Conformational analysis of cyclic sulphites. 2-Oxo 1,3,2-dioxathiane-4-spiro-4-tert-butyl-cyclohexanes

Speetrochimlea Acta, Vol. 43A, No, I1, pp. 1355-1364), 1987.

0584-8539187 $3.00 + 0.00 (~ 1987 Persamon Journals Ltd.

Printed in Great Britain,

Conformational analysis of cyclic sulphites. 2-Oxo 1,3,2dioxathiane-4-spiro-4-tert-butyl-cyciohexanes LouIs CAZAUX, YAHIA KOUDSZ and PIERRE MARONI Synth~se et Physicochimie organique, Unit6 Assoeire au CNRS no. 471, Universit~ Paul Sabatier, 118 rte de Narbonne, 31062 Toulouse Cedex, France

(Received 20 January 1987; in final form 2 April 1987; accepted 10 April 1987) Abstract--Three series of 4-spirosulphites, a new class of cyclic sulphites, were synthesized and five couples of diastereoisomers isolated. Their structural analysis, using 1H NMR coupling constants and SO stretching vibration as conformatioaal probes, shows a large variety of ananchomeric chair forms and multicomponent equilibria for the cyclic sulphite moiety, the cyclohexane part of the molecule remaining in the chair form. Related to the occurrence of severe interactions involving the 5-methyl or 6-tert. butyl substituent, several twist forms were encountered, with 2,5-axis and isoclinal SO or 1,4-axis and axial or equatorial SO as a consequence of the weak free energy difference between chair and twist conformations in the cyclic sulphite series. lacing in the chair form with an equatorial tert-butyl group, the 1'-oxygen (O3) will be axial (compounds I, Ill, IV, VII, VIII) or equatorial (others); (2) isomeries at the sulphite cycle level introduced by the 5-methyl or the 6-tert-butyl suhstituent on the sulphite ring. Thus, each of the two isomers possible for the unsubstituted spirosulphite series (compounds I and II) can be split into two isomers when there is one substitution in the cyclic sulphite moiety. The ratio of diastereoisomers is related to the stereochcmistry of the cyclization process and to the thermodynamic stability of the products.

INTRODUCTION

T h e conformations of six-membered cyclic sulphites are very diverse [1] because the difference in energy ( ~ 13 k J) between chair and twist forms is weak [-1, 2] and therefore they are very sensitive to the substitution pattern. The techniques used in their conformational analysis can be various but i.r. and NMR spectroscopies and chemical equilibrations give the more informative results. Particularly, the possible use o f the SO stretching vibration as a conformational probe has been demonstrated [3] and conformational equilibria with 2,5- and 1,4-axis twist form participation were found for several series of 2-oxo 1,3,2dioxathianes l-1, 2, 4, 5]. In this paper, we describe the structural behaviour of the three series of spirosulphites shown in Table 1: unsubstituted on the sulphite ring (I, II), tertbutylated ( I I I - V I ) and methylated (VII-X) at C-5. 2-Oxo 1,3,2-dioxathianc-4-spirocyclohexanes constitute a new class of sulphites. A spirosulphite with a 5,5-junction is the only example reported in the literature 1-5]. The studied spirosulphites I-X were synthesized from the corresponding 1,3-diols of known configuration by addition of thionylchloride, so that two kinds o f cis-trans diastereoisomeries can be noticed: (1) isomeries at the cyclohexane level; the cyclohexane ring

EXPERIMENTAL

Synthesis Sulphites l-VII were prepared from dio|s of known configuration as previously reported [6-9]. However, the synthesis of new diol precursors was necessary for sulphites VII-X.

Preparation of diols 4-Tert-butyl 1-hydroxy 1-[3'-hydroxy isopropyl] cyclohexane. The two diastereoisomeric diols were prepared by stoichiometric addition of LiAIH, to the corresponding two fl-hydroxyesters [10l in anhydrous ethyl-ether. The yields are nearly quantitative. cis.Diol: white crystals m.p. = 90~ i.r. Iv(era-l}, CCI4 0.0025 M]: 3335, 3625, 3640. 1HNMR ['t5(ppm), DMSO];

Table 1. !

