PURDIE AND PAUL: THE ALKYLATION OF D-FRUCTOSE. 289

By THOMAS PURDTE, F.R.S., and DAVID MCLAREN PAUL, B.Sc., Carnegie Scholar.

P a ~ v r o u s communications from this laboratory (Trans., 1903, 83, 1021 and subsequent papers) have shown tha t the hydroxyl groups of methylaldosides can be completely methylated by methyl iodide and dry silver oxide, and tha t the methylated aldosides so obtained yield on hydrolysis methyl ethers of the respective aldoses. The latter compounds can be reconverted into methylated aldosides by condensa- tion with methyl alcohol containing hydrogen chloride, or by the silver oxide method of alkylation ; in general a mixture of stereoiso- nieric aldosides is thus produced, the former process giving the a-aldoside, the la t ter the p isomeride in larger proportion. The object of the present research was t o ascertain i f ketoses behave similarly t o aldoses in the above series of reactions. The methylfructoside required for our experiments was prepared from pure d-fructose by Fischer’s method (Beg.., 1895, 28, 11 60). Fischer himself failed to obtain the siibstance in the crystalline state, and our efforts towards this end were also unsuccessful. W e therefore used, as starting material, the syrup left after removal of hydrogen chloride and evaporation of the methyl alcohol. This crude material is doubtless, in the main, a mixture of a- and P-methylfructosides, although so far no evidence has been adduced with respect t o the existence of stereoiso- meric alkylketosides. As appears in the sequel, it probably contains also a n isomeric, less lzevorotatory, or possibly dextrorotatory hexo- side and, according t o Fischer, a considerable proportion of unaltered fructose.

W e find tha t the alkylation of this syrupy mixture by means of methyl iodide and silver oxide, and the hydrolysis of the resulting mixture of tetramethyl rnetbylketosides, proceed, in general, in the same manner as in previous similar experiments with aldosides. Greater difficulty was, however, experienced in isolating the pi*oducts, and the rotatory powers observed in different pieparations varied con- siderably. W e at t r ibute these anomalies t o the mixed nature of the initial fructoside material, and to the greater susceptibility of ketoses and their derivatives to oxidation and other chemical changes. By- products are thus introduced which fractional distillation fails to remove completely.

The tetramethyl methylfructoside obtained by alkylation of inethyl- fructoside, and the product of its hydrolysis, tetramethyl fructose, proved t o be uncrystallisable syrups, which could, however, be distilled

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290 PURDIE AND PAUL: THE ALKYLBTION OF D-FRUCTOSE.

without decomposition, and were thus isolated. The syrupy tetra- methyl fructose was reconverted into tetramethyl methylfructoside by the silver oxide process; the liquid fructoside so obtainecl gave on hydrolysis again a syru1)y tetramethyl fructose, from which, however, the pnre methylatecl sugar eveiitudly separated in well-foi-nied crystals. This i n t u r n was converted into the frnctoside by Fischer's method and also by the silver oxide process. Both products being uncrys- tallisable liquids, the a- and p-forms corild not be separated, but, on the evidence of i ts lower Izvorotation, the fructoside from the latter process contained the p-form in greater proportion. Crystalline tetra- methyl fructose mas recovered from both products on hydrolysis.

illeth~!jructositle.

This compound and the parent hexose should, presumably, exist in two stereoisomeric forms corresponding t o the a- and P-alkylaldosides and ddoses. I n order to detect the production of the two methyl- fructosides during the condensation of fructose with methyl alcohol, this process, and the subsequent hydrolybis of the fructoside, were followed by polarirnetric observations. Ordinary fructose shows in aqueous solutions a domnwiwd mutarotation in the lzevo sense [aID - 104" -+ - 92". Assuining on the analogy of glucose t h a t the stable crystallised sugar is the a-form ([a], - 1 0 4 O ) , the specitic rota- tion of the p-fructose should be numerically less than - 92", and the a- and p-fructosicles respectively more m t l less Izi~vorotatory than the corresponding forms of the sugar. P--BIcthylfructoside, like the P-alkylaldosides, should be more rapidly liydrolysed than its a-isomeride, and presumably, therefore, more rapidly formed. Applying these considerations t o the preparation of methylfructoside by Fischer's method, the course of the reaction should be marked by first a decrease and then a n increase of ltcvorotation ; on hydrolysing the product, the reverse optical changes, namely, a n increase and then a diminution of lxvorotation, should be observed.

