Dihydroxy bases

Among the sphingosine bases in the C. sandai sphingolipid mixture, a high content (44% of the total bases) of branched mono-unsaturated dihydroxy bases has been observed.159... 

The major, fatty acid component in these glycolipids is the C22 , acid, and no hydroxy acids were found. The sphingosine bases are represented by dihydroxy bases, among which, the preponderant one is the C,8 sphin-... 

Considerable proportions of cerebroside have been detected in the starfish Asterias rubens.9Ail0 It is a glucosylceramide having a-hydroxy fatty acids (from C16 to C26) and dihydroxy bases whose major components are Cl8 and C22 bases, with one, or two, double bonds. 

Sometimes, the calculation involves a monoprotic acid and a dihydroxy base or another set of conditions in which the relationship is not 1 1. We have to keep track of the various concentrations so that the molarities do not get mixed up. However, stoichiometric calculations involving solutions of specified normalities are even simpler. By the definition of equivalent mass in Chapter 12, two solutions will react exactly with each other if... 

An improved method for the removal of methylenedioxy-groups in alkaloids of the benzylisoquinoline and tetrahydroberberine series has been derived. Alkaloids such as remneine (26) can be converted into the dihydroxy-bases (27) by treatment with boron trifluoride followed by 5-chloro-5-phenyl-l//-tetrazole and subsequent hydrogenolysis of the resulting bis-tetrazolyl ethers. The yields in the process are good.39... 

Because strong bases are completely ionized, the molarity of hydroxide ion in sodium and potassium hydroxide solutions, the monohydroxy bases, is the same as the molarity of the base itself. The OH molarity in 0.10 M NaOH is 0.10 M. In the case of Ca(OH)2, a dihydroxy base, the OH molarity is exactly two times the molarity of Ca(OH)2. Not all hydroxide compounds function well as bases because of their low solubility in water. Aluminum hydroxide, Al(OH)3, and magnesium hydroxide, Mg(OH)2 can both neutralize acids (they are used in several antacids) but neither is very soluble and cannot used to prepare solutions. 

SLs are also polar cell membrane lipids, but they are typically present in much lower concentration than PLs. Soybeans are a relatively rich source of SLs (Vesper et al., 1999), and ceramides and cerebrosides are the primary SL classes in soybeans. SLs contain a sphingoid long-chain (CIS) drhydroxy base and an a-hydroxy fatty acyl chain that is linked to the base by an amide bond. The main soybean ceramide molecular species is a trihydroxy base (4-hydroxy-trans 8-sphingenine) V -acylated with a-hydroxy lignoceric acid (C24 0). The main soybean cerebroside molecular species is a dihydroxy base trans A-trans 8-sphingediene) V acylated with a-hydroxy palmitic acid. The general molecular structures of ceramide and cerebroside are shown in Fig. 10.4. 

Constitution. The empirical formula of thalictrifoline and its ready oxidation indicates that it is a methyl homologue of either canadine or sinactine, that is, corydaline in which two of the methoxyls are replaced by a methylenedioxy group. On oxidation with iodine in methanol it is transformed into a quaternary iodide, which on reduction with zinc and hydrochloric acid yields dZ-thalictrifoline, m.p. 151°, which was also obtained by reduction of the naturally occurring dehydrothalictrifoline chloride. Complete oxidation of thalictrifoline yielded m-hemipinic acid, and therefore its structure was written as LXV. This was confirmed by demethylenation to a dihydroxy base and methylation of the latter. The new base proved to be an optically active stereoisomer of corydaline, presumably that of mesocorydaline, and on racemization by iodine oxidation and subsequent reduction was converted into mesocorydaline. 

It is isomeric with thalictrifoline and has the same functional groups. When demethylenated with phloroglucinol and sulfuric acid it yields a dihydroxy base (m.p. 246°, in vacuo) which on methylation is converted into corydaline. Thalictricavine is therefore either steroisomeric with thalictrifoline or it is thalictrifoline with the pairs of vicinal ether groups interchanged as in LXVI. 

Carbon-13 n.m.r. spectroscopy has been shown to be a useful technique for distinguishing between stereoisomers of spirobenzylisoquinolines. A review of the synthesis and biosynthesis of alkaloids of this group has been published. Methods of synthesis from tetrahydroberberines have been described above. The dihydroxy-base (125 R = = H) has been converted into yenhusomidine (125 ... 

Figure 6.9 Separation of sphingosine bases by silicic acid column chromatography (a) or TLC (b). (a) Solvent system chloroform plus a gradient of 10% 2m NaOH in methanol (see Barenholz and Gatt, 1975). (b) TLC separation of dinitrophenyl derivatives of long-chain bases on layers of silica gel G containing 2% (w/w) boric acid. Solvent system chloroform-hexane-methanol (50 50 14, by vol.). Abbreviations DHB, saturated dihydroxy bases UDHB, A -fran -unsaturated dihydroxy bases THB, trihydroxy bases. Reproduced with permission from Barenholz and Gatt (1975) (a) and Christie (1982) (b).

Figure 8.12. Schematic separation of ceramides by TLC on layers of silica gel G, impregnated with 2 % sodium arsenite (plate A) and 2 % sodium borate (plate B), using a solvent system of chloroform-methanol (95 5, v/v) for development [467], Abbreviations na, normal fatty acid ha, hydroxy fatty acid db, dihydroxy base tb, trihydroxy base sat, saturated.

They can be separated into three groups on layers of silica gel G impregnated with 2% boric acid, i.e. saturated dihydroxy-, unsaturated dihydroxy (with a trans double bond in position 4) and trihydroxy bases, with chloroform-hexane-methanol (5 5 2 by volume) as the mobile phase [465]. When acidic hydrolysis procedures are utilised in the preparation of the bases, unnatural threo- somers of the unsaturated dihydroxy bases are found just below the natural erythro compounds on the TLC plate. Each of the fractions separated by TLC can be recovered from the adsorbent by elution with chloroform-methanol (2 1, v/v), but the eluate should be washed with one quarter the volume of water to remove boric acid which is also eluted. If need be, the bases can be further resolved by silver ion TLC [462] or by HPLC in the reversed-phase mode (reviewed elsewhere [168]). It is possible that HPLC in the adsorption or silver ion modes could also contribute to the problem of analysis, but these do not appear to have been tried. 

Note that aldehyde derivatives prepared from trihydroxy bases will be one carbon shorter than those from equivalent dihydroxy bases, and the number of hydroxyl groups must be determined from the TLC behaviour of the base or its DNP derivative. The proportions of the various isomers within each class of base can be determined with reasonable accuracy by GC, but artefact formation during the hydrolysis stage may distort the apparent relative proportions of the various classes of base to each other. 

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