Structure of a photo-dimer of vitamin A

Translated from french by D A Lerner from the original manuscript of the following article:

Structure d'un photodimère de la vitamine A(*)by Magdeleine Mousseron-Canet, Dan Lerner et Jean-Claude Mani; .published in the Bulletin de la Société Chimique de France, 11, 4639-4641, 1968 (Manuscriptsubmitted on the 2.2.1968) - http://www.societechimiquedefrance.fr/(all authors are from the Laboratoire de Photochimie et de Stéréochimie associé au CNRS andÉcole Nationale Supérieure de Chimie de Montpellier, 8 rue de l'École Normale, 34 296 -Montpellier, Cedex 5, France).

AbstractSpectroscopic data and mainly mass spectrometry allow to specify the mode of dimerisation ofvitamin A or of its acetate under UV irradiation. Theoretical forecasts agreeing with the observedIR results and concerning the stereochemistry of this dimer are reported.

Keywords: dimerisation; vitamin A; retinol; retinyl acetate; photochemistry; dimer; kitol;stereochemistry; mass spectroscopy

Following upon a series of works concerning the photochemical behaviour of polyenes (1), a studywas undertaken on the isomerisation of vitamin A acetate 1 and all-trans retinol 2 under the effetUV light (2, 3).The first experiments done in methanol show the very fast disappearance of the acetate group ofvitamin A acetate, 1, with a flattening of the maximum at 325 nm and the simultaneous growth ofbands on both its low and high energy sides. The infrared shows the weakening of the trans HC =CH group at 960 cm-1 , the total disappearance of the C = O band at 1745 cm-1 and of the C-O at1250 cm-1 and the concomitant appearance of a strong band at 1020 cm-1. The latter is ypical of theC-O vibration of an ether and allowed to conclude to the loss of a molecule of acetic acid and to thelikely binding of methyl alcohol (2).

In solution in hexane, the irradiation takes a different course and leads to the formation of severalproducts which are separated by thin layer chromatography on silica. On one hand, some left overretinyl acetate was isolated, mostly isomerised in position 9 or 13. On the other hand several stripswere separated corresponding to dimers, oxidized or not, and some containing a mixture ofcompounds. Of all these products of irradiation, the most abundant obtained with a yield varyingbetween 30 and 40 % depending on the experimental conditions (concentration, duration ofirradiation, age of the lamp), is an asymmetrical dimer C44H64O4, (M = 656) (3).Compound 3 behaves as a unique species during the various chromatographic separations (changeof support, éluent composition, use of additives such as silver nitrate). All these extracts giveidentical IR and UV spectra and same colour in the Carr-Price reactions. The UV spectrum shows avery wide, asymmetric band, with its maximum at 290 nm (ε= 36 500) and a secondary maximumat 260 nm (ε = 29 000) in hexane. This data suggest the presence of two independent chromophorespossessing respectively four and three double bonds. On the IR spectrum a decrease in the intensityof the trans HC = CH deformation band is seen at 960 cm-1, relative to product 1, while the rest ofthe spectrum is hardly modified.

Dan A. Lerner, Doctorate in Physical Chemistry, University of Montpellier, 1968.

On the mass spectrum of 3 a very weak molecular peak appears at m/e = 656, together with peaks atm/e = 596 and m/e = 536 due to the loss of one and two molecules of acetic acid respectively. Thebase peak found at m/e = 328 corresponds to the monomer and results either from a retro Diels-Alder type fragmentation or from a thermal decomposition of 3. The fragmentation of the ion at m/e= 328 is identical to that of 1.

A priori, a photochemical irradiation should give geometrical isomers and cyclobutane ringcontaining dimers, but the preliminary results obtained for 3, lead us to compare the latter to kitol,4, a natural dimer of vitamin A isolated from fish liver. In particular the UV absorption spectrum is,within the limits of experimental errors, identical to that of kitol (4).Recently, the structure of kitol was solved by NMR (5) and by mass spectroscopy of perhydrokitol(6): it fits with a Diels-Alder's type addition of two molecules of retinol 2, and counters thesymmetric structure 7 (4) previously proposed.

