295

NEIGHBOURING-GROUP EFFECTS IN THE CHEMISTRY OF

3,4-ANHYDRO-D-ALTRITOL

J. G. BUCHANAN AND A. R. EDGAR

Depcvrment of Organic Chedtry, The University, Newcastle upon Tyne NE1 7RU (Great Britain)

(Received October 4th, 1968)

AESTRACT

3,4--Anhydro-D-altritol has been prepared by the partial, acidic hydrolysis of its 1,2:5,6-d&0-isopropylidene derivative_ When treated with 2.5~ sodium hydroxide, 3,4-anhydro-D-altritol yields 1 &anhydro-D-altritol and 1,5-anhydro-L-glucitol as the sole products; 2,3-anhydro-D-iditol is postulated as an intermediate. 2,3-Anhydro- 1,6-di-U-triphenylmethyl-D-iditol, prepared by epoxide migration of 3,4-anhydro- 1,6-di-0-triphenylmethyl-D-altritol, yields 1 &anhydro-D-altritol and 1,5-anhydro-L- glucitol on acidic hydrolysis. Anhydrohexitols are minor products in the hydrolysis of 3,4-anhydro-D-altritol by 0.1~ sulphuric acid. Neighbouring acetoxyl-group participation is evident during the acidic hydrolysis of 1,2,5,6-tetra-O-acetyl-3,4- anhydro-D-altritol.

INTRODUCTION

In 1950, Bladon and Owen’ described the-preparation of 3,Panhydro-1,2:5,6- di-O-isopropylidene-D-altritol (5) and its behaviour towards acidic and basic reagents. Hydrolysis with aqueous sulphuric acid yielded D-mannitol (7) and D-iditol (8). Since cleavage of the epoxide ring in compound 5 by sodium methoxide was very slow, probably for steric reasons, it seemed likely that, in the acidic hydrolysis, 3,4-anhydro-D-altritol (6) would be an intermediate, prior hydrolysis of the acetal residues having taken place. As part of our study of epoxide migrations’ and related reactions3, we wished to prepare the anhydroaltritol6 and to examine its behaviour towards hydrolysis. 3,4-Anhydro-2,5-O-methylene-l,6-di-O-triphenylmethyl-D-alti- to14 was unsuitable for our purpose because of the stability of the methylene bridge.

REWLTS AND DIS(SUSSION

When the acetal 5 was hydrolysed with 80% acetic acid for 10 min at lOO”, 3,4-anhydro-D-altritol (6) could be isolated in 89% yield after silica gel chromato- graphy. The epoxide 6 reduced 2.1 mol. of sodium periodate in 48 h, ‘the fist mol. requiring 15 min. The early products of periodate oxidation were shown by paper chromatography to be 2,3-anhydro-D-ribose [(2) and (l)] and 2,3-aphydro-D-lyxose

Curbohyd. Res., 10 (1969) 295-305

296 J. G. BUCHANAN, A. R. EDGAR

[(3) and (4)]. The longer time required for the reduction of the second mol. of periodate is due to the preferential existence of the ring forms 1 and 4.

CHO CHO

OH HO

0 HCOH

I Heal

CH2OH I

CH,OH ? 4

/OC+k? Me2c\oC I YH20H y-9” tW’-’

HOCf-’ HOCH HOCU I

HOCH $- $;o- ; + “p

HCQH HOCH

HCO, HCOH HCOH HCOH I ,CMe2 I

CH20 I I

CH2OH CH2OH CU20H

= pkiL7 8

H2C ?W-

HOCH ; o/s

s”tQH Y20H

HO H q;

HOCH I

HCO I

3 I I I

HCQ HOCH HCOH HCQH I I

HCOH HCOH HCOH

s’al,/ +H\%OH

I

$ 1

HCOH

CM2 I

=‘-‘2 I

E 1

CH20CPhJ I

CH20CPhj I

HOCH HOCH I I 0’3 I

HCOH I I I

_*

HCO HOCH - “HP0

I I I I HCOH HCOH HCOH

I CH20H

I I I CHpi CH,OCPh~ CH,OCPh,

13 14 ‘15. 16 CH20H I

c

CH2 I

HCO

I

HOCH

HO+ HiOH 11

-6

HCOH I

OCH 1

HCOH

&I2

HCOH

!!$OH

37 18

*In these transformations the Fischer projection formulae have been turned through 180° in order to conform to rules of nomenclature.