• J

3' R2 R I ~ ' ~ . ' , 9 -'

' ' ysv ~

"i"

' 0_2.0 "~"S*'-l

R

( 3

II

111 V

IV VI

RI

H

H

R2

H

H

R3 R4

H H

tBu H

II

O

1355

VII IX

ViII

0 3 - axial series

X

0 3 - equatorial series

H

H

Me

H

Me

H

H tBu

H H

H H

Louis CAZAUXet al.

1356

primary OH, 4.42; tert. OH, 3.84; CH20, 3.26, 3.6; tBu, 0,84; Me, 0.84. trans-Diol: white crystals m.p.= 131~ i.r. [v(cm-t), CC14 0.0025MI: 3340, 3612, 3640. tHNMR [,~(ppm), DMSO]: primary OH, 4.38; tert. OH, 3.99; CH20, 3.4, 3.6; tBu, 0.84; Me, 0.9. The configuration was demonstrated from that found for the secondary tertiary diols derived from the fl-hydroxyesters, following a previously described methodology [.6-].

Preparation of sulphites The two following general procedures were used: (A) thionyl chloride (2.5 ml) was added slowly to a stirred mixture ofdiols (0.02 M) and pyridine (Py:diol = 1.55) in anhydrous dioxane (20 ml) kept to 10~ (B) triethylamine was substituted for pyridine with Et3N:diol = 2.3. After completion ofthe addition, the solution was stirred for 1 h at 25~ Then, the reaction mixture was poured into brine (100 ml) and extracted twice with ether (30 ml). After drying over anhydrous K2COa and MgSO,, the solvents were removed and the crude product obtained with a quantitative yield. Pure isomers were obtained after recrystallization in petroleum ether (l-II) or separation by TLC (petroleum ether-ether 90/10) (Ill-X), Rf (I, Ill, IV, VII, VIII) < Re (II, V, Vl, IX, X). The percentage of isomers estimated by TLC or NMR on the crude product is about 60/30, the higher Rj. compound being the most abundant.

2-Oxo 1,3,2-dioxathiane-4-spiro-4'-tert-butyl cyclohexanes

cis (I) and trans (II). Method A. 1: m.p. = 105~ = 72~

II: m.p.

2-Oxo 6-tert-butyl 1,3,2-dioxathiane-4-spiro-4'-tert-butyl cyclohexanes cis(lll and IV). Method B. II1: m.p. = 136~ (559,~), IV: m.p. = ll0-11YC (45 ~,~).

2-Oxo 6-tert-butyl 1,3,2-dioxathiane-4-spiro-4'-tert-butyl cyclohexanes trans (V and VI). Method B. V: m.p. = 96-100~C (67~,), VI: m.p. = 124-126~ (33 ~o).

2-Oxo 5-methyl 1,3,2-dioxathiane-4-spiro-4'-tert-butyl cyclohexanes cis (VII and VIII). Method A~VII: m.p. = 87~ (63 ~), VIII: m.p. = 80~C (37 ~

2-Oxo 5-methyl 1,3,2-dioxathiane-4-spiro-4'-tert-butyl cyclohexanes trans (IX and X). Method A. IX: m.p. = 99~ (62,%), X: m.p. = 90~ (38','.-o).

PreparatiL'e (TLC) The separations were performed on fluorescent SiO2 plates and with an adsorbent thickness of 1-1.4 mm. The eluent was petroleum ether-ether: 90/10, with one or two elutions as necessary.

Spectra ' H N M R spectra were performed on a Briiker WH-90 apparatus. Infrared spectra were recorded on a Beckman IR-9 spectrometer between 1150 and 1275 c m - ' in 0.05 M sol-

utions (CCI, and CH3CN) using NaCI ceils of 0.5 mm thickness. Multicomponent absorption curves were resolved using a Dupont 310 curve generator ofanalog type simulating Lorentzian functions.

