Epimerization compounds

The epimeric compounds, e.g., 88 and 89 from a Hawaiian Ciocalypta sponge [55] and 90 from a Pohnpei Halichondria sp., may share a common sp2-carbon center in the biosynthesis of the bisabolane skeleton of these compounds. 

FIGURE 8.6 Synthesis of /V-protccted dipeptide amides (A) and methylamides (B) by ami-nolysis of the unisolated succinimido ester obtained through the mixed anhydride in CH2C12.22 The products contained <0.5% of epimerized compound. HONSu = /V-hydroxysuccinimde, NMM = Af-methylmorpholine. 

FIGURE 1.1 Chemistry and stereochemistry of the native cinchona alkaloids quinine, quinidine, cmchonidme, and cinchonine as well as their corresponding C9-epimeric compounds. 

A corresponding sequence from di-O-acetyl-fucal gives a mixture of the precursors of benzyl daunosaminide 88 and the epimeric compound 89 [54]. By this NIS quenching method, enhanced yields of the equatorial 3-azido compounds are obtained. 

Stypodiol, epistypodiol and stypotriol are secondary diterpene metabolites produced by the tropical brown algae Stypopodium zonaie. These compounds display diverse biological properties, including strong toxic, narcotic, and hyperactive effects upon the reef-dwelling fish. In the laboratory of A. Abad an efficient stereoselective synthesis of stypodiol and its C14 epimer, epistypodiol, was accomplished from (S)-(+)-carvone. The key transformations in the synthesis of these epimeric compounds were an intramolecular Diels-Alder reaction, a sonochemical Barbier reaction and an acid-catalyzed quinol-tertiary alcohol cyclization. 

The full paper concerning the reaction of acyl chlorides (benzoyl and toluene-p-sulphonyl chlorides) with the steroidal nitrone derivative (23) of conanine has been published. Additional information concerning the mechanism of these reactions is given. Thus, it was shown that the formation of the a -benzyloxyimine (24) needs acidic conditions which favour the heterolysis of the N—O bond (Scheme 2). At room temperature, under Schotten-Baumann conditions, the epimeric compounds (25) were obtained from (23) with benzoyl chloride. Compound (25) gave (24) on refluxing in neutral solvent or by treatment with acid, whereas (24) was obtained directly from (23) when treated with a benzene solution of benzoyl chloride in the presence of aqueous sulphuric acid. 

In some carbohydrates, such as epimeric compounds, a co-elution is often observed, since the retention behavior of these compounds does not differ very much. With respect to the different dissociation behavior, however, the selectivity of the separation system can be enhanced significantly by lowering the pH of the mobile phase to one that is comparable to the pAT value of the compound. 

In contrast, the separation of the various monosaccharides is much easier, which is surprising because the affinities of these compounds are not very different. However, when the pH value of the mobile phase is lowered to a value corresponding to the pK value of the respective compounds, even small differences in the dissociation behavior contribute to the separation. Thus, the chromatogram in Fig. 3-108 is obtained using a dilute sodium hydroxide solution of c = 0,001 mol/L. Under these conditions, even epimeric compounds such as glucose, mannose, and galactose are baseline resolved. Two important aldopentoses - arabinose and xylose - can also be determined in the same analysis. Remarkable is the elution order, which cannot be correlated with the carbon chain length of the monosaccharides. 

In the early 1960s Battersby and Yeowell (196) isolated two quaternary alkaloids from Strychnos toxifera. These they called macusine A and macusine C, which they claimed to be epimeric compounds and for which they presented structures 66 and 67, respectively (Fig. 2). A little later Orazi et... 

The lyxo isomer of 181 submitted to methoxymercuration-demercuration reaction provided the epimeric compound 188 with full regio- and stereoselectivity [120a]. Its transformation to the corresponding heptulosaric acid derivative 189 (Scheme 40) was easily accomplished applying KBr/TEMPO/NaOCl assisted oxidation. 

The C-17 epimeric compounds with conformations I and V are interesting. In I the 17 -OH experiences Y-effect on C-12 and C-16 where as in V the same Y-effect is noticed from C-20. These interactions are absent in the case of compounds with an -oriented... 

Benzyl 6-isocyanopenicillinate 281 reacted with carbon disulfide in the presence of potassium carbonate to yield a mixture of epimeric compounds 282 (Scheme 81) [149]. 

Unsubstituted glycal 49 underwent cycloaddition reaction with a sp dienophile (maleic anhydride) in refluxing toluene to give a mixture of two epimeric compounds arising from an expected endo attack. The 7 3 ratio in stereoadducts 51 and 52 clearly indicates an stereochemical preference inherent to the pyranosidic substrate (vide supra). 

The C-14 epimeric compound (XXVI) was obtained by partial dioxolanation of the stable diketone XXII-A. Either ketal on treatment with p-toluenesulfonic acid in benzene produced an equilibrium mixture of the two, XXV and XXVI, in nearly equal quantities. The equilibrium is presumably established by the formation of the enol ether XXVII. Thus the configuration at C-14 which is strongly favored in the diketone... 

Acyloxy groups a to a ketonic group can be replaced by hydrogen or allyl under free radical conditions (Scheme 11). Thus either the C-2 epimeric compounds (49) or C-3 epimeric compounds (50) afforded the axial C-allyl products (51) and (52) respectively. (Compound (52) has also been prepared in a different way in Scheme 24, this Chapter.) Compound (52) could readily be epimerized to its C-3 equatorial product on treatment with acid resin in ethanol. The reactions were also applicable to L-sorbose pentaacetate. 

When this is done, the structural formula of an optically active compound appears as the mirror-image of the formula of the corresponding epimeric compound. 

To obtain simple, well-characterized derivatives, the epimeric isohumulones have been transformed into the epimeric humulinic acids (see 8.4.) and further into the epimeric dihydrodeoxohumulinic acids (cis form 72, Fig. 37 trans form 73, Fig. 37). This last transformation is achieved by hydrogenolysis of the humulinic acids (26). The yield can be increased to 80-85% by carrying out the reaction in a 20%-solution of the humulinic acids in acetic acid with 5% Adams catalyst at 55°C during 6-8 h. The optical activity is fully retained. The epimeric compounds 72 and 73 are separated by CCD in the two-phase system ether aqueous buffer pH 7.65 after 100 transfers. The cis isomer 72 has a K-value of 1.36 and a melting point of 179°C. The UV absorption... 

The macrolactone calonyctin A, a plant growth regulator, has been synthesized (Scheme 42) [184, 185], The quinovose acceptor was glycosylated subsequently with the suitably protected quinovosyl, rhamnosyl, and quinovosyl trichloroacetimidates all steps could be performed stereospecifically. Transformation of the tetra-saccharide into the trichloroacetimidate and glycosylation of the racemic hydroxy acid derivative gave two epimeric compounds. Separation and lactonization gave calonyctin A. 

The hydrothermolysis (27.5 MPa, 290 - 400 C) of D-fructose furnished 5-hydroxymethyl-2-furaldehyde, furfural, and l,2,4-benzenetriol. The epimeric compounds S3, separable by h.p.l.c., have been isolated in high yields from the reaction of D-glucose with guanosine under physiological conditions. An analysis of the aroma compounds produced on exposure of riramnose to cysteine under roasting conditions (2(X) - 220 C) revealed the presence of ca. 180 compounds, mosdy derivatives of furan, pyrrole, artd thiophene, 125 of which have been identified. ... 

---