The retention behavior of a wide variety of stationary phases was compared in supercritical fluid chromatography (SFC) and normal-phase high-performance liquid chromatography (NP-HPLC). We also attempted to elucidate the retention behavior in SFC by investigating the selectivity of the different stationary phases. SFC separation conditions with polar stationary phases, such as silica gel (SL) and diol (Diol) phases, operate via adsorptions that include hydrophilic and ionic interactions similar to those in NP-HPLC. Moreover, non-polar stationary phases, such as pentabromophenyl (PBr), pyrenylethyl (PYE), and octadecyl (C18), could be used despite the non-polar mobile phase conditions, because the dispersion and π-π interactions were stronger in SFC than in HPLC. These results reflect the selectivity of the stationary phase and its retention factor, thus providing useful information for the selection of appropriate stationary phases for particular analytes.
A Formal Total Synthesis of Virantmycin: A Modular Approach Towards Tetrahydroquinoline Natural Products. -A novel route to the precursor (X) of virantmycin includes the unprecedented Pd-catalyzed cross-coupling of an unsaturated ketone with the protected propargylic alcohol (I), efficient cyclization of the amino alcohol (V) to the oxazine (VII), and ring closure reaction of the dichloride (VIII). -(KECK, D.; VANDERHEIDEN, S.; BRAESE*, S.; Eur.
A stereoselective approach towards ethyl-substituted conjugated dienoic esters and dienones utilising a Suzuki cross-coupling reaction has been achieved. In addition, a method for their conversion into the corresponding ethyl ketones is presented.Conjugated dienoic esters and conjugated dienones 1 ( Figure 1) are useful building blocks for organic synthesis, being precursors for prominent structural features of a number of natural products, including carotenoids. 1 Figure 1 Alkyl-substituted dienoic esters (only E,E isomer shown for simplicity)The unsubstituted and methyl-substituted compounds (R 2 = H, Me) can be easily prepared by aldol reaction, though care has to be taken to establish the correct stereochemistry across the double bonds. Other possibilities include the utilisation of Wittig-type, 2 Horner-WadsworthEmmons-type, 3 or Julia-type 4 olefination strategies for the formation of the double bonds. However, all these reactions suffer from a sometimes insufficient degree of stereocontrol and low reaction yields in the case of bulkier side chains. To the best of our knowledge, Suzuki crosscoupling strategies involving vinyl triflates have not been applied in the synthesis of ethyl-substituted dienoic ester and dienone derivatives, though there are a number of examples of other cross-coupling reactions, namely Heck and Stille reactions, in the synthesis of retinal and analogues. 5 In this paper we present a novel stereoselective approach towards conjugated dienoic esters featuring a Suzuki cross-coupling reaction. 6 In addition, we present a synthesis of 5-ethyl-7-phenylhepta-4,6-dien-3-one (2) by cross-coupling of a ketone-derived vinyl triflate and a versatile synthetic strategy for the conversion of the corresponding dienoic ester into the ketone 2. The synthetic strategy for 2 is summarised in Scheme 1.Scheme 1 Retrosynthetic analysis of 5-ethyl-7-phenylhepta-4,6-dien-3-one (2) Thus, 2 can be formed by reaction of inexpensive commercially available (E)-2-phenylvinylboronic acid (3) and vinyl triflate precursor 4 or 5. The ester derivative 4 was first synthesised starting from 3-oxopentanoic acid methyl ester (6) following a literature precedence on a very similar system 7 using triethylamine and trifluoromethanesulfonic anhydride, which provided the product in a combined yield of 61% in a 5:7 ratio in favour of the undesired Z isomer (Z)-4 (Scheme 2). The isomers could be separated by column chromatography. When sodium hydride and N-phenylbis(trifluoromethanesulfonimide) 8 were utilised, the desired E isomer (E)-4 could be obtained exclusively in a yield of 71%.Unknown ketone derivative 5 was obtained from heptane-3,5-dione (7) using trifluoromethanesulfonic anhydride and triethylamine in a combined yield of 90% as a 1:1 mixture of both isomers. The isomers could also be separated by column chromatography, but the E isomer (E)-5 decomposed rapidly. R 1 R 2 O R 3 1 R 1 = aryl, alkyl R 2 = alkyl R 3 = alkyl, alkoxy O B OH OH TfO R O 2 3 (E)-4 R = OMe (E)-5 R = Et Scheme 2 Synthesis of vinyl triflates 4 a...
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