Chemical equilibrations In an NMR tube 0.2 g of one isomer was dissolved into 0.33 ml CC14, then traces of thionyl chloride and BF3 etherate were added with a capillary glass. The equilibration was performed with each isomer and analyzed by NMR as a function of time. RESULTS AND DISCUSSION All the I H N M R a n d i,r. results are given respectively in Tables 2 and 3. The configurations of the spirosulpbites are determined from the k n o w n structure o f the starting diols a n d from the c o n f o r m a t i o n a l analysis o f the cyclized products. T h e cyclohexane moiety is very likely in the chair form as f o u n d for the c o r r e s p o n d i n g spirodioxanes [9] while the sulphite p a r t is expected to give multiforms equilibria. T a k i n g into account our previous results on 2-oxo 1,3,2-dioxathianes [1, 2], the following four conformations can be first selected in the O-axial a n d Oequatorial series (scheme 1): chair with axial (C-a) or equatorial (C-e) S=O, and twist with axial (TO-a) or isoclinal (TS-i) S=O. Three different orientations of the S---O b o n d are thus involved with respect to the m e d i u m plane of the ring a n d three frequency ranges for the S = O stretching vibrations were assigned to these three orientations [1, 3]; see b o t t o m of T a b l e 3. Nevertheless, the use o f vS=O as a c o n f o r m a t i o n a l probe must be anticipated by selection o f b a n d s really belonging to this vibrator following criteria previously reported [3]. Moreover, in several cases, Fermi resonances were observed a n d the true frequency calculated (see T a b l e 3). The configurational p r o b l e m for the sulphite ring was solved in a first a p p r o x i m a t i o n by the chemical shift value of the p r o t o n at C-6; as generally observed in cyclic sulphites [11, i ] the less shielded p r o t o n s are cis with regard to the SO group. Three concentrations and solvent conditions were used: 0.05 M CC14 a n d C H 3 C N solutions for i.r.

Table 2. ~H NMR chemical shifts (& ppm) and coupling constants (d, Hz) for spirosulphites I-X Sulphite

Solvent*

6s,

3~,

~6~

t56,

J~,rc

Js,6,

J5,6~

Js,6,

Jss

J66

I II III IV V VI VII VIII IX X

CCI, CCI 4 CDCls CDCI3 CDCI3 CDCIa CCI, CC14 CC14 CCI,

1.73

2.40 1.52 1.95 ~2 1.70 -1.53 -1.17

3.84 3.85 4.09 -3.97 -3.68 3.51 3.55 4.13

3.3 5.3 -2.5 -2.5 3.0 -2.0 --

3.6 4.3 11.9 -~7 -5.1 -1.6 --

11.5 8.5 -11.2

4.4 4.3 2.2

- 14.6

2.20 1.57 ~2 2.10 1.13 -1.25 --

4.90 4.80 -4.63 -4.55 4.76 4.55 5.13 4.50

- 11.6 - 11.6 ----11.4 - 11.6 11.7 - 12.5

*Concentration ~ 0.3 M. tOnly on the 6c and 6t part of the spectrum.

11.1 . . 11.6 . . 3.0

~7 -. 4.0 . 5.8

- 13.6 - 13.7 -- 13.7 . -. --

Conformational analysis of cyclic sulphites

1357

~..O ,"'" 7 R4 I

.-,,,,',,~ U~,l

IIc ,'~"

0 -4 ~'-.~R'~R3

2.~'Oo~Seo

RI

R1 C-a

,

RI

TO-a

TS-i C-e

~2R 4

R2

0

\ ~filllIll t S illllil LL ~ /o 4'i~o/\R 3

"" -/~"~- R2R3

~

RI C-a

R1

TS-i

T O -a

R4

R2~ C ) - / S ~ O RI

C-e

Scheme 1. records and 0.3 M CC6 or CDCI3 for NMR experiments, so that the two effects of solvent polarity and self-association can be expected on conformational equilibria. The position of a contbrmational equilibrium was calculated using quantitative evaluation of the percentage of the three vSO i.r. bands in agreement with the NMR coupling constants as described earlier [1-3]. The integrated molar absorption coefficients are expected invariable with S=O bond orientation as previously shown [3-1.The conformational energies are given in Table 4. The uncertainty is around 15~o on measurements ( A K / K ) and 0.4 kJ mol- ~ on conformational energies (AAG~ ~ 2.5 Aln K = 2.5 AK/K). Unsubstituted spirosulphites cis I and trans I1 The cis isomer 1 presents a conformational equilibrium between C-a and C-e inverted chairs in 0.05 M CC6 or CH3CN solutions as indicated by their i.r. spectra (Table 3). The preponderant C-a chair form ( ~ 95 ~'o) in 0.05 M CC14 is confirmed by the NMR results for sixfold more concentrated solutions, particularly, by the high value (11.5 H z ) o f the J~,-6c coupling constant. This equilibrium is shifted towards the C-e form ( ~ 25 70) by the polar solvent effect I"1,3, 4a]. The free energy differences between the two C-a and C-e conformations are 7.3 kJ tool-~ in 0.05 M CC6 and 2.7 kJ mol- 1 in 0.05 M CHaCN (Table 4). These energies include the SO conformational energy Aso and the difference between the two 1,3-syn axial interactions CH2 . . . OS in C-a and CH2 . . . + S in C-e. Assuming that, in compound I, the Aso energies determined from 4c, 6c- and 4t,6t-dimethyl 2r-oxo