When the anhydroaltritol6 was treated with 2.5N sodium hydroxide for 1.5 h at lOO”, only two products were detected by paper chromatography. They were

Cmbohyd. Res., 10 (1969) 295-305

&‘%AhWYDRD-D-ALTRITOL 297

easily separated by chromatography on a basic ion-exchange resin5, and shown to be 1,4-anhydro-D-altrito16 (14; 53O/a) and LJ-anhydro-t.-gLucitoL (l3; 27O/, by cornpa& son with authentic compounds. No free hexitol was detected. In order to account for the configurations of the products, the most likely reaction sequence involves formation of 2,3-anhydro-D-Iditol (10) by epoxide migration?, followed by intramolecular attack by the C-6 alkoxy-anion on C-3 or C-2. There are several precedents for the formation of a 5-membered ring in this way 8-1o. Study of a Dreiding model of compound 10 shows that ring closure to give a 6-membered ring should also readily occur, since the all-equatorial conformation of the anhydroglucitol13 can be achieved in the transition state. The preference for the five-membered ring is to be expected’ *, since ring-closure is irreversible.

Neither of the products of alkaline hydrolysis of epoxide 6 was derived directly by intramolecular attack by a terminal alkoxy-anion. In ring-opening reactions of cyclohexene oxides, the nucleophile approaches from an axial direction12, and this is the basis of the Fiirst-Plattner rule. A geometrically similar approach should occur in the case of vicinal epoxides which are not fused to another ring, and also in the case of intramolecular ring-opening. A study of Dreiding models shows that it is more difficult to reach a satisfactory position for such intramolecular attack in the anhydroaltritol 6 than in the anhydroiditol 10. This is borne out by some earlier work8’g*‘3- 16. For example, alkaline hydrolysis of l,Zepoxy4butanol yieldedI only 1,2,4-butanetriol. Even in the cases of 5,6-anhydro-1,2-O-isopropylidene-a-n- glucofuranose’4*1 5 and +r_-idofuranose’ 5* ’ 6, where the C-3 hydroxyl group is favourably orientated for intramolecular attack, 3,6- and 3,ianhydrides are relatively minor products.

In principle, epoxide migration in the anhydroaltritol6 can take place in either direction to give either the anhydroiditol 10 or 2,3-anhydro-D-man&o1 (11). The former should be favoured thermodynamically, since it is a tram- epoxide, and kineti- cally, since the C-5 oxygen atom has an erythro relationship to C-4 in the anhydro- altritol 6. The intramolecular nucleophilic attack by the C-6 oxygen atom in the anhydroiditol 10 must occur very rapidly, because no other products were detected, e.g., those which would be expected to arise from the terminal epoxide. We decided to study epoxide migration in a derivative of epoxide 6 in which the primary hydroxyl groups were protected in order to prevent their participation in intramolecular reactions.

Compound 6 was converted into its di(triphenylmethyl)ether 16, which was treated with sodium methoxide in methanol to give 2,3-anhydro-1,6-di-U-triphenyl- methyl-D-iclitol(15) (84% yield). Whereas the anhydroaltritol 16 could be converted back into the epoxide 6 by brief treatment with 80% acetic acid, similar treatment of the anhydroiditol 15 gave only l&anhydro-D-altritol (14) and 1,5-anhydro-I.- glucitol(l3) in approximately the same ratio as in the alkaline treatment of epoxide 6. No anhydroiditol 10 could be detected by paper chromatography_ It is known that vicinal epoxides bearing a suitably situated hydroxyl group are susceptible to intra- molecular scission under acidic3 * ” - ’ ’ as well as alkaline conditions. In particular,

Carbohyd. Res., 10(1969)295-305

298 J. G. BUCHANAN, A. R. EDGAR

2,3-anhydrohexoses have been shown to be readily transformed into 3,6-anhydro- hexoses by acid3, and the products obtained by acidic hydrolysis of epoxide 15 are a clear indication that it has the 2,3-anhydro-D-SO structure shown. Although it is possible to envisage Pmembered oxides (oxetanes) as intermediates in the above transformations, we do not consider this likely, either in these experiments, or in those to be described below.

