CAMPBELL A N D MITCHELL-(+)-trans-CHRYSANTHEMIC ACID 497

THE OPTICAL ROTATION OF (+)-ftans-CHRYSANTHEMIC ACID

By A. CAMPBELL and Wm. MITCHELL

Our previous figures (Campbell & Mitchell. 19jo) for the optical activity of natural (+)-trans-chrysanthemic acid have been questioned by Harper, Reed & Thompson (19 j 1).

Examination of another specimen of the natural acid has given much higher figures than those recorded by Harper et al. (1951) and by Campbell & Harper (1945). though slightly lower than those previously reported by us. It is now suggested that the acid component of pyrethrin I and cinerin I may be racemized to a slight and variable extent in the plant during metabolism or after collection. I t is shown that the acid is not racemized by the conditions of hydrolysis through which we prepared it. I t seems unlikely that Campbell 8 Harper's synthetic (-)-tvans-acid was optically pure ; it is possible tliat the natural (-7)-trans-acid has yet to be isolated in an optically pure form.

In a recent paper (Campbell & Mitchell, 195o), we recorded the optical rotations in ethanol and in chloroform of the (+)-trans-chrysanthemic acids isolated by us from a normal pyrethrum extract and from a polymerized pyrethrum concentrate. Subsequently Harper, Keed & Thompson (1951) have suggested that our figures for the ' normal ' acid were incorrect. These authors report much lower values, in line with those previously given by Campbell 8r Harper (1945) for their synthetic (-)-traits-acid and for a specimen of natural ($)-trans-acid (Table I).

Table I Chvysanlhemic acid

Satural ( -k)- trans . . . [a:= in absolute ethanol . . + I9'4',(2.89)

(Campbell & Mltchell, Igjo) - .. . . . . . . . . .. .. . . . . . . . . . . -1- 14.2" (1.5j)

(Campbell & Harper, 194 j)

(Campbell & Harper, 194 j)

(Campbell & Harper, 1945)

Synthetic (-)-trans .. .. . . . . . . - 14'Oo (1.54)

.. . . . . . . . . - 14.1' (1.92)

[ z ] O in chloroform + 28.5" (3 .08)

(Campbell & Mitchell, 1950) (Harper - 20.7' et al., (6.85) 1951)

-

Concentrations (yo W/V) are in parentheses ; our readings were made at zoo c., the others a t 18" to 19" c .

It seemed to us desirabIe to investigate these rather considerable discrepancies. Ln- fortunately, the acids that we had previously examined were no longer available, and the figures recorded in Table I1 were obtained on a further specimen of acid prepared by us from a normal pyrethrum extract. This acid, m.p. 18.5" c., dZo 0.972 (equivalent, 168 ; calc. for C10H1600, 168) tested 99.57, by the mercury-reduction method.

Table I1 (+)-trans-Chvysanthewiic acid from n normal pyrellivtinz extract

Fez]? in absolute ethanol -: 16.o' (10.56) + 16. l0(j .28) -+ 16.3' (3.01) -+ 15.8' ( 1 . j )

[a:? undiluted + 22.8 ' * - - -

Concentrations (yo w/v) are in parentheses. The observations were made on a Bellingham & Stanley Glass Circle polarimeter. using a Ic-cm. tube. * Staudinger & Ruzicka (1924) recorded -7 20.1' without mentioning a concentration or solvent ; pre-

sumably their value referred to the pure acid.

The specific rotations in chloroform are somewhat lower than those previously obtained by us on an acid derived from a normal pyrethrum extract, but the difference is not very large, while the figures remain much higher than those recorded by Harper et al. (1951). In ethanol, the difference from our previous figure is greater, but the present values remain significantly higher than those recorded by Campbell & Harper (1945). Harper, Keed Pr Thompson suggested that the discrepancies might be due to variation of the optical rotation

J. Sci. Food Agric., 2, November, 1951

498 HIGGONS-DETERMINATIOX OF BORON

with concentration, but it will be noted that no such variation does in fact occur over the range I-IO% (w/v).