A solution of 20 grams of fructose in 188 C.C. of methyl alcohol containing 0.5 per cent. of hydrogen chloride was prepared according to Fischer's directions and kept at the ordinary temperature ; polari- metric observations in a 2 dcm. tube were taken, at first every few minutes, and afterwards at longer intervals. As the solution grndiially became yellow in coloiir, incandescent p s light was used thronghont. A very rapid fa11 of rotation set i n irrimeclintely on the solution being mntle iip, ~ I l i c I i became grndually slo.rver until a miriimuin was reached. This diminution of lxvorotation was followed by a much slower increase, until the rotation became almost constant a t approximately i ts original value. On now heating the solution at

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PURDIE AND PAUL : THE ALRYLA'L'ION OF D-FRUCTOSE. 291

35O, a second less extensive and very slow diminution of lasorotation occurred. The following observations may be recorded in illustration :

Conclemation. Tiiiic after Observed sol nt ion.

4 miriutes - ll*!UO 15 ) ) 8 '99 24 ? 7 6-69

2 honrs 40 ), 14s 3 ,, 60 7 ) 0.99 7 9 ) 1 .32

54 ?, 5 . i 3 190 7 ) 12'06 214 ) ) 12.29

rotation I = 2,

Time of Observed heating at 35". rotation I = 2.

49 hours - l l*lso I S & ,, 9.09 354 Y ) 8-20 39 2 ) 8-05

Time after solutioii. 4 minutes 6 I1011rs

21; ) )

44 I ,

235 ,)

281. 9 ) >

166 ,,

Ol~serveJ rotation 1 = 2.

- 4'40" 6 - 0 5 7.05 7 - 0 4 li -60 6-01 6.01

T h e of Observed heating a t 35". rotation Z=2.

40 ) I 7-01 20 hours - 6.55"

64 Y ? 7'29

T h e extensive diminution and subsequent increase of laevorotation during the condensation at the ordinary temperature accord with the

conclusions indicated above. The smaller firin1 decrease of rotation at higher temperature may be attributable to the condensing effect of

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292 PTJRDIE AND PAUL: THE ALKYLATION OF D-FRUCTOSE.

hydrogen chloride on unaltered fructose, or, as suggested by later observations, to the partial conversion of the fructoside into a less I ~ v o r o t a t o r y isomeric hexoside. The first observation recorded ( A on the curve), a - 11 * 9 2 O gives [ - 56", which approximates to the specific rotation found for a solution of similar strength in pure methyl alcohol [a]: - 57.6'. The minimum yotation, ( B ) - 0 * 9 9 O , indicates that /3-methylfructoside is feebly lzvorotatory, or possibly dextrorotatory. The subsequent maximum rotation, (C) - 12-29", probably marks a n equilibrium between the two fructosides.

To observe the changes of rotation occurring during hydrolysis, the methylfructosicle was procured in the syrupy state, as Fischer describes (Zoc. cit.) and dried in a n exhausted desiccator. The specific rotation of the sprnp in methyl alcohol ( c = 6,466) mas - 36.6", a value approximating to t h a t calculated from the final observation quoted

above, namely, - 35.2' (a - 8*08", c = 11.465, calculated as methyl- f ruc t oside).

The specific rotation of tho syrup in water ( c = 13.867) was [ aIhuer - 34.3". This solution, after being diluted with a n equal volume of water containing 4 per cent. of hydrogen chloride, was left to hydrolyse a t the ordinary temperature and finally heated at 35". Some of the observations made are recorded above.

In qualitative correspondence with the three changes of rotation occurring during the formation of the fructoeide mixture, three oppositely directed changes of rotation occur during its hydrolysis. This is seen by comparison of the two curves which represent the optical changes occurring during the formation and hydrolysis respec- tively of the methylfructoside mixture. The observations, therefore, furnish evidence in support of the view that the mixture contains the two methylfructosides and a third hydrolysable product. The possi- bility of the presence of an acetal is, h o ~ e v e r , not excluded.

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PURDIE AND PAUL : THE ALKYLaTION OF D-FRUCTOSE. 2133

JIetTylatioiL of .Methylfhctoside.