1,3,2-dioxathianes are 10.9 and 7.1 kJ tool-~ respectively in CCI4 and CH3CN, the axial SO being favoured 1,'13], the energy difference between 1,3-syn axial interactions can be derived from the equilibrium I, i.e. 3.6 kJ mol- 1 in CC14 and 4.4 kJ tool- 1 in CH3CN. These values are in good agreement with that previously calculated (4.2 kJ mol- ~in CC14) from trans 4methyl 2-oxo 1,3,2-dioxathiane 12]. The trans isomer 11 exists as a C-a chair form in 0.05 M CC14 solution and as a 0.7 C-a + 0.3 C-e equilibrium in 0.05 M CH3CN solution. From the sum of coupling constants EJs.6, = 8.6 Hz and ds.6~ = 13.8 Hz, vs respectively 8.0 and 14.8 Hz for the corresponding values of compound I, a 0.88 C-a +0.12 C-e equilibrium is found in 0.3 M CC14 solution. This behaviour is very close to that found for the 4,4-dimethyl 2-oxo 1,3,2-dioxathiane: in 0.6 M CC14 solution, a 0.82 C-a+0.18 C.e equilibrium was determined [12] using J5.6,= 8.2Hz and Js.6~ = 13.6 Hz. 6-Tert-butylated spirosulphites III-V or IV-VI Compounds III cannot exist in the C-a chair form related to the M e . . . tBu and M e . . . SO important 1,3-diaxial interactions. So, a TO-a twist form with an axial SO is favoured in 0.05 M CC14 solution as shown on the i.r. spectrum (vSO band at 1192 cm-l). In this solvent the equilibrium is 0.6 TO-a + 0.1 TS-i + 0.3 C-e. In 0.05 M CH3CN the equilibrium is 0.2 TO-a + O.1 Tsi + 0.7 C-e shifted towards the C-e form as expected. In 0.3 M CDCI3, the large NMR coupling constant J5~-6, = 11.9 Hz is compatible with a predominant Ce form; TO-a or TS-i forms must be minor constituents

1358

Louls CAZAUXet al.

Table 3. Vso frequencies (era- t) with, in parentheses, the integrated absorption molar ratio and A = v(CC1,) - v(CH3CN) for sulphites I-X

Sulphite

Solvent FCCI,

I

~.A H~CN

VSO a

1185 1197 1183 1190

(0.25) } (0.70) (0.40)} (0.35)

VSO i

1194' (0.95)

__

1186' (0.75)

--

Vso e

1237 (0.05) 1223 (0.25) 14

8

f- COl, ICH~CN LA

n

F CC14

III

1181 (0.18)} 1197 (0.42)

t CH3CN

/

LA

j. IV

CCI,

] CH3CN

1181 (0,5) ]

1195 (0.5) J 1178 (0.4) ~ 1190 (0.6) J

LA

(" CCI4 t CH3CN

L~ Vl

FCCI,

1187 (0.3) 1195 (0.7) J

] CH3CN

1182 (0.3) } 1193 (0.7)

LA

VII

1183 (o.29/~

CCI#

1195 (0.5) J

CH3CN

1183 (0.19) I 1193 (0.19)J

A cCCI,

VIII

IX

t

CHzCN ,A ( CC14 ~ ACH3CN ( CCI,

X

~ CH~CN

---

1192" (0.6) 1186 (0.2)

1219 (0.1) 1205 (0.1)

6

/

V

1195 (1) 1186 (0.70) 9

14

1188' (1)

1236 (0.3) 1217 (0.49) } 1226 (012 1 )