Table 1

Photo dimer di-acetate 3 Photodimer 4 KitolCH3 in C1 and. C'1 9.00 (12H) 9.00 (12H) 8,99CH3 in C5 and C5' 8.33 (6H) 8.33 (6H) 8.34CH3 in C9 and C13 8.22 (6H) 8.24 (6H) 8.22

CH3 in C'9 8.10 (3H) 8.12 (3H) 8.13COCH3 8.01-8.04 (6H)CH2OAc 5.42-5.86CH2OH 5.85, 6.20, 6.35 (4H) 6.28

H7-H8 & H'7-H'8 3.94-3.98 (4H) 3.88-4.05 (4H) 4.02-4.06H10 4.58 multiplet (1H) 4.58 doubletH12 4.80 multiplet (1H) 4.86 doublet

Peak position in τ units.

Figure 1

A comparison was made between the NMR spectra of 3, that of its reduction product by ALiH4 of3, and of kitol 4. The similarity of the signals draws the fonctional analogy of the compounds.Furthermore the shape of the signal of protons 7 and 8, and 7' and 8' reveals a 9-transstereochemistry for the polyene chains. These protons constitute an A2 system giving a single peak.In comparison, a 9 cis stereochemistry in the whole series of the homologous polyenes of ß-iononeresults in the appearance of a quartet, typical of an AB system characterized by a value of the ratioϕ = ⏐D⏐/J7-8, of the order 2 (15) (*).

The previous results do not allow to choose between structures 3 and 6 resulting respectively to anaddition according to a parallel (A) or antiparallel (B) organization of the monomers (fig. 1)

In the case of kitol, Lederer et al (6) showed that process A only intervened: the mass spectrum ofperhydrokitol (*) shows a peak at m/e = 458, the formation of which is only compatible with theproposed structure. Compound 3 was hydrogenated in acetic acid at atmospheric pressure, in thepresence of reduced platinum oxide. The chromatography on a column of silica allows to separate amono-acetylated dimer (**) as the second acetate underwent an hydrogenolysis, and theperhydrogenated di-acetatylated dimer (**) 5. In the mass spectrum of 5, the molecular peak at m/e= 672 is found as expected. Various fragments were identified (figures 2a and 2b):

(1) The elimination of a molecule of acetic acid gives a fragment at m/e = 612: this fragmentunderwent a second time the process to give the peak at m/e = 552.

2) From these last fragments or from the parent peak, the elimination of the side chain withmigration of the proton in 11 leads to peaks at m/e = 418 and 358.

3) Peaks at m/e 389 and 329 result from the rupture of bond 12'-13' with retention of thepositive charge on the tertiary carbon 13'.

4) The formation of a tertiary carbonium in 13 ' resulting from the rupture of the 13'-14'bond is followed by the breaking of the 11-12 bond with migration of a proton to lead to a fragmentat m/e = 458 in which the positive charge is stabilized by the insaturation in allylic position.

In the spectrum of 5 the presence of the peak at m/e = 458 implies that both alkyl chains of 3 arebound to neighboring carbons of a 6 elements cycle.

(*) 1. A possible photoisomérisation of the all-trans retinyl acetate to give the 9-cis isomer whichcould dimerize later into a π2+π4 Diels-Alder type manner, would lead to a product having a 9-cischain that would however possess the same configuration for the central 6 carbon cycle.2. A possible photoisomérisation of the all-trans monomer into the 13 cis monomer would lead to adimer with a different configuration from that observed by means of IR spectroscopy.(**) For all these compounds the reduction of the numerous insaturations results in the formation ofa mixtures of diastereoisomers the nature of which is not to be envisaged here.

____________________________(*) 1. A possible photoisomérisation of the all-trans retinyl acetate to give the 9-cis isomer whichcould dimerize later into a π2+π4 Diels-Alder type manner, would lead to a product having a 9-cischain that would however possess the same configuration for the central 6 carbon cycle.

2. A possible photoisomérisation of the all-trans monomer into the 13 cis monomer would lead to adimer with a different configuration from that observed by means of IR spectroscopy.

(**) For all these compounds the reduction of the numerous insaturations results in the formation ofa mixtures of diastereoisomers the nature of which is not to be envisaged here.