This process was carried out precisely as described in previous papers O K ~ the alkylation of aldosides. The proportion of the alkylating agents used and the method of isolating the product were also the same ('Trans., 1904, 85, 1074). The action proceeded in a similar manner, and oxidation did not appear t o occur to any considerable extent. The yield of methylated product obtained on distillation mas, however, less than in the case of the aldosides, amountling to only about a third of the weight of fructose originally taken. Thus, in one preparation, the methylfructoside from 60 grams of fructose gave on alkylation only 19 grams of crude distillate boiling at 130-150° under 16--1S mni. pressure; the remainder of the material could not be distilled without decomposition. This residue did not consist of incompletely alkylatecl fructoside, as, although readily soluble in methyl iodide, further treat- ment with the alkylating agents failed to render it capable of being distilled. The crude distillate referred to reduced Fehling's solution slightly, and had a distinctly acid reaction. To remove the acid im- purity the substance was dissolved in ether and treated with barium carbonate. After drying the filtered solution with anhydrous sodium sulphate, removing the ether, and repeatedly distilling the residue, a neutral syrup, without action on 'Fehling's solution, was obtained. Two separate preparations boiling at 132-136' under 10 mm. and 140-146' under 17 mm. pressure respectively gave on analysis :

I. C = 5 2 * S 6 ; H=S*91 ; OMe=61*1. 11. C = 52.84 ; H = 8.77 ; OMe = 59.7. C6H,0(OMe), requires C = 52.80 ; H = 8.80 ; OMe = 62.0 per cent. The substance has, therefore, the composition of a tetramethyl

me thy lfructoside. It mas found tha t much fractionation could be avoided and a much

bette? yield obtained by the following distinct method of preparation. The crude product of the methylation of methylfructoside, without being previously distilled, was directly hydrolysed with dilute hydrochloric acid, as described later ; the methylated reducing sugar so obtained wcis then remethylated by the silver oxide process, and the resulting liquid finally distilled. By this procedur-e, starting with 70 grams of fructose, 39 grams of distillate were collected (b. p. 136-146"), of which 24 grams boiled a t 139-141°under 12 mm. pressure. The substance had no action on Fehling's solution, and i ts analysis gave figures similar to those already quoted.

111. C = 52-65 ; H =: S.70 ; OMe = G 1.3. The molecular weight in benzene solution by the cryoscopic method

VOL. XCI. X mas found to be 330, the calculated number being 250.

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294 PURDIE AND PAUL : THE ALKYLATION OF D-FRUCTOSE.

The specific rotations in ethyl alcohol of the three specimens of the methylated fructoside, analyses of which are recorded above, were [a]r I - 15.6" ; 11 - 18.S0 ; - 19.8O. These figures, in view of the diffi- culties of the preparation and the different methods employed, are fairly concordant, but the examination of a fourth specimen from a larger pre- paration carried out in the same manner as I and I1 showed that the substance was in fact not homogeneous in respect of rotatory power. Three fractions were collected on distillation boiling at 132', 132-1 40°, and 140-150' under 16-18 mm.; the specific rotations of these determined as above were respectively + 1 *2O, - 14", and - 43.29 The three fractions reacted slightly acid, but they reduced Fehling's solu- tion only to a very slight extent even on boiling, and the figures obtained on analysis of each fraction approximated to those already quoted. The variation in rotatory power, which is too great to be accounted for by the traces of impurity indicated by the analyses, is discussed in the sequel.

Ilydrolysis of Tet~umet?Lyl MetlLylfructoside.

This process mas carried out by heating a 5 per cent. solution of the fructoside in 5 per cent. aqueous hydrochloric acid at 100' for about half an hour. The product was isolated as in previous cases (Zoc. cit.) and distilled. The substance thus obtained mas a colourless neutral syrup which reduced warm Fehling's solution vigorously. After repeated distillation a fraction boiling a t 142-146' under 14 mm. pressure gave on analysis :

C = 50.60 ; H = 8-36 ; OMe = 50.3. C,H,O,(OMe), requires C = 50.88 j H = 856 ; OMe = 52.5 pel: cent. The substance has therefore the composition of tetramethyl

The rotatory power observed in a 2-dcm. tube was as follows : fructose.

