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Molecular mechanics application to 1,1,2-trisubstituted cyclohexanes
Journal of Molecular Structure, 268 (1992) 181-190 Elsevier Science Publishers B.V., Amsterdam
Molecular mechanics cyclohexanes
application
181
to 1,1,2-trisubstituted
Tereza Varnali Bogazici University, Bebek 80815, Istanbul (Tiirkiye) (Received 3 June 1991; in final form 28 August 1991)
Abstract Molecular mechanics was used to calculate the energies of the conformers of l,l,Z-trisubstituted cyclohexanes (I-IV). Geometrical parameters of conformational equilibria were investigated. Results showed qualitative accordance with experimental data.
INTRODUCTION
In studies of the conformational behavior of 1,1,2_trisubstituted cyclohexanes, fundamental differences have been observed between compounds I and II, as well as between III and IV (Fig. 1). Compounds I and III show predominance for the equatorial conformer (B), while the axial conformer (A) predominates for compounds II and IV. Experimental results reported for similar compounds are summarized in Table 1. In the present work, the aim is to study the influence of the structure of the 1,1,2-substituents on cyclohexane by carrying out molecular mechanics calculations first on representative model molecules (Ia and IIa) and then on
I-IV (A)
I-IV (B)
k
Fig. 1. Conformations of 1,1,2-trisubstitutedcyclohexane: I: R,R=-(CH,),-; ; III: R,R=-O(CH,),O-; IV: R,R=-O(CH,),O-; a: X=-H; b: X=-CH,; Correspondence
0022-2860/92/$05.00
II: R,R=-(CH,),c: X=-CN.
to: Dr. T. Varnali, Bogazici University, Bebek 80815, Istanbul, Ttirkiye.
0 1992 Elsevier Science Publishers B.V. All rights reserved.
182 TABLE 1 Experimental results reported for similar compounds Compound
Substituent
Preferred conformation
Ref.
I II III III III IV IV IV
-OMe,-OAc,-OSiMe,,-OH -OMe,-OAc,-OSiMe,,-OH -Cl,-Br,-OH,-OMe,-SMe -F,-Cl,-Br,-OMe -CN -Cl,-Br,-OH,-OMe,-SMe -F,-Cl,-Br,-OMe -CN
equatorial axial’ equatorial equatorialb equatorial axial axial” axial
1 1 2,3 4 5 2,3 4 5
“The enthalpy-preferred conformer is the equatorial. bathe percentage of the axial conformer increases with solvent polarity. “The percentage of the equatorial conformer increases with solvent polarity.
X
X
II(A)syn
II(B)syn
II(A)anti
II(B)anti
Fig. 2. Syn and anti orientations of the second ring relative to the X-substituted
ring.
molecules that have been modified step by step. Initially, methyl, a non-polar, bulky substituent is introduced (Ib and IIb ) , followed by cyano, a polar nonbulky substituent (Ic and 11~) , and finally methylene groups are replaced by ether oxygens (IIIa,b,c and IVa,b,c). The QCMP.032 program [6] was used for calculations. The possibility of both syn and anti orientations of the second ring relative to the X-substituted ring was also taken into account (see Fig. 2). RESULTS AND DISCUSSION
The calculations were carried out for all four conformers of each compound with the exception of the series a (X= H). The geometrical parameters resulting from the calculations for the anti conformers are tabulated in Tables 2-4. In almost all cases, the syn conformers had higher conformational energies (see Table 6). Energies, according to the types of interactions that contribute to the total energy, are shown in Table 5. In IIa, both cyclohexane rings flatten around the vicinity of the quaternary
183
TABLE 2 Bond lengths (A) Bond
Ia
IIIa
Ib (A)
Ib (B)
IIIb (A)
IIIIb
Ic (A)
Ic (B)
111~ (A)
111~ (B)
(B) l-2 l-6 l-7 l-10 2-3 3-4 4-5 5-6 7-8 8-9 9-10 2-11 11-12
1.545 1.544 1.549 1.552 1.537 1.534 1.534 1.535 1.534 1.531 1.534
1.528 1.528 1.412 1.409 1.536 1.535 1.535 1.536 1.411 1.507 1.410
1.552 1.548 1.556 1.550 1.543 1.534 1.533 1.536 1.537 1.531 1.533 1.542
1.555 1.549 1.550 1.554 1.544 1.533 1.532 1.534 1.534 1.531 1.535 1.542
1.535 1.529 1.414 1.410 1.541 1.535 1.534 1.535 1.410 1.506 1.410 1.540
1.535 1.531 1.413 1.408 1.540 1.534 1.533 1.535 1.412 1.507 1.410 1.540
1.549 1.546 1.553 1.552 1.540 1.534 1.534 1.536 1.535 1.531 1.533 1.479 1.158
1.549 1.547 1.551 1.552 1.540 1.533 1.533 1.535 1.535 1.531 1.534 1.478 1.158
1.536 1.529 1.413 1.409 1.539 1.535 1.535 1.536 1.410 1.507 1.409 1.480 1.158
1.536 1.529 1.412 1.406 1.539 1.535 1.534 1.535 1.412 1.507 1.409 1.480 1.158
Bond
IIa
IVa
IIb (A)
IIb (B)
IVb (A)
IVh (B)
IIc (A)
IIC (B)
IVc (A)
IVc (B)
l-2 l-6 l-7 l-11 2-3 3-4 4-5 5-6 7-8 8-9 9-10 lo-11 2-12