The purity of the acids used both in our earlier and present determinations was such as practically to preclude the possibility of error being introduced by the presence of a contaminant of high optical rotation. I t is unlikely that the optical rotation would be affected by small variations in the strength of the ethanol used, and even less likely that it would vary in chloro- form from different sources. We conclude, therefore, that the optical rotation of (+)-trans- chrysanthemic acid is in fact higher than stated by Harper, Reed & Thompson. They suggest that Campbell & Harper's synthetic (-)-trans-acid and natural (-+)-trans-acid were optically pure, but this seems to us doubtful and, in any event, incapable of proof.

It was considered possible that the natural (+)-trans-acid prepared by alkaline hydrolysis of pyrethrin I and cinerin I might be more or less racemized, depending on the severity of the conditions of hydrolysis. Accordingly the pure acid (2 g.) in alcohol (IS ml.) containing potassium hydroxide (0.9 g.) was boiled under reflux for S hours (these conditions being similar to those used in the initial hydrolysis by which we prepared the acid). The ethanol was then removed and the acid recovered almost quantitatively by the usual methods. The recovered acid showed [a]: + 16.2" [c, 3.0% (w/v) in absolute ethanol] and [a]: + 27.9" [c, 3.0"/, (w/v) in chloroform]. Thus there is no suggestion that the acid is in fact racemized in the conditions that we used in its preparation both in this hydrolysis and in that described in our previous paper.

Wc can only conclude that racemization of the acidic component of pyrethrin I and cinerin 1 occurs to a small and variable extent in the plant during metabolism, or during harvesting, drying or storage. This is a phenomenon quite commonly observed with essential oils, alkaloids and other naturally-occurring, optically active substances. I t now seems clear that optically pure synthetic (-)-trans-chrysanthemic acid has not yet been obtained ; and i t is possible that this applies also to the natural (-+-)-trans-acid.

Stafford Allen & Sons Ltd. Wharf Koad, London, N.I

Received 23 May, 1951

References Campbell, A. c'k Mitchell, W. ( I g j O ) . J . Sci. Food Harper, S. H., Reed, H. W. B. & Thompson, H. A.

Campbell, I . G. M. BC Harper, S. H. (1g-)5). J . Staudinger, H. r t Iiuzicka, L. (1924) . Helv. chiin. AgYiC. 1, 137. ( I g j I ) . ,I. Sci. Food d g ~ i c . 2 , 94.

diem. Soc., p. 283. d r t a 7, 201.

A RAPID COLORIMETRIC ,METHOD FOR THE DETERMINATION OF BORON, WITH ITS APPLICATION TO SOILS AND CROPS

B y D. J. HIGGONS

Waxoline purple AS lias been established as an efficient colorimetric reagent for the determination of boron in 8404 sulphuric acid. Data are presented showing the loss of boron during the orthodox ashing techniques of both plant residues and soil extracts. Conditions of ignition have been established whereby it is possible to check thc relative boron status o f any crop or soil when compared against standards from the same crop or similar soil type known to have a satisfactory boron status.

With the increasing importance of boron in the growth of agricultural crops, methods for the determination of boron are urgently needed which have both a high degree of precision and are rapid to operate with a large number of samples without the need for lengthy separation techniques. For many years, the quinalizarin method has been used for this type of work, but i t has been found in this laboratory that it is extremely sensitive to traces of water and is difficult to operate with large numbers, requiring very frequent standardization to give any degree of accuracy in the results.

Several alternative methods have recently been published for the colorimetric determination of boron. Dickinson (1943) reported the use of alizarin S in 98% sulphuric acid and this was followed by a paper by Evans & McHargue (I947), who developed a method based on the use

J. Sci. Food Agric., 2, November, 1951