I n water ( c = 5,1405)' [ a]$' - 18.1' -p - 20.9'. I n ethyl alcohol ( c = 5*1005), [a]:" - 13.9" -+ 20.2".

The mutarotation indicated above was accelerated in the case of the alcohol solution by adding a trace of alcoholic ammonia, and the permanent stage was reached only after three and a half days. I n aqueous solution the rotation became permanent in three hours, and remained so on adding a trace of ammonia, The alcoholic solution was made up immediately after the substance was distilled ; the aqueous solution later ; hence probably the difference in the range of the muta- rotation in the two cases. The permanent specific rotations observed on two other preparations in alcoholic solution mere - 21.7' (c = 4-47), and - 25' (c = 5*28), the initial rotation in the latter case beiug - 18'. It will be seen that the mutarotation observed is in the opposite sense

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PURDIE AND P A U L : 'L'HE ALKYLATION OF D-FRUCTOSE. 295

to that exhibited by the parent sugar. This apparent anomaly is explained later.

It has been repeatedly shown in previous papers that the mixture of stereoisomeric alkylated aldosides, prepared from alkylated aldoses by the silver oxide process, contains the more readily hydrolysable P-isomeride in such large proportion that its presence can be detected by a diminution and subsequent increase of rotatory power during hydrolysis. This method of ascertaining the presence of two isomeric ketosides was accordingly applied to specimens of tetramethyl methyl- fructoside prepared by the two distinct processes already described.

A 4.5 per cent. solution, in 5 per cent. aqueous hydrochloric acid, of the substance obtained by the direct methylation of methylfructoside, was heated at loo", and polarimetric observations were taken every ten minutes in a 2-dcm. tube, using a Welsbach light. The initial specific rotation, [ uIAuer - 32*7", diminished until after forty minutes it had attained the constant value, - 21*2O, or - 22.5' calculating on the assumption that the methylated fructoside was entirely hydrolysed to tetramethyl fructose. This value agrees approximately with the values given above for the isolated product. The decrease of rotation was uniform, showing no such fluctuations as mere recorded in hydrolysing the methylfructoside from which the substance mas prepared.

Asimilar experiment was made with the tetramethyl methylfructoside obtained from tetramethyl fructose by the silver oxide process. The observations were made on a solution of the fructoside ( c = 5,332) in 2.5 per cent. aqueous hydrochloric acid which was heated in a thermostat a t 50'. The initial specific rotation, [uIAuer - 3 8 * 2 O , diminished until after four hours it had attained nearly the same constant value as before, - 23*2", or, calculated for tetramethyl fructose, - 24.5'. The observations, therefore, furnish no evidence of the presence in either specimen of isomeric fructosides which differ widely in rotatory power and rate of hydrolysis.

CT*ystaZline I'etramethyl Fructose.

A specimen of the distilled syrupy tetramethyl fructose described in the last section eventually showed signs of crystallisation. The avail- able material, 17 grams in all, was accordingly nucleated and left to crystallise. By drairing the resulting semi-solid mass on a porous plate and recrystallising repeatedly from light petroleum, 4 grams of pure substance were obtained in square plates, which melted sharply a t 98-99'. Analysis gave :

C = 50.46 ; H = 8.72 ; OMe = 52.44.

The molecular weight found in aqueous solution by the cryoscopic C,H,O,(OMe), requires C: = 50.88 ; H = 8.56 ; OMe = 52.54 per cent.

x 2

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method was 2 IS, the calculated number for tetramethyl fructose being 236. The compound was therefore identical in composition and molecular weight with the syrup from which i t was obtained. It was very soluble in water and all organic solvents, except light petroleum, in which it dissolved sparingly. It acted vigorously on slightly warmed Fehling's solution. I3eing readily recoveretl in the crystalline state from its solutions, the ninterial at our disposal, although small, sufficed for the observations recorded bclow.

From the analogy of tetramethyl glucose (Trans., 1904, 85, 1052) crystallisation from light petroleum should give the stable tetramethyl a-fructose, showing in solution, like crystallisecl fructose, a downward mutarotation. On heating this for some time above i ts melting point and rapidly cooling, the sugar should then contain a considerable proportion of the unstable @form, which should be detected, if present i n suflicient quantity, by mutarotation in the opposite sense. The observations tabulated below confirm these conclusions.