1.550 1.547 1.547 1.550 1.536 1.533 1.534 1.536 1.536 1.534 1.533 1.536
1.536 1.533 1.412 1.413 1.536 1.534 1.540 1.537 1.415 1.522 1.522 1.415
1.562 1.549 1.553 1.551 1.543 1.533 1.533 1.536 1.536 1.532 1.532 1.537 1.543
1.562 1.553 1.548 1.553 1.543 1.532 1.531 1.535 1.537 1.534 1.534 1.537 1.542
1.545 1.534 1.415 1.413 1.542 1.534 1.533 1.536 1.415 1.521 1.521 1.415 1.542
1.544 1.535 1.412 1.414 1.541 1.533 1.533 1.536 1.415 1.522 1.522 1.415 1.542
1.557 1.548 1.550 1.551 1.540 1.534 1.533 1.536 1.536 1.533 1.533 1.536 1.479
1.556 1.550 1.548 1.551 1.539 1.533 1.532 1.536 1.537 1.534 1.534 1.536 1.478
1.545 1.533 1.413 1.412 1.540 1.534 1.534 1.537 1.415 1.522 1.522 1.415 1.481
1.544 1.533 1.411 1.412 1.539 1.534 1.533 1.536 1.414 1.522 1.522 1.415 1.482
carbon atom. The endocyclic angle at the quaternary carbon decreases to 107.8” compared with cyclohexane (111.4” ) [ 71 and l,l-dimethylcyclohexane (109.9’) [7]. The anglesbandj? (113.8” and 113.9”) display the bending of the whole “flap” of the chair. Flattening [ 81 decreases gradually as one moves away from the quaternary carbon. The rings are twisted, with (Y= 112.6” and (Y’ = 108.0” (Fig. 3). In compound IVa, when compared with IIa, the cyclohexane ring is less flattened, yet more twisted (Fig. 4). Angles p and j? show less bending at the “flap” of the chair. The dioxane ring also flattens (OCO angle = 109.3 o and ccc=114.0”). In Ia, the endocyclic angle at the quaternary carbon in the six-membered
184 TABLE 3 Bond angles (deg.) Angle
Ia
IIIa
Ib (A)
fi (B)
IIIb (A)
IIIb (B)
Ic (A)
Ic (B)
111~(A)
111~(B)
2-l-6 2-l-7 2-l-10 6-l-7 6-l-10 7-l-10 l-2-3 2-3-4 3-4-5 4-5-6 l-6-5 l-7-8 7-8-9 8-9-10 l-10-9 l-2-11 3-2-11
108.8 111.8 110.1 111.6 110.2 104.2 11X6 111.3 110.5 110.5 112.5 106.1 102.3 102.2 106.2 109.9 109.4
110.4 110.3 109.3 109.7 110.0 107.0 111.9 110.9 110.7 110.8 111.7 107.6 102.0 101.4 107.0 110.0 109.7
109.7 112.0 111.8 110.4 109.7 103.1 111.4 112.5 110.5 111.2 113.2 107.5 103.2 101.3 105.8 114.0 110.2
108.9 113.0 112.0 110.6 108.0 104.2 112.4 112.6 110.1 109.8 113.1 106.3 102.2 102.9 106.5 115.2 108.3
111.7 109.9 109.9 108.8 110.0 106.5 110.6 112.5 110.9 111.0 111.7 107.7 101.7 101.4 107.4 113.3 111.3
110.4 110.9 109.9 109.3 109.3 106.8 111.0 111.6 110.5 110.5 112.1 108.0 102.7 101.5 106.9 113.5 109.8
109.1 112.1 110.9 111.1 110.0 103.7 112.4 111.7 110.4 110.7 112.8 106.9 102.6 101.7 106.1 112.4 110.4
109.0 112.4 111.0 111.1 108.9 104.3 112.7 112.0 110.3 110.2 112.9 106.5 102.7 102.1 105.9 113.4 110.3
110.9 110.2 109.2 109.6 110.3 106.7 110.8 111.4 110.8 110.8 111.7 107.7 101.9 101.4 107.4 112.9 111.3
110.3 110.5 109.4 109.7 110.0 107.0 111.6 111.0 110.7 110.8 111.8 108.0 102.7 101.5 106.9 111.9 109.9
IIa
IVa
IIb
IIb
IL%(A)
IVb (B)
IIc
IIC
IVc (A)
IVc (B)
(A)
(B)
(A)
(B)
108.9 109.3 110.2 111.6 109.8 107.0 112.1 112.4 110.5 111.2 114.3 114.7 110.9 110.1 111.1 114.7 114.8 109.8
107.3 112.0 110.0 111.7 108.4 107.3 112.8 112.2 110.1 110.3 114.5 114.2 111.0 110.9 110.8 114.2 116.0 108.1
108.0 110.0 109.2 112.2 110.3 107.2 112.7 111.4 110.7 111.2 114.0 114.3 111.0 110.3 111.0 114.3 113.4 109.8
107.7 111.0 108.8 112.0 109.4 107.9 113.1 111.6 110.5 110.8 114.2 114.0 111.1 110.7 110.8 113.9 114.0 110.3
108.7 106.6 105.7 113.4 113.1 108.9 111.6 111.2 110.4 111.1 112.3 114.2 109.4 109.3 109.2 114.3 113.3 111.1
108.9 106.3 105.6 113.4 112.4 109.8 112.0 111.3 110.4 110.8 112.5 114.3 109.4 109.2 109.3 114.3 113.1 110.7
2-l-6 2-l-7 2-l-11 6-l-7 6-l-11 7-l-11 l-2-3 2-3-4 3-4-5 4-5-6 l-6-5 l-7-8 7-8-9 8-9-10 9-10-11 l-11-10 1-2-12 3-2-12
107.8 110.3 108.0 112.6 110.3 107.8 113.8 110.8 110.5 111.1 113.9 113.9 111.1 110.6 110.8 113.8 110.3 109.3
108.1 106.7 105.8 113.6 112.8 109.3 112.8 110.6 110.3 111.0 112.4 114.0 109.4 109.3 109.3 114.0 109.9 109.3
109.5 106.1 106.5 112.9 112.8 108.7 111.4 112.2 110.4 111.2 112.5 114.5 109.4 109.1 109.3 114.4 114.1 111.1
108.3 107.1 106.4 113.2 112.1 109.3 111.8 111.4 110.2 110.7 112.6 114.2 109.4 109.2 109.3 114.3 113.2 109.5
ring is 108.8” and in the five-membered ring is 104.2’. Since angles j? and /I?’ are unsymmetrical, the bending at the “flap” in the chair at carbons 2 and 6 is not the same, thus the six-membered ring adopts a somewhat distorted chair conformation. Compound IIIa compared with Ia shows less flattening and adopts a much less distorted chair conformation (Fig. 5 ) .