In the light of these results, we re-investigated the hydrolysis ’ of the diacetal5 by dilute sulphuric acid. Paper chromatography showed that the main product was a mixture of hexitols, and that smaller amounts of anhydrohexitols were also present. The products were separated, by chromatography and by crystallisation, to give D-iditol (8; 45%, as the hexa-acetate), D-mannitol (7; 33%), 1,4-anhydro-D-altritol (14; 4%), 1,5-anhydro-~glucitol (13; 3%), 3,6-anhydro-D-altritol (12; 3%), and 1,4-anhydro-D-mannitol (9; 2%). The isolation of D-iditol (8) and D-mannitol (7) as the major products is in complete agreement with the results of Bladon and Owen’. These products clearly arise by simple, acid hydrolysis of the 3,4-epoxide ring. The mixture of anhydrohexitols was more complex than that arising from the alkaline reaction. Two of the compounds, 13 and 14, were the major products from the treat- ment of the anhydroaltritol 6 with alkali, and it would appear that some epoxide migration to the anhydroiditoll0 has occurred under acidic conditions. 3,6-Anhydro- D-altritol(l2) would then arise from the 2,3-anhydromannitol 11, the other possible epoxide-migration product, and 1,4-anhydro-D-mannitol (9) by intramolecular ring- opening in the 3,4_anhydroaltritol 6 itself. There has been no previous indication of epoxide migration under acidic conditions in the chemistry of epoxides of cyclic sugars3 or of inositols’. It is possible, however, that such reactions might be more favoured in an acyclic case where free rotation may assist in the attainment of the required transition state. Hudson and Barker 22 have recently irvoked vicinal epoxide formation in order to explain the multiplicity of anhydropenritols resulting from the acid-catalysed dehydration of D-arabinitol which, due to steric factors, reacts more slowly than ribitol or xylitol.

It is interesting that, among the anhydrohexitol products arising from treatment of epoxide 6 with acid, no 2,6-anhydro-D-glucitol (17) or 1,4-anhydro-D-iditol (18) were isolated. The anhydroglucitol 17 is the six-membered ring product expected to arise from the 2,3-anhydromannitol 11. Its possible presence cannot be excluded, because an authentic sample was not available. The l+anhydroiditol 18 might be expected to be formed, together with the 1,4-anhydromannitol9, by intramolecular attack of a terminal hydroxyl group on the 3&epoxide ring of compound 6. It was shown not to be a product by paper-chromatographic comparison of hydrolysis mixtures with the authentic compound. When the two cyclisation processes (6-+9; 6+18) are studied by means of Dreiding models, the one leading to compound 9 is more favourable than that leading to compound 18. In the latter case, hindrance to attack at C-4, due to the C-5 hydroxyl group, occurs in the extended conformation of the carbon chain.

We considered the possibility that some of the anhydrohexitol products from

Carbohyd. Res., 10 (1969) 295-305

3,4-ANHYDRO-D-ALTRlTOL 299

the acidic hydrolysis of epoxide 6 might arise through the formation of sulphuric esters (cJ ref. 23). It was found, however, that the use of dilute perchloric acid, whose anion is a very weak nucleophile, or additions of sulphate ion had no effect on the product distribution. It is concluded that ester intermediates are not involved.

It was of some interest to examine the acid hydrolysis of the tetra-acetate 19 of the 3,4_anhydroaltritol6. It would be expected that hydrolysis would occur much more easily than in the parent epoxide 6, because of neighbouring-group participation by a vicinal acetoxy-group 24-26. In cyclic systems, there is a stereochemical require- ment that the neighbouring acetoxy-group must be trarrs to the epoxide ring. In the case of compound 19, either the C-5 acetoxy-group or that at C-2 should be capable of participation, with the formation of tetraacetates (20,21) of D-iditol and D-mannitol, respectively. It would be expected that the C-5 acetoxy-group in compound 19, having the erythro-relationship to C-4, would participate more readily than the C-2 acetoxy group. These expectations were borne out when it was found that hydrolysis of the acetate 19 with 80% acetic acid at 100” was complete in 15 min, conditions which hardly affect compound 6. The crude product, presumably a mixture of com- pounds 20 and 21, was deacetylated and examined by paper chromatography. Iditol(8) and mannitol(7) were the sole products, and were characterised as D-iditol hexaacetate (80%) and D-mannitol (15%). In the acidic hydrolysis of epoxide 6, the iditokmannitol ratio was 45:33.