To obtain a solution in which the p-form should be in excess of the proportion contained in the equilibrium mixture, the sugar, imme- diately before making up tlie solution, mas heated for two hours at 1 15-120°, and then solidified by cooling. The mutarotaiion was observed in approximately 5 per cent. solutions, a 2-dcm. tube being used.

Altcr recrys tallisation from light petrolenm.

7--\

Ethyl alcohol . . , .. . ... ... - 94'2" - 86.7" - 70.9" - 87.0" Methyl alcohol.. . , , , . , . ... 99.0 95'6 - - Water ... ... ... ... ... .. . ... 124'7 121.3 112.4 121 -0

After hcating for two hours at 115-120".

[al;'. [a]'," '

Solvent. Iiiitinl. Final, n.

The oppositely directed mutarotatious, it mill be seen, reach approxi- mately the same final values in similar solutions. The mutarotation in water was much more rapid than in the other solvents. The two upward changes of rotation in water and ethyl alcohol required two hours and seventy-nine hours respectively for completion. The rotatory power, as in the case of fructose, is higher in water than in tlie alcoholic solvents, I n view of the observations here recorded, the apparently anomalous mutarotation of the syrupy tetramethyl fructose previously referred to finds ready explanation. The liquid having been recently distilled, i t contained a larger proportion of' the @form than is present when equilibrium is reached in solution.

With the view of obtaining evidence of the production of a- and P-fructosides when the crystalline hexose is methylated, the compound was reconverted into tetramethyl methylfructoside by the silver oxide process and also by Fischer's method. The two products were then separately hydrolysed and the course of the action followed by means

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PURDIE AND PAUL : 'THE AT,T<YLA'I'ION OF D-FRUCTOSE. 297

of the polarimeter, as in similar experiments described in the last section.

Two grams of the ketose were used for methylation by the silver oxide process. The distilled liquid product, weighing 1 *3 gram, was too small in quantity t o be purified. I t s fructoside nature was, how- ever, evinced by tho absence of action on Fehling's solution, and i t s composition approximated t o tha t of tetramethyl methylfructoside. Analysis gave :

C = 52.15 ; H = S.92. C,H,O(OMe), requires C = 52.80 ; H = 8-80 per cent.

Its sFecific rotation in ethyl alcoholic solution (c = 5.03) was [u]; - 103.2". The substance was hyclrolysed by heating a solution of it (c = 2.SS) in 8.5 per cent. hydrochloric acid at 50". The specific rotation, however, contrary to expectation, did not increase and then decrease to approximately t'he value for tetramethyl fructose, bu t diminished uniformly during the process from [u].~,,~, - 114.7" to - 100*3", or, calculating the concentration for the theoretical yield

of tetramethyl fructose, t o - 106.1". That the ketose had been pro- dticed by the liydroljsis was proved, hqwever, by i ts recovery in the crystalline s ta te from the product.

For niethylation by Fischer's process, 1 gram of the crystalline ketoFe was used. The condensation proceeded rapidly at 50", the specific rotation of the solution increasing uniformly during the process from - 97.7' t o the constant value, - 135.6". The isolated syrupy product had no action on Fehling's solution. On hydrolysis, under the eonditions indicated above, the specific rotation decreased uniformly to the value recorded in the parallel experiment, namely, to - 106.2". I n this case also the product of the hydrolysis yielded the crystalline ketose. The final rotations reached on completion of the hydrolyses were less than the value for tetramethyl fructose, - 1 2 1 * 3 O , but this is probably attributable to a secondary action of the

hydrogen chloride on the sugar. On adding a solution of phenylhydrazine acetate to tetramethyl

fructose an oil is quickly deposited. This oil, after being washed with water, has no action on Fehling's solution even on boiling, bu t on treatment with hydrochloric acid and subsequent neutralisation tho product shows distinct reducing action. An alcoholic solution of the oil is dextrorotatory ; on adding aqueous hydrochloric acid the aolu- tion gradually becomes lzvorotatory, and simultaneously acquires reducing power. The oil could not be crystallised, bu t from these observations i t is doubtless a hydrazone.

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298 PURDIE AND PAUL: THE ALKYLATION OF D-FRUCTOSE.

Discussion of Results.

