Threo compounds

The higher selectivity observed with the threo-compound has been rationalized to arise from less steric hindrance in the five-membered transition state. 

In elimination from the threo compound 285, the situation is different because the stereoelectronic and conformational factors are not cooperative. Since expulsion of the leaving group from the stereoelectronically preferred carbanion 2tH is rendered difficult by severe Ar-ArS02 steric interactions, the anti elimination is slowed down and syn elimination via carbanions 292 and 293 become competitive."... 

The erythro compound shows little or no kinetic isotope effect, but the threo compound has a moderate one, H/fcD 2-3-3-3, for both syn and anti processes. This suggests that an E2 process is involved. Eliminations from the cyclic bromides may produce trans alkenes by syn eliminations or by anti eliminations, if n 8 in(189). 

The four related compounds oxalomycin (157), neooxalomycin (158), cur-romycin A (159), and curromycin B (160) were reported in 1985 157 and 158 were isolated from a yet to be identified Streptomyces species (79, 80) and 159 and 160 from an ethidium bromide-treated strain of S. hygroscopicus (81, 82). The absolute configuration of oxalomycin (157) and neooxalomycin (158) has been determined by application of a combination of X-ray crystallography and chemical correlation to degradation products, the important derivatives being the p-bromobenzoate 161, obtained from 157 by ozonolysis-reduction, acetylation, partial hydrolysis, and reacylation with p-bromobenzoyl chloride, and the erythro acetate 162 which was obtained along with the threo compound 163 after acetylation of the ozonolysis products of 157 (79, 80). No stereochemical infor-... 

Chloro-2-butyl Tellurium Trichloride15 A 100 ml flask fitted with a magnetic stirrer is flushed with nitrogen, charged with 1.67 g (6.2 mmol) of freshly sublimed tellurium tetrachloride, 0.36 g (6.4 mmol) of (Z)-2-butcnc, and 20 ml of ethanol-free chloroform, and immersed in an ice bath. The mixture is stirred for 3 h, filtered, and the filtrate is evaporated yield 1.35 g (67%), mixture of threo (80%) and erythro (20%) isomers. The mixture is recrystallized to yield the pure threo compound m.p. 107°. The erythro compound was not obtained in an isomerically pure state. 

Oleic acid was converted to the erythro diol 264 or the threo diol 258 by reaction with permanganate or hydrogenperoxide respectively. The threo compound 258 was converted to the 9,11-dioxahomoprostanoids. Reaction with paraformaldehyde formed 259 or 260, phosgene converted the diol into 261 whereas reaction with thiophosgene gave compound 262. 

Further supporting evidence is provided by the observation that, after the acetolysis has been allowed to go to 50 per cent completion, the remaining erythro compound, ET, is optically pure, while the remaining threo compound, TT, is racemized. This is considered as evidence also for the intermediate formation of an fintimate ion pair, since it is found that tlio reactions are not inhibited by even high concentrations of HOTs. Such a test is not very meaningful, however, since HOTs is extraordinarily little ionized in the low dielectric, acetic acid. 

The smallest member of such homologous series are the 2,3-dihydroxy succinic (tartaric) dialdehydes. Three stereoisomeric forms are possible, i. e. one meso (erythro) and two enantiomeric (d- and L-threo) compounds (i. e. 3). Because of the enantiotopic nature of the termini of a meso chain, any twofold aldol addition under reagent control imposed by the same chiral biocatalyst would eliminate the element of cj-symmetry and thus effect a terminus differentiation (cf. Scheme 4). For the same reason, enzymes that cannot easily differentiate the two enantiotopic aldehyde groups would lead to the formation of two different. 

Less acidic than Ti and Zi chloroderivatives, MeTi(OPr )3 perfoims chelation-controlled addition to chiral alkoxy ketones as well as or better than organomagnesium compounds, but fails to chelate to aldehydes or hindered ketones. Should the formation of a cyclic chelation intermediate be forbidden, the reaction is subject to nonchelation control, according to Ae Felkin-Anh (or Comforth) model. Under these circumstances, the ratio of the diastereomeric products is inverted in favor of the anti-Cram product(s). In the case of benzil (83 Scheme 7) this can be accounted for by the unlikely formation of a cyclic intermediate such as (85), and thus the preferential intermediacy of the open chain intermediate (86) that leads to the threo compound (88). This view is substantiated by the fact that replacement of titanium with zirconium, which is characterized by longer M—O bonds, restores the possibility of having a cyclic intermediate and, as a consequence, leads to the erythro meso) compound (87) thus paralleling the action of Mg and Li complexes. 

We started our investigation with the reduction of (Z)-styryltrimethylsilane, (Z)-13, R = Ph [21] When brought to reaction with lithium metal in diethyl ether the usual product of reduction 23 is found, trapped as the dimethyl derivative 24 after work-up with dimethyl sulfate 24 is isolated in 87% yield as a 1 3 mixture of erythro and threo compounds. On the other hand, upon heating (Z)-13, R = Ph, for 10 hours in toluene 28 is obtained derived from the 1,4-dilithium intermediate 27 (Scheme 5). 

Sol 3. The syn character of pyrolytic eliminations has been demonstrated in many ways. For example, pyrolysis of erythro and threo isomers of 1-acetoxy-2-deutero-l,2-diphenylethane gave in each case frans-stilbene as a major product, but the stilbene from the erythro compound retained nearly all its deuterium, whereas the stilbene from the threo compound had lost most of its deuterium. Either the hydrogen or deuterium could be syn to the acetoxy group, but the preferred conformations are those in which the phenyl groups are as far removed from each other as possible. 

Malspeis and Hung [85] investigated the isomeric N-nitro-soephedrine (erythro compound) and N-nitrosopseudoephedrine (threo compound) ... 

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