CyOAc

I CH,OAc I

CH,OAc

I AcOCH

_ 1

OCH Ac.H i

HCO

I -8 HOCH ‘HOCH

I I I H COAc HCOAc HCOAc

I I I C+OAc CHpc CH20H

19 20

I

CH20AC I

CH=OAC

i

I ,,O-CH OCH

Mech-{H _ Ac.H 1

OCH

I -7

HCOH HCOH I I

HCOAc HCOAC

I I CQOAc cH=oAc

21

*In this transformation the formula has been turned through 180°.

There has recently been renewed interest in vicinal epoxides derived from hexitols27*2s.

Curbohyd. Res., 10 (1969) 295-305

300 I. G. BUCHANAN, A. R. EDGAR

EXPERIMENTAL

General ntethods. - Evaporations were carried out under diminished pressure with a bath temperature below 40”. Z&i-ared spectra were measured for potassium bromide discs. Melting points are uncorrected. Comparison of materials with authentic substances was made, unless stated otherwise, by mixed m-p. determination, infrared spectra, and paper chromatography. “80% Acetic acid” refers to acetic acid-water (41, v/v).

Chromatographic methods. - Adsorption chromatography was carried out with silica gel (Hopkin and Williams). Triphenylmethyl ethers were chromatographed on silica gel which had been neutralised with ammonia and reactivatedz5. CeIIulose columns were made from Whatman cellulose powder (Standard Grade; W. and R. Balston Ltd). Thin-layer chromatography (t.1.c.) was carried out with Kieselgel G (Merck) as adsorbent; compounds were detected with anisaldehyde-sulphuric acid2g. Paper chromatograp y h was by the descending technique, using Whatman No. 1 paper, except in the case of hexitols, when No. 4 paper was used. Solvent A was butanone, saturated with water, the paper being pre-equilibrated in the solvent vapour before irrigation; solvent B was butyl alcohol-pyridine-water (3:l:l by vol). Hexitols and reducing sugars were detected by alkaline silver nitrate3’, vicinal glycols by periodate and SchifYs reagent3 r, and vi&al epoxides by sodium iodide and methyl red25. The latter spray was also used on t.1.c.

3,4-Anhydro-D-altritol(6). - The acetal’ 5 (6 g) was heated for 10 min at 100” with 80% acetic acid (10 ml). Paper chromatography in solvent A showed that the major product gave a purple colour rapidly with periodate and Scbiff’s reagent, and a yellow colour with sodium iodide-methyl red. The solution was evaporated to dryness, the residual syrup dissolved in chloroform (50 ml), and the solution chromato- graphed on a column of silica gel (65 g). Chloroform-ethanol (3:2) eluted compound 6 which was obtained as a syrup (3.5 g, 89%), [a],-, +7.8” (c 2.12, water) (Found: C, 43.5; H, 7.6. C6H1205 talc.: C, 43.9; H, 7.4%).

The compound reduced periodate3’ as follows: 0.98 mol. (15 min), 1.4 mol. (45 min), 1.6 mol. (6 h), 2.1 mol. (2 and 4 days).

3,PAnhydro-D-altritol (6; 30 mg) was treated with a solution of sodium periodate (40 mg) in water (1 ml). After 1 h, periodate and iodate were removed by precipitation with barium chloride. Paper chromatography of the product (solvent A) showed the presence of two compounds, each reacting with alkaline silver nitrate and with sodium iodide-methyl red. They had the same RF value as 2,3-anhydro-L- ribose (0.50) and 2,3anhydro+lyxose (0.57), chromatographic samp!es of which were prepared by brief, acid hydrolysis3 (0.1 N sulphuric acid for 20 min at 100°) of the corresponding methyl /3-r_-glycopyranosides2*33.

Alkaline Jlydroiysis of 3&anhydro-D-altritoi (6). - Compound 6 (3.0 g) was heated for 1.5 h at 100” with 2.5~ sodium hydroxide (10 ml). The solution was passed through a column of Dowex-50 (NH,+) resin, and the combined eluate and washings evaporated to a syrup. Paper-cbromatographic examination of the

Cwbohyd. Res., 10 (1969) 295-305

3,4-ANHYDRO-D-ALTRITOL 303

bined with fractions 3 and 4 from the cellulose column (total 23 mg, 3%). The syrup was treated overnight with acetone (10 ml) containing cont. sulphuric acid (0.1 ml). Isolation with chloroform yielded a syrup (10 mg), which crystallised. 3,6-Anhydro- 1,2:4,5-di-O-isopropyhdene-D-altritol had m.p. 45”, undepressed when mixed with an authentic sample, m-p. 48-52”. Its infrared spectrum was indistinguishable from that of an authentic sample3* 23.