Ia
54.4 - 58.3 36.0 35.3 54.8 104.8 - 133.3 -55.2 -53.0 70.7 - 173.9 57.2 -69.0 175.7 - 133.0 106.7 - 13.2 -43.5 -14.1 - 175.9 - 52.2 63.2
Angles
l-2-3-4 l-6-5-4 l-7-8-9 l-10-9-8 2-l-6-5 2-l-7-8 2-l-10-9 2-3-4-5 3-2-l-6 3-2-l-7 3-2-l-10 3-4-5-6 5-6-l-7 5-6-l-10 6-l-7-8 6-l-10-9 7-l-10-9 7-8-g-10 8-7-l-10 6-1-2-11 7-l-2-11 10-l-2-11
Dihedral angles (deg. )
TABLE 4
55.8 -56.5 29.1 33.8 55.7 109.8 - 136.3 -55.8 - 55.4 66.0 - 176.6 56.1 -66.1 176.5 - 128.3 102.3 - 16.8 - 38.3 -9.0 - 177.6 -56.2 61.2
IIIa 55.7 -55.7 27.6 41.6 54.2 118.2 - 144.8 -55.8 -53.3 69.7 - 175.2 54.9 -69.7 177.4 - 119.2 93.3 -24.4 -41.9 -2.1 72.2 - 164.8 -49.7
Ib (A) 53.8 - 59.2 36.6 32.8 55.4 105.5 - 132.6 - 55.8 -51.7 71.6 - 171.1 57.5 - 69.3 177.3 - 132.2 107.5 - 10.2 - 42.3 - 16.3 - 176.3 -53.0 64.3
n, (B) 54.3 -55.9 30.7 33.1 55.7 107.8 - 133.9 - 55.0 - 54.226 66.6 - 176.6 55.1 -65.7 178.0 - 129.7 102.7 - 15.0 -38.7 -11.2 71.6 - 167.6 -50.8
IIIb (A)
- 56.7 24.3 35.1 56.0 116.5 - 141.6 -56.3 -55.0 66.4 - 175.7 56.1 - 66.4 177.0 - 121.4 97.1 -21.1 -36.1 -3.2 - 179.3 -57.9 60.0
55.9
IIIb (B) 55.5 -57.3 32.5 38.9 54.9 111.1 - 139.2 -55.9 -53.7 69.8 - 174.9 56.3 -69.1 176.7 - 126.6 100.1 - 18.8 -43.3 -8.5 71.6 - 165.0 -49.6
Ic (A) 54.4 -58.2 33.0 37.4 54.8 110.5 - 138.3 -55.9 -52.4 71.2 - 172.4 57.2 -69.6 176.1 - 127.1 101.7 - 17.0 -42.9 -9.9 - 178.6 - 55.0 61.4
Ic (B) 56.0 -56.1 29.4 33.4 56.0 108.7 - 135.2 -56.0 - 55.6 65.9 - 177.3 55.7 -65.8 177.1 - 129.1 102.8 - 16.1 -38.1 -9.7 70.1 - 168.5 -51.7
111~ (A)
55.6 -56.5 24.1 34.9 55.6 115.3 - 140.1 - 56.2 - 54.8 66.9 - 176.0 56.2 -66.2 176.4 - 122.5 98.1 -21.0 -35.8 -3.2 179.4 -59.0 58.1
111~ (B)
l-2-3-4 l-6-5-4 l-7-8-9 l-11-10-9 2-l-6-5 2-l-7-8 2-l-11-10 2-3-4-5 3-2-l-6 3-2-l-7 3-2-1-11 3-4-5-6 5-6-l-7 5-6-1-11 6-l-7-8 6-l-11-10 7-l-11-10 7-B-9-10 8-7-1-11 B-9-10-11 4-3-2-12 11-1-2-12 7-1-2-12
56.6 - 56.0 - 55.9 56.7 53.2 170.9 - 172.9 -55.9 - 56.6 69.7 - 172.8 55.6 -68.7 170.9 - 68.6 69.5 -53.7 55.5 53.3 -55.8
IIa
TABLE 4 (Continued)
57.3 -57.3 -57.2 57.4 56.2 173.3 - 174.0 - 55.4 -56.5 66.1 - 177.6 55.6 -62.1 172.9 - 67.6 67.9 - 59.5 52.4 59.4 -52.5
IVa
55.7 -55.2 - 56.6 56.2 53.0 171.9 - 171.0 - 55.9 -52.4 69.8 - 172.8 54.7 -67.8 173.8 -67.6 69.1 -52.2 56.0 52.5 -55.9 -73.1 -46.6 - 164.0
IIb (A) 56.0 -57.7 -56.2 56.3 54.5 174.6 - 176.0 -56.2 -52.7 70.3 - 170.5 55.9 -68.6 173.4 -65.0 66.9 - 53.9 54.9 53.7 -54.9 - 174.3 64.1 -55.2
IIb (B) 55.7 - 56.7 -57.3 57.7 55.9 173.2 - 173.0 -55.0 -54.8 67.3 - 177.1 54.9 -62.0 174.3 - 66.9 66.8 -59.2 52.3 59.0 -52.5 - 72.7 -50.3 - 166.0
Ivb (A)
57.0 -57.8 -57.2 57.1 56.8 173.7 - 174.2 -55.7 - 56.0 66.5 - 176.7 55.6 -61.9 173.9 - 67.0 67.6 - 58.8 52.6 58.8 -52.6 - 176.7 59.0 -57.8
IVb (B)
-55.6 -56.5 56.6 53.8 171.7 - 172.2 -55.8 -54.1 68.7 - 174.1 54.7 -67.5 173.1 -68.1 69.3 -53.1 55.7 53.1 - 55.8 - 70.6 -48.5 - 165.7
56.9
IIc (A)
56.3 - 56.5 -55.8 56.4 53.9 172.0 173.8 -56.0 -53.3 69.7 - 171.8 55.5 - 68.4 172.0 -67.5 68.8 -53.3 55.5 52.9 -55.8 - 174.6 61.1 -57.4
IIc (B)
57.2 -57.0 -57.2 57.6 56.6 173.0 - 173.8 -55.6 -56.6 66.0 - 178.3 55.1 -61.7 173.7 -67.5 67.4 -59.6 52.2 59.4 - 52.3 - 70.2 -52.0 - 167.8
IVC (A)
56.5 -57.4 -56.8 57.0 56.1 171.9 - 172.4 -55.7 -55.5 67.0 - 176.4 55.8 -61.9 172.8 - 68.5 69.0 -58.3 52.8 58.2 -52.9 - 176.3 57.7 - 58.9
IVc (B)
!G
Compression Bending Stretch-bend Van der Waals 1,4 energy other Torsional Dipole Final steric energy Dipole moment
16.16
1.19 1.89 0.28 9.08 -2.31 6.04
IIa
1.24 5.30 0.59 11.71 -2.22 4.47 2.09 23.17 1.850
IVa
Type of interaction
0.97 3.17 0.33 9.55 -2.87 9.22 2.03
IIIa
0.757
19.41
1.06 3.19 0.04 7.57 -2.17 9.71
Ia
Compression Bending Stretch-bend Van der Waals 1,4 energy other Torsional Dipole Final steric energy Dipole moment
Types of interaction
Energies for types of interaction (kcal mol-‘)
TABLE 5
20.22
1.70 2.74 0.40 9.82 - 1.85 7.39
IIb (A)
22.85
1.46 3.92 0.15 8.23 - 1.86 10.95
Ib (A)
19.68
1.71 2.96 0.40 9.68 - 1.72 6.66
IIb (B)
22.31
1.48 4.01 0.15 8.18 -2.08 10.57
Ib (B)
and dipole moments (D)
1.60 5.96 0.70 12.38 -2.15 5.57 2.09 26.15 1.840
I?% (A)
1.27 3.73 0.43 10.28 -3.01 10.27 2.02 23.53 0.801
IIIb (A)
1.54 5.56 0.65 12.24 -2.40 4.78 2.08 24.45 1.839
IVb (B)
1.21 3.39 0.40 10.07 -3.26 9.66 2.06 24.98 0.739
IIIb (B)
17.23 4.150
1.44 2.19 0.27 9.54 -2.12 5.91
IIC (A)
20.02 4.150