In the conditions where the hydrogenation takes place, the fast hydrogen addition at the beginning,becomes very slow when an equivalent of about one double bond left is reached. The massspectrum of 3 hydrogenated up to the stage where no UV absorption above 210 nm was observedwas recorded. The analysis of this spectrum shows that for every ion of mass P containing thecentral cycle, corresponds a peak at mass P-2, and that at mass P-4 there is no significant peak, oreven no peak: we thus have at most a double bond per molecule of dimer in the mixture (the lattercontains between 60 to 70 % of totally hydrogenated product). Finally, we note that there is no peakat m/e = 458-2: thus the central cycle carries the residual double bond.

Figure 2a

This constitutes an additional proof in favour of structure 3, and eliminates the possibility ofexistence of cyclobutane ring containing compounds in measurable quantity that could have beenmixed to 3. It is important to notice that this mono-unsaturated product gives by a retro Diels-Aldermechanism, peaks at m/e = 334 and m/e = 336, which do not exist in the spectrum of 5.

Figure 2b

Compound 5 was then reduced by aluminum-lithium hydride, and the resulting diol 8 wasdehydrated to form a tetrahydrofuran cycle by-product 9. In the mass spectrum of 9 the molecularpeak is seen at m/e = 570 (C40H74O). An intense peak is found at m/e = 347: it results from the bondbreaking that gave the peak at m/e = 329 in 5. Other fragmentations on the side chains are obtainedin the same conditions as for 5 at m/e 376, 555, 446 and 418. The retro Diels Alder process on thecentral cycle gives either the peak at m/e = 458, which confirms the structure of the tetrahydrofuran(which is fused to a cyclohexane ring) or peaks in m/e = 293, 265 and 276, which could beobtained as shown in figure 3 (fig. 3). A peak at m/e 319 is also observed for the terahydrofuranderivative of kitol (6).

Figure 3

All these results do not however provide information on the configuration relative to the centralcycle of 3 (or 5).It seemed to us interesting to try to deduct this configuration using the models proposed amongothers by Woodward and Hoffman (7) on cycloaddition reactions by considering that we had a herea one-step Diels-Alder addition. In this case the quoted authors showed that correlation diagramslinking various molecular orbitals (MO) of the diene and dienophile, could be interpreted in acoherent way if the reaction occurred in the ground state (8).

These two molecules must each possess an orbital the symmetry of which is such that it wouldfavour a charge transfer interaction in the transition state which mixes them. This interaction shouldcorrespond to a lowering of the energy of the transition.state to allow the reaction to proceed. Theseconditions are met if we consider that it takes place between the highest occupied MO of the dieneand the lowest unoccupied MO of the dienophile and vice versa. Furthermore, in the case of theaddition of a pair of molecules which are both simple dienes, or among which one is a simple dieneand the other one a classical dienophile, the so-called endo addition is favored (7) (fig. 4a). Otherauthors, such as Fukui for example, have particularly detailed these points of view (9).

Figure 4a,b

The symmetries of the MOs of retinyl acetate were determined by the method of Hückel, completedby an iteration process (see experimental section). These symmetries are the same for all isomers ofretinyl acetate (fig. 4b). The resulting MOs are such as the highest occupied is symmetric and thelowest empty is antisymmetric with regard to the mediating plan of the combined chains. It must beemphasized in the present specific problem, that the diene and dienophile are only parts of a muchmore extended conjugated systems. Accordingly it should be necessary, in the phase of approach of

both molecules situated in parallel plans, not only to take into account interactions situated near thereaction center, but also to consider those that can occur further on the polyenes chains.An examination of molecular models allows to notice that the overlapp of the MO in 12,12 'necessary for the classical endo addition, requires for the chains a V shaped orientation thatprevents any possibility of further interactions along the rest of their length (fig. 5).

Figure 5

Such an addition leads then to the compound having the following stereochemitsry: chains are cis e-a and the -CH2OAc groups are trans. On the other hand if we position both reacting molecules so asto form the other possible dimer in which the neighbouring position of the chains is respected, thelatter orient themselves in a parallel mode and are almost superimposed.. It is observed that the lackof interaction in 12-12' is compensated by two interactions in 9-11' and in 7-9' which are binding,and consequently lower the energy of the transition state. Furthermore, no steric factor preventssuch an approach (fig. 6).

