Almost seventy years ago, chemist Dorothy Jordan noted that "gels are easier to recognize than to define"."] We will attempt a definition nonetheless : Gels are semi-rigid colloidal systems rich in liquid. Protoplasm ranks as the most widespread example of this peculiar state of matter. A gel is formed by a molecular network, often fibrous in nature, that entrains the dispersing medium within its capillary spaces. Most organic gelators are polymeric (for example, gelatin, ['] poIy(acry1ic acid) ,I3] and hyaluronic acidr4]), but small molecules (for example. cholesteryl 4-(2-anthryloxy)b~tanoate, [~~ 5-hexadecyl-2,4,6-triaminopyrimidine,l6] N-octylglucona~nide,~'~ and arborolsr8]) can induce gelation as well.We have long been interested in a little-known but remarkable gelator. dibenzoyl-L-cystine (DBC).19] Although most gels require 1 -1 0 O i O gelator, DBC in water rigidifies at concentrationsas low as 2 mM (0.1 YO). While attempting to learn how such miniscule quantities of DBC manage to self-assemble into a macroscopic network, we performed an X-ray analysis of a gel "fiber". Structural data at the atomic level is virtually unavailable for gel networks, and it is only through good fortune that these data became accessible. But before giving the details, let us summarize the properties of acylcystines in water.DBC forms a firm, translucent gel in a thermally reversible process at a concentration of 4 mM in ethanol (5%)/water. Ethanol was used because the DBC tends to crystallize from pure water. There is, clearly. a delicate balance between gelation and crystallization. The gelating properties are destroyed when the S-S linkage is replaced by CH,CH, or CH=CH groups."'] This implies that the CH,-S-S-CH, dihedral angle["] of about 90 'C plays a key role in the formation of the molecular network. Gels d o not form from esterified DBC or at basic p H values, which suggests the presence of either a favorable -COOH/-COOH attraction or a deleterious -COY /-CO; repulsion. Addition of NaCl (0.1 M) softens the gels, whereas more than 10 YO dioxane induces crystallization.Diacetyl-L-cystine fails to gelate in ethanol ( 5 %)/water. Di-noctanoyl-L-cystine (4 mM) forms only a soft and cloudy gel. The superior quality of the DBC gel leads us to suspect that aromatic K -~T stacking stabilizes the creation of fibers. This conclusion is appealing. because intermolecular hydrogen bonding between DBC molecules in water would seem too unfavorable to serve as the sole adhesive force. Assembly of DBC molecules appears to be a cooperative event much like the micellization of surfactants. This was affirmed by a primitive yet informative experiment in which 2.0 x 2.0 cm2 squares of Parafilm were placed on the surface of aged DBC gels. Known weights were then positioned carefully at the center of a film until the film submerged. A plot of "supportable weight" vs. DBC concentration (not shown) gave a sharp break at 1.6 KIM, a value that might be qualitatively regarded as the "critical gelation concentration".Spin-lattice relaxation stud...
The three-step manufacturing process used in the synthesis of tenofovir disoproxil fumarate (1) was studied and optimized, leading to a more productive and robust process. The yield was improved from about 13% overall to 24%. Key process improvements identified included implementation of a telescoped process for the second stage that obviated the need for an extraction and solvent exchange, and significant optimization of the final reaction, including the beneficial effect of adding a quaternary ammonium salt to the alkylation reaction and development of a nonaqueous process for removal of NMP and triethylamine from the product mixture to decrease the level of decomposition of product during the isolation.
[structure: see text] The unusual [5.3.2]-bicyclic structure of the insecticidal Amaryllidaceae alkaloids cripowellin A (1) and B (2) has been synthesized for the first time via a sequence of Sharpless dihydroxylation, ring-closing metathesis, and intramolecular Heck reaction. The asymmetric synthesis of the 1-epi-aglycon 82 proceeds with virtually complete diastereo- and enantioselectivity (de, ee > or = 98%) in 13 steps and an overall yield of 5.6%. In addition, three alternative approaches toward the aglycon 3 are also described focusing on (1) the alkylation of the 2-benzazepinedithianes 35 and 36 with the electrophile 11, (2) a radical cyclization of the precursor (R/S,S,S)-39, and (3) an intramolecular arylation reaction of the aryl ketone 47.
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