1.25 3.43 0.03 8.02 -2.23 9.53
Ic (A)
17.03 4.150
1.41 2.18 0.27 9.43 -2.16 5.90
IIC (B)
19.94
1.23 3.38 0.03 7.95 -2.32 9.67
Ic (B)
1.45 5.61 0.55 12.11 -2.41 4.11 3.05 24.48 4.914
IVC (A)
1.15 3.37 0.32 9.94 -3.11 8.88 3.54 24.09 4.589
IIIC (A)
1.39 5.66 0.52 12.04 -2.64 3.98 2.91 23.86 5.671
IVC (B)
1.10 3.20 0.32 9.81 -3.13 8.82 3.02 23.14 4.100
IIIC (B)
188 TABLE 6 Total energies for the 2-substituted Compound
Ib IIb IIIb I# IC IIC IIIC IVC
molecules
(kcal mol-‘)
Anti conformer
Syn conformer -
(A)
(B)
(A)
(B)
22.85 20.22 24.98 26.15 20.02 24.09 17.23 24.48
22.31 19.68 23.53 24.45 19.94 23.14 17.03 23.86
22.43 20.22 25.15 26.21 20.43 24.09 17.25 26.02
22.38 23.43 23.58 31.75 19.98 23.16 19.10 28.15
9
10
Fig. 3. Numbering
system and angles to illustrate deformation
Fig. 4. Newman projections oxygen.
of compounds
IVa
(right)
of the cyclohexane
ring.
and IIa (left): LP, lone pair on ether
When the series I-IVb is studied, a slight increase in bond lengths l-2,2-3 and l-6 is observed. Systematic changes from Ia to Ib are the increase in a’, /I’ and y and the decrease in cx,p and y’ . The dihedral angles do not show large
189
Fig. 5. Newman projections of compounds IIIa (left) and Ia (right): LP, lone pair on ether oxygen.
deviations in (B ) , but angles l-7-8-9 and l-10-9-8 in (A) change drastically; both angles are approximately 350” in Ia, but are 27.60” and 41.62”, respectively, in Ib(A). This behavior shows that carbons 8 and 9 are moving away from 2 and coming closer to 6 when Me substitution takes place. Introduction of the Me group to IIa, (IIb ), causes a systematic increase in (Y’ and /?’ and a decrease in (Y and p without much alteration in the dihedral angles. Inspection of IIIb and IVb shows that the presence of the oxygen atoms does not change the previously observed tendency of the molecules. However, the rings are less flattened and more twisted. When the series I-IVc is studied, the changes are in general the same as they are in I-IVb, but smaller in magnitude. The most striking difference is the polarities of the 111~conformers, where conformer (A) is more polar than conformer (B ). In contrast, for IVc, conformer (B ) is more polar than conformer (A). This behavior is in accordance with the experimental data given for similar compounds by Zefirov et al. [ 41. Another general feature observed is that the equatorial conformers (B ) have a greater tendency to be more twisted. The remaining geometric parameters also changed but to a lesser degree and not so systematically. Data in Table 4 reveal that types of interactions which contribute to the total energy are almost of the same magnitude for different molecules. In general, interactions seem to be of the same kind in these molecules. In Table 6, the total energies for all the conformers are summarized. For 2substituted I and III, the predominance of the equatorial conformer is a straightforward consequence of the two equatorial conformers being more stable than the two axial conformers. For 2-substituted II and IV, according to Table 6, the lowest enthalpy is found for the equatorial (anti) conformer, and the equatorial (syn) conformer is highly unstable compared to the equatorial (anti). The orientation of the second ring in the axial conformers is of minor significance, so that the energies of the axial (syn) and (anti) conformers are similar. As a result, the two conformers with axial X and the one with equa-
190
torial X are relatively stable. Thus, the axial conformation should have greater entropy (mixing entropy), i.e. the content of the axial conformer in the equilibrium mixture should increase with increasing temperature. This is in accordance with experimental results [ 11. For 111~ and IVc, polarity of the solvent is expected to play an important role in conformational equilibria. Compound 111~ is predicted to display an axial shift in polar solvents, whereas IVc is predicted to show an equatorial shift. These predictions are in parallel with experimental results reported for similar compounds [ 11. ACKNOWLEDGMENTS
I thank Prof. Hadi &bal for encouragement, advice and most valuable discussions, and also for critically reading the final manuscript prior to publication. This work was supported by Bogazici University Research Funds, grant number 88B0545.