Figure 6

In this case the following stereochemistry is obtained: both chains are in a trans di-equatorialpositions and the CH2-OAc groups linked to carbons 14' and 14 are respectively e and a, in cisposition (fig. 7).

Figure 7Stereochemistry of photodimer 3.

This Diels-AIder's dimerisation of 3 is thermally reversible and gives back the all–trans monomers.The latter being thermodynamically the most stable, its stericity cannot make foresee that of the sidechains of the dimer. This mode of dimerisation cannot at first sight explain the lack of success of asynthesis by a thermal way, noticed until now; as the dimer is only obtained following UVirradiation. It is necessary to note that the energy of activation for a thermal reactions should growup with the total number of ! electrons, since the number of bonds undergoing a change in theirtheir length in the transition state increases. So, the hypothesis could be put forward according towhich one of the molecules is excited to the first singlet excited state and would then undergo anisoenergetic relaxation to a highly excited vibrational state of the ground state, allowing its reactionwith another ground state monomer. This would simply require a reaction rate superior to the onethat is observed on average for bimolecular reactions due to the probable formation of a weakground state association of the dimer.

The luminescence of 1 and 2 was also examined. Spectra were recorded in a non-polar solvent(hexane) and in a polar one (ethanol), in fluid solution at 25 °C or in rigid matrix (EtOH or EPA) at77 °K, at various concentrations. In every case fluorescence only was observed. Attempts to favourany phosphorescence, by vacuum degassing to eliminate the oxygen, or using ethyl-iodide to favouran heavy atom effect remained negative. This agrees with the general observation that the firstexcited singlet and triplet states of these polyenes have a ! - ! character and the energy gap betweensuch levels is generally too large to allow an easy conversion of spin. For example in a 3.10-5 Msolution in hexane, at 25 °C, the values of 355 nm for the excitation maximum and 495 nm for thefluorescence maximum (uncorrected values) are recorded: the height of 1S1 is thus of the order of 60kcal above 1S0, and the relaxation of 1S1 to 1S0 leads to a very hot fundamental state.The conclusions on the structure find a support in the examination of the IR spectrum of theheterocyclic tetrahydrofuran cycle 9 prepared by dehydration of the perhydrogenated diol 8.Previous publications indeed allow to establish a correlation between the value of the IR frequencyof the C-O-C vibration and the configuration of the junction of a tetrahydrofuran with an hexene orhexane cycle. These results were established in particular for tetrahydrofuran heterocycles derivedfrom myrcene and presenting an olfactive interest (10). The same is true for other simplercompounds (11) and in the perhydrophenanthrenic series (12). In summary, the C-O-C vibration issituated in the neighborhood of 1020 cm-1 for a trans junction and 1060 cm-1 for a cis junction. Alsothe more or less complete saturation of the alicyclic six elements ring does not bring notablechanges (Table II). These results are perfectly fit for a comparison with compound 9, the relatedstructure of which was proved.

Table 2

The reduction of the diacetylated dimer 5 by LiAlH4 leads to the saturated diol 8 showing afterdilution an OH band bound at 3507 cm-1 in the IR. This diol was dehydrated according to threedifferent processes: in benzene in the presence of traces of p-toluenesulfonic acid, in acetone in thepresence of traces of sulphuric acid, and finally by reduction by means of LiAlH4 of thecorresponding dimesylate (figure 8). In every case, a saturated, low polarity product that had noalcohol group is obtained. It shows in the IR a band at 1060 cm-1 and another one at 890 cm-1.

Figure 8

Integration on the NMR spectrum gives 4 protons in the form of a very wide multiplet, around 6,25τ; the centesimal analysis fits with the formula C40H74O. These results imply the formation of thetetrahydrofuran 9 with a cis connection, and hence for the photodimer, the cis relation for the -CH2OAc substituents. It should be noted that the attempts of deshydration of the trans diols inacetone in presence of sulphuric acid lead to by-products of acetone almost quantitatively). Thismethod leads exclusively to tetrahydrofuran heterocycles from the cis diols, giving a supplementaryproof of the proposed stereochemistry.The theoretical predictions agreeing with the infrared results allow to attribute the cis configurationto the neighboring methylols groups of photodimer 8. Besides, it seems reasonable to assign a transdiequatorial configuration to the the cumbersome side chains. The structure proposed for the

photodimer 3 of Vitamin A is displayed on figure 7. It seems that one could attribute to theperhydrogenated dimer of vitamin A 3, a structure similar to that of perhydrogenated kitol.For the moment, the data obtained do not allow to draw conclusions concerning the identity of thephoto-dimer itself with kitol. It is not impossible that in spite of the use of quasi monochromaticlight, an easy secondary photoisomerisation on the residual polyenic chains (1) introduces aplurality of structure at this level.The interesting conclusion of this work to be retained is that a common dimer structure results froman enzymatic or a photochemical process.