TABLE II

FRACTIONATION OF PRODUCTS OF ACID HYDROLYSIS OF COMPOUND 5.

Fraction Vol. of fraction Content of fraction No. (ml)

500 1,4-Anhydro-D-altritol (14). l&anhydro-D-mannitol (9) 50 14,9, 3,6-Anhydro-D-altritol (lZ) 75 12 75 12

75 12, 1.5~Anhydro-L-glucitoi (13) 25 13

125 Hexitol 975 H&to1

Water next eluted I &anhydro-D-mannitol (9) (12 mg, 2%), which crystallised from ethanol; m-p. 140-142”, indistinguishable from an authentic sample?6 prepared from methyl 3,6-anhydro-a-D-mannopyranoside3’ by acid hydrolysis and boro- hydride reduction_

Finally, water eluted 1,4-anhydro-D-altritol (14) (30 mg, 4%). Recrystallised from ethanol, it had m.p. 105-106”, [a]n + 14.2” (c 1 .O, water), and was indistinguish- able from an authentic sample.

Fractions 7 and 8 from the cellulose column were evaporated to a syrup (805 mg), which was dissolved in hot ethanol (5 ml)_ On cooling, D-mannitol(7) (320 mg, 33%) crystallised, m.p. 160-l 62”, indistinguishable from an authentic sample. Paper chromatography of the mother liquors in solvent B showed the presence of id&o1 with onIy a Iittle mannitol. The liquors were evaporated to a syrup (450 mg) which was acetylated with acetic anhydride (5 ml) in pyridine (30 ml) overnight. Isolation by means of chloroform yielded D-iditol hexaacetate (809 mg, 45%), m.p. 122-123”, indistinguishable from an authentic sample3.

Small samples of epoxide 5 were treated severally with 0.1~ sulphuric acid, 0.1~ sulphuric acid plus sodium sulphate (2 mol.), and 0.1~ perchloric acid for 4 h at 100”. The products in each case were examined by paper chromatography in solvent A and were indistinguishable.

1,2,5,6-Tetra-O-aceryZ-3,4-a~r~-~-~~~rjf~~ (19). - Compound 6 (0.5 g) was acetylated with acetic anhydride (3 ml) in pyridine (15 ml) overnight at room temper- ature. Isolation by means of chloroform yielded a syrup (0.9 g, 87%) (which gave a positive reaction with the. sodium iodide-methyl red spray). The acetate, b.p.

Carbohyd. Res., 10 (1969) 295-305

304 J. G. BUCHANAN, A. R. EDGAR

140”/10- 3 mm, had [c&, +4.4” (c 10.3, chloroform) (Found: C, 50.2; H, 5.7. C14H2,,09 talc.: 50.6; H, 6.1 “A).

Hydrolysis of acetate 19 with aceiic acid. - Acetate 19 (0.5 g) was heated with 80% acetic acid (3 ml) at 100’. Examination of the reaction mixture by t.1.c. (benzene-

ether, 1:l) showed that no epoxide remained after 15 min. The solution was evapo- rated, and the syrupy product was deacetylated with methanolic sodium methoxide. Paper chromatography in solvent I3 showed that the product contained mainly iditol, with a small proportion of mannitol. The hexitol mixture (0.27 g) was acetyl- ated with acetic anhydride (3 ml) in pyridine (10 ml) overnight at room temperature. AchloroformisolationprocedureyieldedD-iditolhexaacetate(443mg,80%)m.p. 122-4” (from ethanol), indistinguishable from an authentic sample3. The mother liquors were deacetylated, with sodium methoxide in methanol, to yield D-mannitol (7)

(50 mg, 15%), m-p. 165O (from ethanol), identical with an authentic sample.

ACKNOWLEDGMENTS

We thank the Northumberland Education Committee for a Postgraduate Award (to A. R. E.), and Dr. Robert Barker for a gift of 1,4-anhydro-D-altritok

REFERENCES

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CurborZyd. Res., 10 (1969) 295-205

3,4-ANHYDRO-D-ALTRKOL 305

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Curbohyd. Res., 10 (1969) 295-305