REFERENCES 1 I.G. Mursakulov, E.A. Ramasanov, M.M. Guseinov, N.S. Zefirov, V.V. Samoshin and E.L. Eliel, Tetrahedron, 36 (1980) 1885. 2 I.G. Mursakulov, M.M. Guseinov, N.K. Kasumov, N.S. Zefirov, V.V. Samoshin and E.G. Chalenko, Tetrahedron, 38 (1982) 2213. 3 N.S. Zefirov, E.G. Chalenko, I.G. Mursakulov, M.M. Guseinov, N.N. Kasumov and E.L. Ramazonov, Zh. Org. Khim., 14 (1978) 1560. 4 I.G. Mursakulov, V.V. Samoshin, R.V. Binnatov, N.K. Kasumov, M.I. Povolotskii and N.S. Zefirov, Zh. Org. Khim. 19 (1983) 2527. 5 H. Gzbal, T. Varnali and A.E. Getin, Can. J. Chem., submitted for publication. 6 N.L. Allinger, MM2 (85) -PC, converted and modified by A.B. Buda. Dept. of Chem. Princeton University, Princeton NJ, USA. 7 U. Burkert and N.L. Allinger, Molecular Mechanics, American Chemical Society, Washington, DC, 1982, p. 96. 8 U. Burke& Tetrahedron, 35 (1979) 691.
Molecular mechanics cyclohexanes
application
181
to 1,1,2-trisubstituted
Tereza Varnali Bogazici University, Bebek 80815, Istanbul (Tiirkiye) (Received 3 June 1991; in final form 28 August 1991)
Abstract Molecular mechanics was used to calculate the energies of the conformers of l,l,Z-trisubstituted cyclohexanes (I-IV). Geometrical parameters of conformational equilibria were investigated. Results showed qualitative accordance with experimental data.
INTRODUCTION
In studies of the conformational behavior of 1,1,2_trisubstituted cyclohexanes, fundamental differences have been observed between compounds I and II, as well as between III and IV (Fig. 1). Compounds I and III show predominance for the equatorial conformer (B), while the axial conformer (A) predominates for compounds II and IV. Experimental results reported for similar compounds are summarized in Table 1. In the present work, the aim is to study the influence of the structure of the 1,1,2-substituents on cyclohexane by carrying out molecular mechanics calculations first on representative model molecules (Ia and IIa) and then on
I-IV (A)
I-IV (B)
k
Fig. 1. Conformations of 1,1,2-trisubstitutedcyclohexane: I: R,R=-(CH,),-; ; III: R,R=-O(CH,),O-; IV: R,R=-O(CH,),O-; a: X=-H; b: X=-CH,; Correspondence
0022-2860/92/$05.00
II: R,R=-(CH,),c: X=-CN.
to: Dr. T. Varnali, Bogazici University, Bebek 80815, Istanbul, Ttirkiye.
0 1992 Elsevier Science Publishers B.V. All rights reserved.
182 TABLE 1 Experimental results reported for similar compounds Compound
Substituent
Preferred conformation
Ref.
I II III III III IV IV IV
-OMe,-OAc,-OSiMe,,-OH -OMe,-OAc,-OSiMe,,-OH -Cl,-Br,-OH,-OMe,-SMe -F,-Cl,-Br,-OMe -CN -Cl,-Br,-OH,-OMe,-SMe -F,-Cl,-Br,-OMe -CN
equatorial axial’ equatorial equatorialb equatorial axial axial” axial
1 1 2,3 4 5 2,3 4 5
“The enthalpy-preferred conformer is the equatorial. bathe percentage of the axial conformer increases with solvent polarity. “The percentage of the equatorial conformer increases with solvent polarity.
X
X
II(A)syn
II(B)syn
II(A)anti
II(B)anti
Fig. 2. Syn and anti orientations of the second ring relative to the X-substituted
ring.
molecules that have been modified step by step. Initially, methyl, a non-polar, bulky substituent is introduced (Ib and IIb ) , followed by cyano, a polar nonbulky substituent (Ic and 11~) , and finally methylene groups are replaced by ether oxygens (IIIa,b,c and IVa,b,c). The QCMP.032 program [6] was used for calculations. The possibility of both syn and anti orientations of the second ring relative to the X-substituted ring was also taken into account (see Fig. 2). RESULTS AND DISCUSSION
The calculations were carried out for all four conformers of each compound with the exception of the series a (X= H). The geometrical parameters resulting from the calculations for the anti conformers are tabulated in Tables 2-4. In almost all cases, the syn conformers had higher conformational energies (see Table 6). Energies, according to the types of interactions that contribute to the total energy, are shown in Table 5. In IIa, both cyclohexane rings flatten around the vicinity of the quaternary
183
TABLE 2 Bond lengths (A) Bond
Ia
IIIa
Ib (A)
Ib (B)
IIIb (A)
IIIIb
Ic (A)
Ic (B)
111~ (A)
111~ (B)
(B) l-2 l-6 l-7 l-10 2-3 3-4 4-5 5-6 7-8 8-9 9-10 2-11 11-12
1.545 1.544 1.549 1.552 1.537 1.534 1.534 1.535 1.534 1.531 1.534
1.528 1.528 1.412 1.409 1.536 1.535 1.535 1.536 1.411 1.507 1.410
1.552 1.548 1.556 1.550 1.543 1.534 1.533 1.536 1.537 1.531 1.533 1.542
1.555 1.549 1.550 1.554 1.544 1.533 1.532 1.534 1.534 1.531 1.535 1.542
1.535 1.529 1.414 1.410 1.541 1.535 1.534 1.535 1.410 1.506 1.410 1.540
1.535 1.531 1.413 1.408 1.540 1.534 1.533 1.535 1.412 1.507 1.410 1.540
1.549 1.546 1.553 1.552 1.540 1.534 1.534 1.536 1.535 1.531 1.533 1.479 1.158