EXPERIMENTAL PART

General indications.The NMR spectra are recorded on a Varian A 60 with CDCl3 as solvent, and the chemical shifts aregiven in τ units, with tetramethylsilane serving as internal reference; IR spectra are are taken on aPerkin-Elmer 221 or an IR 8 Beckman, and UV spectra on a Unicam SP-800. The mass spectrawere recorded on an AEI MS 9 and a Varian M 66. Measurement of luminescence were made on aJobin-Yvon spectrofluorimeter (Bearn type).

Photodimer of all-trans retinyl acetate, 3

100 mg of all-trans retinyl acetate 1 (Hoffmann-La Roche) are dissolved in 130 cm3 of hexane (*)(MCB, spectrographic grade), and irradiated for 12 minutes in a Pyrex reactor. The solution iscooled by water running in a double wall unit dipping in the centre of the reactor and containing thelamp. The latter is a Philips HPK 125 and a stream of nitrogen R is bubbled through the solution towhich magnetic stirring is applied. The rest of the operations is done in a glove box, kept under anitrogen R atmosphere and located in a darkroom (A 40 W illuminator with a KD chloro 2 filter isused were necessary). After vacuum evaporation with cooling of the irradiated solution, theproducts are deposited on a silica IG plate according to Stahl (20 x 20 cms, 0,5 mm in thickness)and eluted with a (1:10) methyl-ethyl-ketone:heptane mixture. After elution the plate reveals thepresence of several products among which some are fluorescent (excited at 360 nm). Two productsare less eluted that the dimer, the one fIuorescent, in too small amount to be studied, and the otherone not fIuorescent, the analysis of which corresponds to the fixation of a molecule of oxygen onthe dimer:

(*) Hexane: not the best solvent for 1, but used because of its photochemical neutrality: it canbehave neither as sensitizer, nor as quenching agent.

Analysis C44H64O6: Calc. %: C 76,7 H 9.36 O 13,93 found 76.36 9,34 14,72.

The spectroscopic study (IR, UV, NMR) of this compound suggests the formula of a dimer, but itsstructure was not determined. It is probably a mixture.Products having Rf superior to 0.45 approximately, are fIuorescent and correspond to monomers.The most important fraction is constituted by the initial acetate and its isomers.The dimer 3 forms a non fIuorescent wide yellow strip on the plate, with a Rf close to 0.35. In thepresence of the Carr-Price reagent, a dark blue colour is obtained, while in the same conditions theacetate monomer gives a lively blue. This band is scraped from the plate and extracted withanhydrous ether (40 mg are obtained).

UV: λex = 290 nm ε≈ 36500: λ ex = 260 nm, ε≈29 000 (hexane).IR: 1730-1240-1030 cm-1 for acetates groups - 970 cm-1 for- trans-CH = CH groups.NMR: `See table 1.

Molecular distillation of the photodimer 3.

By distilling the photodimer under a of 10-4 Torr vacuum at 220 °C approximately, a fraction of all-trans retinyl acetate is recovered (approximately 1,1 mole from a mole of dimer 3).

Synthesis of the photodimer of trans-retinol

In the same conditions as above, starting from all-trans retinol 2, but eluting with a methyl-ethyl-cetone/heptane mixture (v/v:40/60).

UV: λ. = 290 nm, ε# 38500λ = 260 nm, ε# 32000 (EtOH).

IR: 3630 cm-1 (free O-H group), 965 cm-1 (trans CH = CH).NMR: See table 1.

Reduction of the di-acetate photo-dimer by AlLiH4.

