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Polypeptides
Published in Stanislaw Penczek, H. R. Kricheldorf, A. Le Borgne, N. Spassky, T. Uryu, P. Klosinski, Models of Biopolymers by Ring-Opening Polymerization, 2018
The original version of the Leuchs method, i.e., the cyclization of N-ethoxycarbonyl or N-methoxycarbonyl-amino acid chlorides (Equations 1 and 2) requires prolonged heating at temperatures in the range of 70 to 90°C.1-3 At these reaction temperatures amino acid NCAs may begin to decompose and hydrogen chloride causes cleavage of the NCA ring. Therefore, several improvements of the Leuchs method were reported. Originally, thionyl chloride was used for the preparation of the acid chloride function.1-3 This reagent has the advantage of gaseous by-products (Equation 4); yet it requires long reaction times at low temperatures or reaction temperatures >40°C. Phosphorus pentachloride is more reactive,4 but the phosphorus oxide chloride formed as a by-product (Equation 4) may affect the crystallization of NCAs. Dichloromethyl methyl ether (Equation 5) was recommended5 for the high purity of the resulting NCAs. However, this reagent was not commercially available in previous decades. The treatment of N-benzyloxycarbonyl-amino acid with phosgene yields at low temperature in the presence of triethylamine N-benzyloxycarbonyl-aziridine-2-ones which, upon catalytic hydrogenation, yields NCAs (Equations 7 and 8).7,8 This method is not recommendable for preparative purposes because the phosgenation of the free amino acids gives NCAs in a simpler and more direct way. The most useful halogenating reagent is phosphorus tribromide.9 The bromide ion is a better leaving group than chloride in the cyclization step (Equation 1), and it is a better nucleophile in the dealkylation step (Equation 2). Therefore, the bromination of N-alkoxycarbonyl-amino acids proceeds well even at temperatures below 25°C.
Reactions between lithiated 1,3-dithiane oxides and trialkylboranes
Published in Journal of Sulfur Chemistry, 2021
Basil A. Saleh, Keith Smith, Mark C. Elliott, Gamal A. El-Hiti
Reactions of trialkylboranes with various trisubstituted methanes such as chloroform (CHCl3), dichlorofluoromethane (CHCl2F), chlorodifluoromethane (CHClF2) and 1,1-dichloromethyl methyl ether (DCME) in the presence of a strong base result in the transfer of all three alkyl groups from boron-to-carbon in a single process [9,10]. Even trialkylboranes having a tertiary alkyl group, such as a tert-butyl or thexyl moiety, on reaction with DCME and lithium triethylcarboxide at 25°C, transfer all three groups successfully [9,10]. A reagent with three different leaving groups attached to a central carbon atom could, in principle, be used as an alternative to DCME, opening up possibilities for asymmetric induction to generate enantiomerically enriched chiral tertiary alcohols. Compounds having two sulfur-containing leaving groups have been used successfully to perform up to two 1,2-boron to carbon migrations [11–15], and in principle, a third leaving group could be incorporated to allow a third migration. A potential advantage of using sulfur-based leaving groups might be that stereoselectivity could be controlled, as it can, for example, in reactions of various electrophiles with metalated 1,3-dithiane oxides [16–21]. Such reagents might be able to offer possibilities for the generation of appropriately substituted chiral reagents for reactions with trialkylboranes. However, some basic studies are needed in order to underpin such possibilities.