1.549 1.547 1.551 1.552 1.540 1.533 1.533 1.535 1.535 1.531 1.534 1.478 1.158
1.536 1.529 1.413 1.409 1.539 1.535 1.535 1.536 1.410 1.507 1.409 1.480 1.158
1.536 1.529 1.412 1.406 1.539 1.535 1.534 1.535 1.412 1.507 1.409 1.480 1.158
Bond
IIa
IVa
IIb (A)
IIb (B)
IVb (A)
IVh (B)
IIc (A)
IIC (B)
IVc (A)
IVc (B)
l-2 l-6 l-7 l-11 2-3 3-4 4-5 5-6 7-8 8-9 9-10 lo-11 2-12
1.550 1.547 1.547 1.550 1.536 1.533 1.534 1.536 1.536 1.534 1.533 1.536
1.536 1.533 1.412 1.413 1.536 1.534 1.540 1.537 1.415 1.522 1.522 1.415
1.562 1.549 1.553 1.551 1.543 1.533 1.533 1.536 1.536 1.532 1.532 1.537 1.543
1.562 1.553 1.548 1.553 1.543 1.532 1.531 1.535 1.537 1.534 1.534 1.537 1.542
1.545 1.534 1.415 1.413 1.542 1.534 1.533 1.536 1.415 1.521 1.521 1.415 1.542
1.544 1.535 1.412 1.414 1.541 1.533 1.533 1.536 1.415 1.522 1.522 1.415 1.542
1.557 1.548 1.550 1.551 1.540 1.534 1.533 1.536 1.536 1.533 1.533 1.536 1.479
1.556 1.550 1.548 1.551 1.539 1.533 1.532 1.536 1.537 1.534 1.534 1.536 1.478
1.545 1.533 1.413 1.412 1.540 1.534 1.534 1.537 1.415 1.522 1.522 1.415 1.481
1.544 1.533 1.411 1.412 1.539 1.534 1.533 1.536 1.414 1.522 1.522 1.415 1.482
carbon atom. The endocyclic angle at the quaternary carbon decreases to 107.8” compared with cyclohexane (111.4” ) [ 71 and l,l-dimethylcyclohexane (109.9’) [7]. The anglesbandj? (113.8” and 113.9”) display the bending of the whole “flap” of the chair. Flattening [ 81 decreases gradually as one moves away from the quaternary carbon. The rings are twisted, with (Y= 112.6” and (Y’ = 108.0” (Fig. 3). In compound IVa, when compared with IIa, the cyclohexane ring is less flattened, yet more twisted (Fig. 4). Angles p and j? show less bending at the “flap” of the chair. The dioxane ring also flattens (OCO angle = 109.3 o and ccc=114.0”). In Ia, the endocyclic angle at the quaternary carbon in the six-membered
184 TABLE 3 Bond angles (deg.) Angle
Ia
IIIa
Ib (A)
fi (B)
IIIb (A)
IIIb (B)
Ic (A)
Ic (B)
111~(A)
111~(B)
2-l-6 2-l-7 2-l-10 6-l-7 6-l-10 7-l-10 l-2-3 2-3-4 3-4-5 4-5-6 l-6-5 l-7-8 7-8-9 8-9-10 l-10-9 l-2-11 3-2-11
108.8 111.8 110.1 111.6 110.2 104.2 11X6 111.3 110.5 110.5 112.5 106.1 102.3 102.2 106.2 109.9 109.4
110.4 110.3 109.3 109.7 110.0 107.0 111.9 110.9 110.7 110.8 111.7 107.6 102.0 101.4 107.0 110.0 109.7
109.7 112.0 111.8 110.4 109.7 103.1 111.4 112.5 110.5 111.2 113.2 107.5 103.2 101.3 105.8 114.0 110.2
108.9 113.0 112.0 110.6 108.0 104.2 112.4 112.6 110.1 109.8 113.1 106.3 102.2 102.9 106.5 115.2 108.3
111.7 109.9 109.9 108.8 110.0 106.5 110.6 112.5 110.9 111.0 111.7 107.7 101.7 101.4 107.4 113.3 111.3
110.4 110.9 109.9 109.3 109.3 106.8 111.0 111.6 110.5 110.5 112.1 108.0 102.7 101.5 106.9 113.5 109.8
109.1 112.1 110.9 111.1 110.0 103.7 112.4 111.7 110.4 110.7 112.8 106.9 102.6 101.7 106.1 112.4 110.4
109.0 112.4 111.0 111.1 108.9 104.3 112.7 112.0 110.3 110.2 112.9 106.5 102.7 102.1 105.9 113.4 110.3
110.9 110.2 109.2 109.6 110.3 106.7 110.8 111.4 110.8 110.8 111.7 107.7 101.9 101.4 107.4 112.9 111.3
110.3 110.5 109.4 109.7 110.0 107.0 111.6 111.0 110.7 110.8 111.8 108.0 102.7 101.5 106.9 111.9 109.9
IIa
IVa
IIb
IIb
IL%(A)
IVb (B)
IIc
IIC
IVc (A)
IVc (B)
(A)
(B)
(A)
(B)
108.9 109.3 110.2 111.6 109.8 107.0 112.1 112.4 110.5 111.2 114.3 114.7 110.9 110.1 111.1 114.7 114.8 109.8
107.3 112.0 110.0 111.7 108.4 107.3 112.8 112.2 110.1 110.3 114.5 114.2 111.0 110.9 110.8 114.2 116.0 108.1
108.0 110.0 109.2 112.2 110.3 107.2 112.7 111.4 110.7 111.2 114.0 114.3 111.0 110.3 111.0 114.3 113.4 109.8
107.7 111.0 108.8 112.0 109.4 107.9 113.1 111.6 110.5 110.8 114.2 114.0 111.1 110.7 110.8 113.9 114.0 110.3
108.7 106.6 105.7 113.4 113.1 108.9 111.6 111.2 110.4 111.1 112.3 114.2 109.4 109.3 109.2 114.3 113.3 111.1
108.9 106.3 105.6 113.4 112.4 109.8 112.0 111.3 110.4 110.8 112.5 114.3 109.4 109.2 109.3 114.3 113.1 110.7
2-l-6 2-l-7 2-l-11 6-l-7 6-l-11 7-l-11 l-2-3 2-3-4 3-4-5 4-5-6 l-6-5 l-7-8 7-8-9 8-9-10 9-10-11 l-11-10 1-2-12 3-2-12
107.8 110.3 108.0 112.6 110.3 107.8 113.8 110.8 110.5 111.1 113.9 113.9 111.1 110.6 110.8 113.8 110.3 109.3
108.1 106.7 105.8 113.6 112.8 109.3 112.8 110.6 110.3 111.0 112.4 114.0 109.4 109.3 109.3 114.0 109.9 109.3
109.5 106.1 106.5 112.9 112.8 108.7 111.4 112.2 110.4 111.2 112.5 114.5 109.4 109.1 109.3 114.4 114.1 111.1
108.3 107.1 106.4 113.2 112.1 109.3 111.8 111.4 110.2 110.7 112.6 114.2 109.4 109.2 109.3 114.3 113.2 109.5
ring is 108.8” and in the five-membered ring is 104.2’. Since angles j? and /I?’ are unsymmetrical, the bending at the “flap” in the chair at carbons 2 and 6 is not the same, thus the six-membered ring adopts a somewhat distorted chair conformation. Compound IIIa compared with Ia shows less flattening and adopts a much less distorted chair conformation (Fig. 5 ) .