The cooled solution in ether, under an atmosphere of nitrogen R and protected from light leads tothe corresponding diol, identical to that obtained by the photochemical dimerisation of retinol 2.Oxidation tests on this diol by MnO4 suspended in ether in the conditions where retinol is oxidizedinto retinal leave sthe alcohol functions untouched. A prolonged action of MnO4 leads to adegradation of the molecule.

Tests of thermal dimerisation

Solutions of all-trans retinyl acetate in hexane (1-2 mg / cm3) are maintained at varioustemperatures varying from 30 to 60°C, protected from light and blanketed by nitrogen. Variousheating times were tried, from 5 mn to 1 hour. In these experimental conditions we were not able toisolate a compound presenting physico-chemical characteritics comparable to those of thephotodimer.

Perhydrogenation of the photodimer 3300 mg of 3 are dissolved in 20 cm3 of acetic acid in the presence of 200 mg of platinum oxide(reduced beforehand) and are hydrogenated at ambient temperature and pressure. After 36 hoursapproximately, 80 cm3 hydrogen are absorbed and the hydrogenation is ended. After settling of theplatinum oxide and extraction with ether, the ether phase is washed with a 2N NaOH solution untilneutralization, then washed with water, dried and evaporated. A viscous colorless oil (280 mg) isobtained and chromatographied on a silica column. Three fractions are obtained:

- elution by hexane: 8 mg of a hydrocarbon, resulting from the hydrogenolysis of the acetatefunctions.

Table 3

Detailed examination of the result of a chromatography on a preparative plate

Products approximateweight

Rf Luminescenceunder 360 nm

excitation

Carr-Pricereaction

Retinyl acetatesisomers 15 0.56 strong yellow green blue-green

Retinyl acetateall-trans 30 0.52 strong yellow green blue

Oxidized Retinylacetate 20 0,45 weak green fluorescence greenish grey

Unidentifiedcompounds (*) 15 0.40(**) non fluorescent dark blue

Dimer di-acetate 40 0.34(**) non fluorescent dark blueUnknownfluorescentproduct

3 0,24strong yellow greenish

Oxidized dimers 8 0.18(**) non fluorescent greenishUnidentifiedpolymers 10 0.00 non fluorescent brown

(*) This apparently inhomogeneous fraction was not studied and could contain other types ofdimers, cyclobutanic for example. (**) In the center of the eluted strip.

- Elution within an ether:hexane (1:99) mixture: 60 mg of a mono-acetate the structure of which isunder consideration.

Analysis C42 H78O2: Calc. %: C 82,01 H 12,78 O 5,20 Found : 81,37 12,61 5,66.

IR: 1730-1240-1040 cm-1.NMR: 5,95 (m) CH2- OAc (2 H); 8,05 (s) OCOCH3 (3 H). No olefinic protons.

- Elution with an ether/hexane mixture (v:v/6:94): 160 mg of diacetate 5.Analysis C44 H80O4: Calc. %: C 78,51 H 11,98 O 9,51

Found 78,49 11,73 9,70.

IR: 1730-1240-1040 cm-1 bands, much more intense than in the previous fraction.NMR: 5,90 (m) CH2 - OAc (4 H); 8,06 (s) OCOCH3 (6 H) - no olefinic protons

For the tests on product 3 incompletely hydrogenated, hydrogenation is stopped as soon as there isno more UV absorption above 210 nm, that is after 10 h.

Reduction of 5.

200 mg 5 are reduced by LiAlH4 ( 80 mg ) in ether and 130 mg of the diol 8 are obtained afterpurification.

IR: 3639 cm-1 (free O-H), 3507 cm-1 (bound O-H).

