Stereochemistry acetamidation

The second method involves the direct reaction ofbromo- or lodofluoroolefinx with zinc powder [110] (equation 81) in solvents such as DMF, NJV-dimethyl-acetamide (DMAC), tnglyme, tetraglyme, THF, and CH3CN. In all cases, the stereochemistry is retained Table 4 shows some typical examples of this method ology Similar results are obtained with l-chloro-2 lodotetrafluorocyclobutene [III] and l-iodo-2-chlorohexafluorocyclopentene [112]... 

Based on their unique stereochemistry in which two nitrogens and two oxygens are bound to each rhodium in a ris-2,2 fashion, dirhodium(II) car-boxamidates, exemplified by dirhodium(II) acetamidate [Rh2(acam)4],... 

Dorzolamide contains two chiral centers, and is therefore capable of existing in four diastereomers. The stereochemistry at the C-6 position of the starting material is preserved during the various chemical reactions which take place during the synthesis. The stereochemistry at the C-4 position (absolute configuration being 5) results from the Ritter substitution reaction (Scheme 1, Steps I-II) used to transform the alcohol to an acetamide. The Ritter reaction yields mostly the rra j-diastereomer, and the c/s-diastereomers are easily separated as their maleate salts. The potential sulfonamide positional isomer (3-sulfonamide) has not been observed at levels greater than 0.1% in HPLC analyses. 

Unlike the other N5 modifications described previously, a 5-azido thioglycoside was found to be less reactive than the simple amide as is clear from the measured relative reactivity values (RRVs) (Fig. 5.4) [59], However, it was subsequently reported by Teruaki Mukaiyama and coworkers that a 5-azido sialyl fluoride was more reactive than both the corresponding acetamide and the V,V-diacctyl derivatives toward activation with the combination of SnCl4 and AgB(C6F5)4, although this may be a consequence of the differing anomeric stereochemistries employed [60],... 

Lower 1,2-asymmetric induction was observed when the cyclization of the same substrates was carried out using either phenylselenenyl chloride or bromide. However, the diastereoselectivity increased in the case of acetamide (R = Ac)220. The stereochemistry of the products 3 was assigned from 13C-NMR data and confirmed by NOE experiments. 

The key steps in the first total synthesis of (+)-momilactone A by P. Deslongchamps et al. were a highly diastereoselective transannular DIels-Alder cycloadditlon and the Prdvost reaction The 3-ketolactone moiety was installed by first treating the tricyclic alkene with A/-bromo acetamide and silver acetate to obtain the trans bromoacetate with excellent diastereoselectivlty. The cIs stereochemistry of the lactone was achieved a few steps later by the intramolecular nucleophilic displacement of the bromide with the carboxylate ion on the adjacent six-membered ring. 

N.Q.R. The C1 n.q.r. spectra of cyclic 1,3,2-/i -diazaphosphorines correlate with parameters estimated by CNDO/2 calculations. The structures of the intermediates produced in the reactions of formamides and acetamides with phosphorus pentachloride and phosphoryl trichloride have been studied by n.q.r. spectroscopy. The stereochemistry of dichlorodiazadiphosphetane has been studied, and evidence on the polarity of radial chloride atoms in chlorophosphoranes discussed. ... 

Stereochemistry — The conformational analysis of various deuteriated ethyl-phosphine-borane adducts and dimethyl methylphosphonates were based on vibrational spectral data. The stereochemistry of diethylphosphonyl acetamide,the unusual vinyl compounds (60 X = L.E.P., O, and a number of l,3,2-dioxaphosphorinanes have been studied. It was found that the Raman-active ring vibration is related to the orientation of the phosphoryl group. Conformational data for dioxaphosphepane was compared with calculated parameters. A low-temperature study of several cyclopropyl-phosphonates (61) revealed rotational isomerization about the P-O bonds but not about the P-C bond. The variable temperature i.r. and Raman spectra of the silyl phosphates (62) also revealed rotational isomerism. ... 

A change to a less basic solvent can influence the stereochemistry of unimolecular elimination. In nitrobenzene or acetic acid, erythro-3-deutcro-2-butyl tosylate yields deuterium-free m-2-butene and monodeuterated /ra 5-2-butene, suggestive of a jy/i-elimination (104). In more basic solvents, such as water, acetamide and 80% aqueous ethanol, the labelling of olefins is reversed and a/i/i-elimination is followed. The change in stereochemistry is attributed to the relative basicities of the tosylate anion and solvent and hence the tendencies of these species to accept a beta proton from the carbonium ion (105). 

An early synthesis, published in 1983 and 1984 [83], requires three steps from 2-chloro-N-(2,4-dichlorophenyl)acetamide (Scheme 17.17). First, the amide group is transformed into imidoyl chloride with phosphorus pentachloride, followed by displacement of the two non-aromatic chlorine atoms by two equivalents of the potassium salt of 1,2,4-triazole. The triazolyl group borne by the imino bond being the best leaving group is replaced when reacted with a good nucleophile such as (4-chlorophenyl)methanethiol. The stereochemistry (E or Z) of the double bond is not stated in the literature. 

The low vapor pressure and high thermal stability of CILs render them suitable for enantioseparations in gas chromatography (GC). Recently, CILs have been used as chiral stationary phases (CSPs) in GC [40]. Armstrong and coworkers carried out enantiomeric separation of chiral alcohols and diols, chiral sulfoxides, some chiral epoxides and acetamides using a CIL based on ephedrinium salt. Using an ephedrinium CIL (4) as the CSP, enantiomeric separation of alcohols and diols was achieved (Fig. 1). The presence of both enantiomeric forms of ephedrine makes it possible to produce CSPs of opposite stereochemistry, which could reverse the enantiomeric elution order of the analytes. This offers an additional advantage that may not be easily achieved with common and widely used chiral selectors in GC such as the cyclodextrins. However, there was a decrease in enantiomeric recognition ability of the CSP after a week which the authors attributed to dehydration-induced... 

Key steps in two formal syntheses of racemic 8a-ep -dendroprimine rac-787) are shown in Scheme 103. Hsu et al. employed the stepwise [3 + 3] cycloaddition of the dianion of N-benzyl-2-(p-toluenesulfonyl)acetamide 797 to methyl crotonate (798) to yield the 3-sulfonylglutarimide 799, the iw 5-stereochemistry of which was established by X-ray analysis. Replacement of the JV-benzyl substituent by a 3-chloropropyl chain was followed by regiospecific reduction of imide 800 with sodium borohydride... 

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