Ia
54.4 - 58.3 36.0 35.3 54.8 104.8 - 133.3 -55.2 -53.0 70.7 - 173.9 57.2 -69.0 175.7 - 133.0 106.7 - 13.2 -43.5 -14.1 - 175.9 - 52.2 63.2
Angles
l-2-3-4 l-6-5-4 l-7-8-9 l-10-9-8 2-l-6-5 2-l-7-8 2-l-10-9 2-3-4-5 3-2-l-6 3-2-l-7 3-2-l-10 3-4-5-6 5-6-l-7 5-6-l-10 6-l-7-8 6-l-10-9 7-l-10-9 7-8-g-10 8-7-l-10 6-1-2-11 7-l-2-11 10-l-2-11
Dihedral angles (deg. )
TABLE 4
55.8 -56.5 29.1 33.8 55.7 109.8 - 136.3 -55.8 - 55.4 66.0 - 176.6 56.1 -66.1 176.5 - 128.3 102.3 - 16.8 - 38.3 -9.0 - 177.6 -56.2 61.2
IIIa 55.7 -55.7 27.6 41.6 54.2 118.2 - 144.8 -55.8 -53.3 69.7 - 175.2 54.9 -69.7 177.4 - 119.2 93.3 -24.4 -41.9 -2.1 72.2 - 164.8 -49.7
Ib (A) 53.8 - 59.2 36.6 32.8 55.4 105.5 - 132.6 - 55.8 -51.7 71.6 - 171.1 57.5 - 69.3 177.3 - 132.2 107.5 - 10.2 - 42.3 - 16.3 - 176.3 -53.0 64.3
n, (B) 54.3 -55.9 30.7 33.1 55.7 107.8 - 133.9 - 55.0 - 54.226 66.6 - 176.6 55.1 -65.7 178.0 - 129.7 102.7 - 15.0 -38.7 -11.2 71.6 - 167.6 -50.8
IIIb (A)
- 56.7 24.3 35.1 56.0 116.5 - 141.6 -56.3 -55.0 66.4 - 175.7 56.1 - 66.4 177.0 - 121.4 97.1 -21.1 -36.1 -3.2 - 179.3 -57.9 60.0
55.9
IIIb (B) 55.5 -57.3 32.5 38.9 54.9 111.1 - 139.2 -55.9 -53.7 69.8 - 174.9 56.3 -69.1 176.7 - 126.6 100.1 - 18.8 -43.3 -8.5 71.6 - 165.0 -49.6
Ic (A) 54.4 -58.2 33.0 37.4 54.8 110.5 - 138.3 -55.9 -52.4 71.2 - 172.4 57.2 -69.6 176.1 - 127.1 101.7 - 17.0 -42.9 -9.9 - 178.6 - 55.0 61.4
Ic (B) 56.0 -56.1 29.4 33.4 56.0 108.7 - 135.2 -56.0 - 55.6 65.9 - 177.3 55.7 -65.8 177.1 - 129.1 102.8 - 16.1 -38.1 -9.7 70.1 - 168.5 -51.7
111~ (A)
55.6 -56.5 24.1 34.9 55.6 115.3 - 140.1 - 56.2 - 54.8 66.9 - 176.0 56.2 -66.2 176.4 - 122.5 98.1 -21.0 -35.8 -3.2 179.4 -59.0 58.1
111~ (B)
l-2-3-4 l-6-5-4 l-7-8-9 l-11-10-9 2-l-6-5 2-l-7-8 2-l-11-10 2-3-4-5 3-2-l-6 3-2-l-7 3-2-1-11 3-4-5-6 5-6-l-7 5-6-1-11 6-l-7-8 6-l-11-10 7-l-11-10 7-B-9-10 8-7-1-11 B-9-10-11 4-3-2-12 11-1-2-12 7-1-2-12
56.6 - 56.0 - 55.9 56.7 53.2 170.9 - 172.9 -55.9 - 56.6 69.7 - 172.8 55.6 -68.7 170.9 - 68.6 69.5 -53.7 55.5 53.3 -55.8
IIa
TABLE 4 (Continued)
57.3 -57.3 -57.2 57.4 56.2 173.3 - 174.0 - 55.4 -56.5 66.1 - 177.6 55.6 -62.1 172.9 - 67.6 67.9 - 59.5 52.4 59.4 -52.5
IVa
55.7 -55.2 - 56.6 56.2 53.0 171.9 - 171.0 - 55.9 -52.4 69.8 - 172.8 54.7 -67.8 173.8 -67.6 69.1 -52.2 56.0 52.5 -55.9 -73.1 -46.6 - 164.0
IIb (A) 56.0 -57.7 -56.2 56.3 54.5 174.6 - 176.0 -56.2 -52.7 70.3 - 170.5 55.9 -68.6 173.4 -65.0 66.9 - 53.9 54.9 53.7 -54.9 - 174.3 64.1 -55.2
IIb (B) 55.7 - 56.7 -57.3 57.7 55.9 173.2 - 173.0 -55.0 -54.8 67.3 - 177.1 54.9 -62.0 174.3 - 66.9 66.8 -59.2 52.3 59.0 -52.5 - 72.7 -50.3 - 166.0
Ivb (A)
57.0 -57.8 -57.2 57.1 56.8 173.7 - 174.2 -55.7 - 56.0 66.5 - 176.7 55.6 -61.9 173.9 - 67.0 67.6 - 58.8 52.6 58.8 -52.6 - 176.7 59.0 -57.8
IVb (B)
-55.6 -56.5 56.6 53.8 171.7 - 172.2 -55.8 -54.1 68.7 - 174.1 54.7 -67.5 173.1 -68.1 69.3 -53.1 55.7 53.1 - 55.8 - 70.6 -48.5 - 165.7
56.9
IIc (A)
56.3 - 56.5 -55.8 56.4 53.9 172.0 173.8 -56.0 -53.3 69.7 - 171.8 55.5 - 68.4 172.0 -67.5 68.8 -53.3 55.5 52.9 -55.8 - 174.6 61.1 -57.4
IIc (B)
57.2 -57.0 -57.2 57.6 56.6 173.0 - 173.8 -55.6 -56.6 66.0 - 178.3 55.1 -61.7 173.7 -67.5 67.4 -59.6 52.2 59.4 - 52.3 - 70.2 -52.0 - 167.8