1220' (0.7) 16

--

1185' (1) 3 1192 (0.75) 1187 (0.16) 5

--1217 (0.05) 1205 (0.08) 12

1193' (1)

--

1190' (1) 3

--

1190" (0.79) 1188" (0.38) 2

1188 (0.45)1 1196 (0.45)J

1192' (0.9)

1182 (0.46) ~ 1193 (0.29) J

1186" (0.75)

1218 (0.07) --1217 (0.05) --

6 1193 (0.74) 11876 (0.73)

-1218 (0.08) __--

1193 (0.48)

1218 (0.13)

--

--

LA VSO mean

~CCI 4

1190.5_+3.5

1220+2

frequencies from Rcfs ['2, 3]

i CH3CN

1184.5 + 3 7 • 2.5

1209 • 1 11 + 2

L A

1224 (0.3)

n

1238 (0.2) 1220 (0.76) 18

m

1237 (0.14) 1223 (0.62) 14 1243 (0.05) 1220 (0.25) 23 1243 (0.18) 1220 (0.27) 23 1240 (0.39) 1222 (1) 18 1242 • 2 1225 -t- 1 17•

*Vso frequency taking into account the Fermi resonance. because autoassociation is favoured in the C-e form [2]. Compounds IV and VI present only one CH2 . . . SO 1,3-diaxial interaction in the C-a form, thus leading to the expected observation of such chair forms either in CCI4 or CH3CN. Another equilibrium is found for the spirosulphite V. Two 1,3-syn axial interactions, i.e. M e . . . tBu and C H z . . . SO destabilizes the C-a form. Here also the TO-a form is favoured due, in part, to its ring unstrained by substituents. The equilibria are: 0.75 TO. a+0.05 TS-i+0.2 C-e in 0.05 M CC14 and 0.16 TO-a +0.08 TS-i+0.76 C-e in CHaCN.

A difference in energy of 4.4 or 6.7 kJ mol-~ between TO-a and TS-i forms for 111 or V in CC14 (Table 4) compared to around 0.8 kJmo1-1 for 4methyl 2-oxo 1,3,2-dioxathiane [2] arises from substituent interactions. The equilibrium is shifted towards C-e in 0.3 M CDCI3; it can be relevant of an autoassociation effect as previously observed [4a, 2]. Chemical equilibrations III,-~-~IV and V~---VI in 2 M C C I , confirm that the C-a chair forms IV and VI are more stable than conformations of equilibria III and V. Two very close values of 35/65 and 30/70 were obtained respectively for the two ratio l l l / I V and V/VI, indicating a weak difference in energy between

Conformational analysis of cyclic sulphites Table 4. Conformational energies* (AGes kJmol -l) for spirosulphites I-X Sulphites

Solvent'l

C-a

I

CCI4 CHsCN

0 -0 -0 . 0 . 0 -0 . 0 --2.5 0 . 0 . 0 . -0 -3.8 0 . 0 . 0 . 0 -1.2 -. . . 0 -0 -0 . 0 -0 -0 . -0 . . . . . .

CC14~

II III

IV V

Vi VII

VIII

IX

X

CCI4 CHsCN CC14~ CC14 CHsCN CC14 CHsCN CDCls:I: CC14 CHsCN CC14 CHsCN

CDCIs~ CCla CHsCN CC14:1: CCI4 CHsCN CC14~ CC14 CHsCN CC14~ CCI( CHaCN CC14:I:

TO-a TS-t TO-e --. . .

. . .

. . .

.

.

. .

--. . --

-. . 4.4 -6.5 -. . . . . . 6.7 -5.6 -. . . . . . 6.0 ---. . 7.1 ---. . 5.5 3.5 -2.5 . . 3.2 -. .

C-e 7.3 2.7

2.1 5.4 1.7 0

3.3 0

4.3 0 0 7.1 2.7

-0.5 0 0

* Uncertainty is around 0.4 kJ tool- 1. t0.05 M solutions. :1:0.3 M solutions.

the two couples of isomers only differentiated by the

cis or trans configuration of the substituents on the cyclohexane ring.