The results of previous researches show tha t the nlkylation of the alcoholic hydroxyl groups of aldoses does not produce any striking change in their rotatory powers. Thus, for example, the specific rotation of a-glucose is + 106O, tha t of tetramethyl a-glucose + 1 0 1 O . This statement also holds true in the case of fructose. The initial specific rotation of crystalline tetramethyl fructose in water is about - 1 2 4 O , that of the parent ketose about - 1 0 4 O . Fructose, like other

reducing sugars, is, no doubt, capable of assuming two stereoisomeric forms of the y-oxidic type, and, according to the now generally accepted view, the downward mutarotation of ordinary a-fnictpse implies a partial transformation into the less I~evorotatory p-form. This form has, however, not been isolated, nor has a solid mixture of the two forms been obtained which in solution shows the oppositely directed mutarotation due to the change p -+ a. I n the case of tetramethyl fructose, on the other hand, a solid mixture of this kind is readily obtained from the a-form by fusion as already described. Evidence is thus furnished of the existence of both forms of this sugar in the solid state.

Wi th respect to alkylfructosides, neither the a- nor the p-form of methylfructoside has as yet been isolated, but the changes of rotatory power which we observed during the production and hydrolysis of Fischer’s substance are probably attributable to its being a mixture of the two stereoisomerides. I n the case of tetramethyl methylfructoside no similar fluctuation of rotatory power was observed during its pro- duction from the syrupy or from the crystalline tetramethyl fructose by condensation with methyl alcohol. Observations on the change of rotatory power occurring during hydrolysis also failed to indicate the presence of stereoisomeric forms which hydrolyse at different rates. I n every case, whether the compound was prepared from methylfructoside by direct alkylation, or from the syrupy or crystalline tetramethyl fructose by Fischer’s process, or by the silver oxide process, its hydrolysis was accompanied by a uniform diminution of rotatory power. The failure to detect the more easily’hydrolysable p-form by an initial rise of rotation during the reaction may be accounted for either by its not being present in sufficient quantity or, more probably, by the rotatory powers of the two isornerides not being sufficiently wide apart.

The relative rotatory powers of the fructosides obtained from crystal- line tetramethyl fructose by the two methods ([a]: - 135.6’ and - 103.2O in alcoholic solution) are, however, in accord with previous experience of similarly prepared aldosides. The product of the silver oxide process

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PURDIE AND PAUL : THE AT,JCYT,ATION OF D-FRUCTOSE. 299

is the less I,levorotatory, and contains therefore the /I-isomeride in larger proportion.

The rotatory powers of the tetramethyl methylfructoside mixtures, which were obtained by direct methylation of methylfructoside, varied considerably, but were in every case much lower than the values just quoted for the frnctosides prepared from crystalline tetramethyl fructose. This cannot be accounted for by the former containing the p-form in larger proportion, as the syrupy ketose obtained from them and the ketoside mixtures prepared in t u r n from this ketose also showed correspondingly low I~vorotat ions. W e a t tribute the anomaly to the presence of dextrorotatory hexosides in the mixtures in question, which owed their origin to the occurrence of iso,neric changd during t.he prepnration of the original methylfructoside. Irvine and Cameron (Trans., 1905, 87, 907) encountered a similar anomaly in the prepara- tion of tetrametbyl methylgalactoside and tetriimethyl galactose from a syrupy mixture of methylgalnctosides. Our supposition is borne out by the following observations. The original methylfructoside syrup showed a n abnormally low rot:-Ltory power, [aIbue,. - 31.3", in aqueous solution, and the lz?vorotntion of the product of i ts hydrolysis was much less than t h i t of fructose. It was also previously mentioned that , on distilling one of the preparations of tetramethyl methyl- fructoside, a small slightly lower boiling fraction was collected which possessed a feeble dextrorotation. Further, a dextrorotstory compound, probably a tetramethyl hexose, was found to be presEnt in the syrupy mother liquor of the cryst:~lline tetramethyl fructose. On recovering this syrup from the porous porcelain in which it mas absorbed and dis- tilling it, a fraction collected at 140' under 16 mm. pressure showed the specific rotation" + 4".

W e propose making a further study of the composition of the methylfructoside mixture which results from Fischer's method of condensat ion.

CHEMICAL RESEARCH LABORATORY, UNITED COLLEGE OF ST. SALVATOR AND ST. LEONARD,

UNIVERSITY OF ST. ANDREWS.

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