Witkop

Pruckner and Witkop have compared the ultra-violet absorption spectra of a series of alkaloids and alkaloidal nuclei, with special reference to the effects produced by substitution on the harman group leading to yobjuine (p. 505) and its derivatives. 

Processes for the isolation and purification of yohimbine have been described by Thoms, Feldhoff, Chemnitius, Schwyzer, Raymond-Hamet and others. Some of these processes have been modified to provide for estimation of the yield of yohimbine, usually as the hydrochloride, from the bark, or to facilitate the identification of this alkaloid in commercial yohimbe bark, which is apt to vary both in quality and in botanical origin.According to Witkop (1943) modern technical yohimbine hydrochloride may contain a little isoyohimbine but no allo-yohimbine. 

In addition to the natural isomerides of yohimbine listed in the foregoing table, several resulting from chemical reactions with yohimbine are known. Witkop has described e-yohimbine, C21H26O3N21 m.p. 203° dec.), [a] " 29-8° (pyridine) produced along with yohimbine, by the... 

Constitution of Yohimbine and its Isomerides. The yohimb alkaloids are methyl esters of acids. Yohimbine, yohimbene, mesoyohimbine (isoyohimbine) and y-yohimbine (table, p. 502) are hydrolysed to four, distinct, monocarboxylic acids, C2,yH2402N2, each of which on decarboxylation by heating with soda-lime yields yohimbol, long supposed to be a secondary alcohol, Ci,yH240N2, but which Witkop has shown to be a ketone and has re-named yohimbone, C18H22ON2, m.p. 307° (dec.),... 

Witkop applied the lead tetracetate process to opoyohim-bine, and obtained tetradehydroapoyohimbic acid, ... 

Other contributions to this discussion have been made by Harvey, Miller and Robson, and by Dewar and King, and important additional experimental evidence for the location of the. CHOH group at has been provided by Witkop, who explains the improvement in the yield of yohimbone, which occurs when the decarboxylation of yohimbic acid takes place under oxidising conditions, as due to the primary formation of a j8-ketocarboxylic acid, which is readily decarboxylated. This may be represented, using part of the yohimbine formula fXIV), as follows MeO. OC. CH. 1 CHOH. ->HO. OC. CH. CHOH. ... 

The same author has shown by repetition of the early experiments already referred to (p. 507) that when yohimbine hydrochloride is distilled with zinc dust there is formed in addition to harman (XVII), p-cresol, which must originate from ring E of the yohimbine formula (XIV) by inclusion of C as the methyl group. Witkop s formula for yobyrine (IV6) received prompt confirmation by Clemo and Swan s synthesis of this base by... 

Recently Waters and Witkop observed that irradiation with ultraviolet light of a solution of cholest-4-en-3-one during reduction with NaBH4 in 2-propanol increases the rate of reduction to almost that of cholestan-3-one. The yield of cholest-4-en-3)S-ol is not much decreased by the irradiation, although the mixture of products is more complicated (see page 74). These results may be applicable to problems of selective reduction of a 4-en-3-one system. 

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