5-Methylated spirosulphites V I I - X The major forms in compound VII and VIII are C-a chair forms with only one significant S O . . . CH2 1,3syn axial interaction. Typical C - a ~ T S - i ~ C-e equilibria exist in 0.05 M CC14 respectively 0.79:0.07:0.14 for VII and 0.9:0.05:0.05 for VIII while the Ts-i form disappears in 0.05 M CH3CN, i.e. 0.38 C-a + 0.62 C-e for V l l and 0.75 C-a + 0.25 C-e for VIII. Very close equilibria are observed in 0.05 M or 0.3 M CC14 by i.r, or N M R spectroscopy. The C-a ~ TS-i ~ C.e equilibrium is modified by the greatly strained chair forms C-a for X and C-e for IX (two C H 2 . . . Me 1,3-syn diaxial interactions). So that these two forms can likely be largely replaced by two twist forms TO-a for X and TO-e for IX where only one gauche CH2 . 9 Me interaction is substituted for the two 1,3-syn axial ones. Two M e . . . Me 1,3-syn axial interactions are estimated to be around 30.9 kJ m o l - ~ while one gauche M e . . . Me interaction is around 3 . 8 k J m o l ~t 1,14] with 27.1 kJ m o l - ~ in difference. Ts-i twist forms are

1359

also strained with one C H 2 . . . Me 1,3-syn axial interactions (15.5 kJ cal m o l - 1) for each of the two compounds. The difference in free energy between C-a and twist forms being 13.0kJmol - t for TO-a and 13.8kJmo1-1 for TS.i in the 4-Me trimethylene sulphite [-2"1. Thus, the occurrence of only the TO-a twist form instead of the C-a form is a reasonable hypothesis from an energy point of view. In the same way, the participation of a TO-e twist form in the equilibrium can be reasonably considered for compound X; this form can be slightly distorted towards a boat form giving the best release of substituent interactions. Then, the equilibria will be for X 0.48 TO-a+O.13 TS-i + 0.39 C-e in 0.03 M CC14 shifted towards C-e in 0.05 M C H a C N or 0.3 M CC14. And for IX: 0.74C-a +0.08 TS-i+ 0.18 TO-e in 0.05 M CC14 and 0.79 C-a +0.27 TO-e in 0.05 M CH3CN while the very small coupling constants (2.0 and 1.6Hz) in 0.3M CC14 indicate a predominant C-a form for this trans spirosulphite. The new TO-e form of IX is very close in energy to the TO-a form of X if we assume a difference of around 8.4 kJmo1-1 1-13] between the C-e and C-a forms of compound II used as references and taking into account both the X and IX equilibria and the gauche and 1,3-syn axial interactions occurring in C-a and C.e chair forms. Now, some remarks can be made on the equilibria of the two last series differentiated by substituting a tertiobutyl group at C-6 by a methyl group at C-5: (1) complex equilibria are observed due to gauche or 1,3-syn axial M e . . . CH2 interactions. (2) In the 6-substituted series a variable increase of the TS-i form is noticed when passing from 0.05 M CC1, to CHsCN, while in the 5-substituted series the TS-i form disappears in CHsCN. This result can be explained by the M e - 5 . . . CH2 gauche interaction (q~ = 40 o in TS-i) which is released in the C-e forms (~b -- 60~ (3) Most stable TS-i forms of compounds VIII and X are inverted ones related to those illustrated in Scheme 1. Inverted TO-a form must also be considered for compound X. (4) The most shifted equilibria (VII, X) by the solvent effect present a preferential conformation in 0.3M CCI4 similar to that found in 0.05 M CHaCN. Thus, the concentration effect is analogous to the polar solvent effect and can be explained by autoassociation acting most easily on the C-e groups as previously noticed for other 6-membered cyclic sulphites [4a] and comparable to the solvent effect. CONCLUSION Three series of spirosulphites, a new class of sulphites, were synthesized and five couples of diastereoisomers isolated. Configurations were determined by filiation from diols of known structure and as a consequence of the conformational analysis,

1360

Louis CAZAUXet al.

