed1980

Photoreceptor signals at visual threshold.

Electrical responses of cone photoreceptors in the retina of the freshwater turtle have been characterised for flashes and steps of light in darkness and in the presence of background light. These intracellular measurements have been combined with the behavioural increment threshold curve to yield an estimate of 5-10 muV for the signal developed in a cone when the turtle can just detect an increment flash. The signal developed when the cones under the stimulus image are dark-adapted is of interest, for its measurement would help to explain how known physiological processes subserve visual detection for a variety of photic conditions. The effective quantal absorption of dark-adapted, red-sensitive cones of Pseudemys scripta elegans for a stimulus that the turtle can just detect is reported here. By combining this result with previous electrical measurements on red-sensitive cones of this species, an estimate of 35-70 muV is obtained for the signal developed in a dark-adapted cone at behavioural threshold. This larger signal required for detection of a flash in darkness is of particular interest in view of the recent observation that the intrinsic noise of turtle cones in darkness is larger than that of illuminated cones.

Animals↗
PubMed1975

Methyl 5(6)-phenylsulfinyl-2-benzimidazolecarbamate, a new, potent anthelmintic.

1. Human hepatic "acid" beta-galactosidase preparations, which had been purified approximately 250-fold, were examined for activities toward 4-methylumbelliferyl beta-galactosylceramide, lactosylceramide, galactosyl-N-acetylgalactosaminyl-[N-acetylneuraminyl]-galactosyl-glucosylceramide(GM1-ganglioside) and galactosyl-N-acetylgalactosaminyl-galactosyl-glucosylceramide (asialo GM1-ganglioside). 2. The enzyme was active toward the synthetic substrate, GM1-ganglioside and asialo GM1-ganglioside but was inactive toward galactosylceramide. Under our assay conditions, optimized for lactosylceramidase II, the preparations were as active toward lactosylceramide as toward GM1-ganglioside or its asialo derivative. The apparent Km values for the three natural substrates were similar. When determined by the assay system of Wehger, D.A., Sattler, M., Clark, C. and McKelvey, H. (1974) Clin. Chim Acta 56, 199-206, lactosylceramide-cleaving activity was 0.2% of that determined by our assay system. This confirmed our previous suggestion that the Wenger assay system determines exclusively the activity of lactosylceramidase I, which is probably identical with galactosylceramide beta-galactosidase. 3. Crude sodium taurocholate was far more effective than pure taurocholate in stimulating hydrolysis of the three glycosphingolipids by the beta-galactosidase. However, crude taurocholate could largely be replaced by smaller amounts of sodium taurodeoxycholate, suggesting that the unique activating capacity of the crude taurocholate might be due to taurodeoxycholate present as the major impurity. 4. Cl- was generally stimulatory for hydrolysis of the natural glycosphingolipids by our enzyme preparation. Effects of additional oleic acid and Triton X-100 were generally minor in either direction. 5. When the enzyme preparation was diluted with water, activity toward the synthetic substrate declined rapidly while those toward the natural substrates were essentially stable. Activity toward the synthetic substrate remained much more stable when the enzyme was diluted with 0.1 M sodium citrate/phosphate buffer, pH 5.0. 6. These observations provide insight into the complex relationship among the human hepatic beta-galactosidases.

Animals↗