Clathration-induced asymmetric transformation of cefadroxil

[structure: see text] The cephalosporin antibiotic Cefadroxil can be epimerized at the alpha-carbon of its amino acid side chain using pyridoxal as the mediator. By clathration with 2,7-dihydroxynaphthalene, the desired diastereomer can be selectively withdrawn from the equilibrating mixture of epimers. In this way, an asymmetric transformation of Cefadroxil can be accomplished. This opens the possibility of the production of Cefadroxil starting from racemic p-hydroxyphenylglycine, in contrast to the current industrial synthesis that employs the D-amino acid in enantiopure form.     

Chromatographic study of the thermodynamics of solutions of hydrocarbons in liquid crystal solvents. Evidence for order disturbance by the solutes

Gas-chromatographic experiments were carried out in various phases of the solvents 4-acetoxy-N-[4-methoxy-benzylidene]-aniline, dibutoxyazoxybenzene, lithium stearate, dihexoxyazoxybenzene, and diheptoxyazoxybenzene. The solutes were linear, branched and cyclic alkanes, and substituted benzenes. Excess enthalpies, entropies, and free entropies were calculated from net retention volumes. In the nematic liquid crystalline phases the effect of order disturbance was significant in and but it was, by enthalpy-entropy compensation, not demonstrable in . Differences in flexibility and degree of expansion of the solutes did not result in significantly different values of the excess quantities.     

Sommelet-hauser rearrangement of an ammonium ylide derived from the HIV-1 reverse transcriptase inhibitor nevirapine

Functionalization at the 3-position of the dipyridodiazepinone nevirapine ( 1 ) has been accomplished by Sommelet-Hauser rearrangement of an ylide derived from 1 . Treatment of N-cyanomethylpyrrolidinium salt 4 with potassium tert-butoxide in a mixture of dimethylsulfoxide and tetrahydrofuran at ?10°, followed by acid hydrolysis, afforded a mixture of compounds 5 and 6 in a ratio of 1:1.8. Upon treatment of 4 with sodium amide in liquid ammonia, 5 and 6 were obtained in a ratio of 1.5:1 and a combined yield of 83%. Compound 5 is the desired product resulting from Sommelet-Hauser rearrangement of 4 , whereas 6 derives from competing Stevens rearrangement and intramolecular cyclization of the aldehyde produced upon hydrolysis. Baeyer-Villiger oxidation of 5 afforded the 3-hydroxy derivative 2 , a recently identified metabolite of nevirapine.     

Chemistry of strained polycyclic compounds—IX: The synthesis and homoketonization of 4-substituted homocuneane acetates

The synthesis of three bridgehead homocuneanes acetates viz 1-bromopentacyclo [4.3.0.0^2.4.0^3.8.0^5.7]nonan-9-one ethylene ketal 4-acetate (10), 1-bromopentacyclo[4.3.0.0^2.4.0^3.8.0^5.7]nonane 4-acctate (22) and pentacyclo[4.3.0.0^2.4.0^3.8.0^5.7]nonane-9-one ethylene ketal 4-acetate (31) is described, starting from the readely available homocubane carboxylic acid (4). The base and acid catalyzed homoketonization reaction of these acetates has been studied. Under basic conditions the acetates (10, 22 and 31) are converted into tettacyclo[4.3.0.0^2.4.0^3.8]nonane derivatives by a cyclopropanol ring cleavage. This homoketonization reaction is a stereospecific process proceeding with retention of configuration. The effect of cage strain on the stereochemistry of base induced homoketonization of bridgehead cage alcohols is discussed.The acid catalyzed homoketonization of acetate (10) was also found to occur exclusively with retention of configuration.     

Integrated optical microcavities for enhanced evanescent-wave spectroscopy

The use of integrated optical microcavities (MCs) for enhanced optical spectroscopy and sensing is investigated. The MC sustains high- Q whispering-gallery modes, in which the energy of the optical field can be efficiently stored. The resulting enhanced field can be used to probe fluorescent molecules in the cladding of the MC. Enhanced fluorescence excitation with an integrated optical MC is demonstrated experimentally for what is believed to be the first time. A comparison between a MC and a straight waveguide shows that the MC may give an increase of 40 times in fluorescence excitation. Because of the ultrasmall size of the MC (15 microm in radius), the fluorescence signal may be observed from only 20 molecules in the cladding.     

Facial selectivity in addition reactions to the highly strained enone in tricyclo[5.2.1.0]deca-2(6),8-dienone and tricyclo[5.2.1.0]dec-2(6)-enone

Tricyclo[5.2.1.0^2,6]deca-2(6),8-dien-3-one contains a highly strained central double bond due to geometrical constraints imposed by the tricyclic skeleton which does not allow optimal sp² hybridization at the C₂ and C₆ bridgehead positions. Michael addition of various nucleophiles (alkoxides, cyanide, and malonate) under protic conditions resulted in an exclusive exo-facial selectivity. This preference can be explained by steric and electronic factors. Michael additions using lithium dialkylcuprates resulted in predominant formation of endo-products, but also some exo-products were obtained. These exo-products arising from endo-approach may be the result of coordination of the cuprate with both the enone moiety and the olefinic C₈-C₉ bond. Michael additions to tricyclo[5.2.1.0^2,6]dec-2(6)-en-3-one, which lacks this C₈-C₉ double bond showed exclusive exo-facial selectivity to give exo-products. Besides these additions were all considerably slower than those to tricyclo[5.2.1.0^2,6]deca-2(6),8-dien-3-one proving significant electronic participation of the C₈-C₉ double bond in reactions with this substrate.