The cyclohexane part of the spirosulphite keeping in a chair form, a large variety of conformational behaviour is found by i.r. spectroscopy in 0.05 M CC14 and CH3CN solutions and by NMR experiments in 0.3 M CC14 or CDCla for the sulphite moiety. Unsubstituted spirosulphites I and II present an inverted chair equilibrium shifted by solvent effect from the C-a form (95 ~ in CC14) to the C-e form (~ 3 0 ~ in CH3CN). Their free energy differences (7.3 kJ tool- t in CC14 and 2.7 in CH3CN) agree with previously calculated values of SO conformational energies (10.9 and 7.1kJmo1-1 respectively) and energy difference in 1,3-syn axial interactions involving the + SO group (~ 4.2 kJ mol- :). 6-Tert-butylated derivatives III-VI show both ananchomeric C-a forms (IV, VI) and TO-a,-~-TSi ~ C - e equilibria (III, V). TO-a forms exist instead of C-a forms due to large M e . . . CH2 1,3-syn axial interactions taking place in the latter. Equilibria are shifted towards either the TS-i or the C-e form as normally expected for polar solvent effect. 5-Methylated derivatives VII-X exist either as equilibria: C-a ~ TS-i ~ C-e (VII, VIII); TO-a ~ TSi ~ C-e (X) or C-a ~ TS-i ~ - TO-e (IX). The participation of TO-a and TO-e forms in these equilibria proceeds from the instability of the corresponding C-a and C-e forms where severe M e . . . Me 1,3-syn axial interactions are acting and results from a weak difference in energy between the three twist conformations TS-i, TO-a and TO-e. When passing from 6.05 M CC14 to CH~CN the TS-i form disappears as a consequence of the released 5M e . . . CH2 gauche interaction in both the C-e and TO-e forms. These results give new arguments for the existence of twist forms in cyclic sulphite series, confirm TS-i and TO-a forms and, particularly, reveal the TO-e form as a new kind of conformation for such compounds. The predominant conformation observed at 0.3 M CC14 or CDCls by ~H NMR spectroscopy is very close to that found by i.r. spectroscopy in 0.05 M CC14 for

no or relatively few shifted equilibria (I, II, I V . . . ) whereas for greatly shifted equilibria (VII, X) the major conformation is that found in 0.05 M CHsCN. Thus, the concentration effect will be related to the autoassociation ability of the molecule which compares well with a polar solvent association. Chemical equilibrations confirm the greater stability of compounds IV and VI. Nevertheless, the three techniques, i.r. and NMR spectroscopy and chemical equilibrations, used for these conformational analyses involve limitations which are reached in these series. So, new methods will be necessary to advance in this area.

REFERENCES

[1] L. CAZAUX, G. CHASSAING and P. MARONI, Spectrochlm. Acta 40A, 519 (1984) and references therein. [2] L. Cnznux, G. CNASSAINGand P. MARONI,J. chem. Res. $68 (1982); ibid. J. chem. Res. M, 0816 (1982). [3] L. CAznux, J. D. BASTIDE, G. CHASSAmGand P. MARONI,Spectrochim. Acta 35A, 15 (1979). [4] P. MARONIand L. CAZAUX,C. r. hebd. Stanc. Acad. Sci., Paris (a), 272C, 1660 (1971), (b) 272C, 2065 (1971), (e) 273C, 156 (1971). 1"5] A. A. BOLOTOV,A. A. RODIN, K. A. V'YUNoV, A. I. GINAKand YU. S. SARKISOV,Zk. Org. Khim. 18, 2060 (1982), C.A. 98, 107261m (1983). [61 M. PERRYand Y. MARON1,Ball. Soc. ehlm. Ft. 2372 (1969). [7"1 Y. KOUDSl and Y. MARONI,Tetrahedron Lett. 4454 (1973). [8"1 R. CANTAORELandY. MARONI,C. r. hebd. S~anc. Acad. Sci., Paris 276C, 1679 (1973). [9-1 L. CAZAUX, J. P. GORRICHON,P. MARONIand M. PERRY, Can. J. Chem. 56, 2998 (1978). [101 N. IDRISS,M. PERRYand Y. MARONI,Tetrahedron Lett. 4447 (1973), [I1] P. MARONI,L. CAZAUX,J. P. GORRICHON,P. TISNES and J. G. WOLF,Bull. Soc. chim. Fr. 1253 (1975). [121 H. NtKANDER,Ph.D. thesis, Turku University (1980). 1-13] G. CHASSAING,th~se Doctorat d'Etat, Toulouse (1976). [14-1 J.L. PmRRa,in Collection Intersciences. Armand Colin, Paris (1971).