Tetrahydrofuran derivative 9

1) 110 mg of 8 with 20 mg of para-toluene-sulfonic acid dissolved in 50 cm3 benzene are heated at80 °C during 4 hours under a stream of pure nitrogen. After neutralization, washing with water,drying and evaporating the solvent, a chromatography on a silica column gives 35 mg of product 9eluted with an hexane/benzene (v:v/1: 1). (The remaining 60 mg of 8 give back 9 by renewing thewhole process).2) 80 mg of 8 and 60 mg of mesyle chloride are added to 20 cm3 of pyridine at 0 C ° and left soduring 48 hours. Th sample is diluted with ether, washed with 1 N HCl until reaching a slightlyacidic pH, then neutralized by sodium carbonate and finally washed with water up to neutral pH. 90mg of the dimesylate of 8 are recovered.IR: characteristic narrow band at 1175 cm-1 and modification of the bands at 1350 et 930 cm-1.The dimesylate is reduced by AlLiH4 at room temperature in a few minutes. After the treatment 50mg from a raw product are recovered and chromatographied on alumina of activity II-III. Theelution of the mixture with hexane gives 8 mg of a hydrocarbon, and then with hexane/benzene(v:v/1: 1) gives 20 mg of 9.3) We add 0,002 cm3 of H2SO4 to a solution of 20 mg of 8 in 2 cm3 anhydrous acetone in thepresence of 200 mg of powdered anhydrous Na2SO4. After one night at room temperature,neutralization with a carbonate solution, washing with water, a chromatography on alumina(activity II-III) gives 10 mg of 9 eluted with the petroleum ether /benzene mixture (v/v: 1/ 1).

Analysis C40 H74O: Calc. %: C 84,13 H 13,06 O 2,80Found : 84,24 13,23 2,83.

IR: 1065 (C-O-C) 885, 1 470 et 1 375 cm-1.UV: No absorption.NMR: the protons of the tetrahydrofuran ring give a braod multiplet at 6,25 the intensity of whichcorresponds to 4 protons.

Calculation of the MOs. (*).

The calculation is initiated by adopting the following values:

- For the so-called coulombian integral α, relative to a 2p electron of carbon, α= 7,00 eV- For the exchange integral ß: ß = - 0,78 eV.

The iteration brings in the following relations:

a)drs = 1,52 - 0,19 lrs (13)

which relates the distance between neighbouring carbon atoms sp2 hybridized as a fonction of thecomputed bond index lrs.

b) βrs = β exp (7,00 -5,26 drs) (14)

which leads to the new value of the exchange integral as a function of the calculated distance.

The convergence is very fast, and the calculation is stopped at the fifth iteration. This simplecalculation does not aim at searching exact values of the energy, as the symmetry of the orbital onlyis of interest here.

(*) These calculations were made by one of us (D.A L.) in the Computing Center (CUTI) of theFaculty of Sciences of Montpellier.

BIBLIOGRAPHY.

(1) M. Mousseron, Advances in Photochemistry, John Wlley and Sons, New York, Vol. IV, p. 195.(2) M. Mousseron, J. C. Mani, C. Favié et D. Lerner, C.R. Acad. Sci., 1966, 262-C, 153.(3) M. Mousseron, J. C. Mani et D. Lerner, Bull. Soc. Chim . 1966, 3043.(4) R. Kaneko, Nippon Kagaku Zasshi., 1959, 80, 177. - H. Chatain et M. Debodard, C.R. Acad.Sci., 1951, 2Jl1233, 105.(5) B. V. Burger, C. F. Garbers, K. Pachler, R. Bonnett et B. C. L. Weedon, Chem. Comm., 1965,588.(6) C. Giannotti, B. C. Das et E. Lederer, Chem. Comm., 1966, 28; Bull. Soc. chim., 1966, p. 3299.(7) R. B. Woodward and R. Hoffman, J. Am. Chem. Soc., 1965, 87, 2047.(8) P. Millie, Bull. Soc. Chim., 1966, p. 4031.(9) K. Fukui in, Molecular Orbitals In Chemistry, Physics and Biology, P. O. Löwdin and B.PULLMAN Eds, Academic Press Inc., New York, 1964, p. 525.(10) M. Mousseron, M. Mousseron and M. Granier, Bull. Soc. Cbim., 1960, p. 1418.(H) H. Christol, A. Donche and F. Plenat, Bull. Soc. Chim., 1966, p. 2535.(12) H. Christol, M. Levy and Y. Pietasanta, Bull. Soc. Chim, 1965, p. 746.(13) A. Julg, Tetrahedron, 19 suppl. 2, 25, 1963.(14) from R. S. Mulliken, Phys. Rev., 1948, 74, 1963. - cf. Pullman B et A., les ThéoriesElectroniques de la Chimie Organique, Masson (1952) page 200.(15) Mousseron-Canet and J. C. Mani, Bull. Soc. Chim. 1966, p. 3291.