IVC (A)
56.5 -57.4 -56.8 57.0 56.1 171.9 - 172.4 -55.7 -55.5 67.0 - 176.4 55.8 -61.9 172.8 - 68.5 69.0 -58.3 52.8 58.2 -52.9 - 176.3 57.7 - 58.9
IVc (B)
!G
Compression Bending Stretch-bend Van der Waals 1,4 energy other Torsional Dipole Final steric energy Dipole moment
16.16
1.19 1.89 0.28 9.08 -2.31 6.04
IIa
1.24 5.30 0.59 11.71 -2.22 4.47 2.09 23.17 1.850
IVa
Type of interaction
0.97 3.17 0.33 9.55 -2.87 9.22 2.03
IIIa
0.757
19.41
1.06 3.19 0.04 7.57 -2.17 9.71
Ia
Compression Bending Stretch-bend Van der Waals 1,4 energy other Torsional Dipole Final steric energy Dipole moment
Types of interaction
Energies for types of interaction (kcal mol-‘)
TABLE 5
20.22
1.70 2.74 0.40 9.82 - 1.85 7.39
IIb (A)
22.85
1.46 3.92 0.15 8.23 - 1.86 10.95
Ib (A)
19.68
1.71 2.96 0.40 9.68 - 1.72 6.66
IIb (B)
22.31
1.48 4.01 0.15 8.18 -2.08 10.57
Ib (B)
and dipole moments (D)
1.60 5.96 0.70 12.38 -2.15 5.57 2.09 26.15 1.840
I?% (A)
1.27 3.73 0.43 10.28 -3.01 10.27 2.02 23.53 0.801
IIIb (A)
1.54 5.56 0.65 12.24 -2.40 4.78 2.08 24.45 1.839
IVb (B)
1.21 3.39 0.40 10.07 -3.26 9.66 2.06 24.98 0.739
IIIb (B)
17.23 4.150
1.44 2.19 0.27 9.54 -2.12 5.91
IIC (A)
20.02 4.150
1.25 3.43 0.03 8.02 -2.23 9.53
Ic (A)
17.03 4.150
1.41 2.18 0.27 9.43 -2.16 5.90
IIC (B)
19.94
1.23 3.38 0.03 7.95 -2.32 9.67
Ic (B)
1.45 5.61 0.55 12.11 -2.41 4.11 3.05 24.48 4.914
IVC (A)
1.15 3.37 0.32 9.94 -3.11 8.88 3.54 24.09 4.589
IIIC (A)
1.39 5.66 0.52 12.04 -2.64 3.98 2.91 23.86 5.671
IVC (B)
1.10 3.20 0.32 9.81 -3.13 8.82 3.02 23.14 4.100
IIIC (B)
188 TABLE 6 Total energies for the 2-substituted Compound
Ib IIb IIIb I# IC IIC IIIC IVC
molecules
(kcal mol-‘)
Anti conformer
Syn conformer -
(A)
(B)
(A)
(B)
22.85 20.22 24.98 26.15 20.02 24.09 17.23 24.48
22.31 19.68 23.53 24.45 19.94 23.14 17.03 23.86
22.43 20.22 25.15 26.21 20.43 24.09 17.25 26.02
22.38 23.43 23.58 31.75 19.98 23.16 19.10 28.15
9
10
Fig. 3. Numbering
system and angles to illustrate deformation
Fig. 4. Newman projections oxygen.
of compounds
IVa
(right)
of the cyclohexane
ring.
and IIa (left): LP, lone pair on ether
When the series I-IVb is studied, a slight increase in bond lengths l-2,2-3 and l-6 is observed. Systematic changes from Ia to Ib are the increase in a’, /I’ and y and the decrease in cx,p and y’ . The dihedral angles do not show large
189
Fig. 5. Newman projections of compounds IIIa (left) and Ia (right): LP, lone pair on ether oxygen.
deviations in (B ) , but angles l-7-8-9 and l-10-9-8 in (A) change drastically; both angles are approximately 350” in Ia, but are 27.60” and 41.62”, respectively, in Ib(A). This behavior shows that carbons 8 and 9 are moving away from 2 and coming closer to 6 when Me substitution takes place. Introduction of the Me group to IIa, (IIb ), causes a systematic increase in (Y’ and /?’ and a decrease in (Y and p without much alteration in the dihedral angles. Inspection of IIIb and IVb shows that the presence of the oxygen atoms does not change the previously observed tendency of the molecules. However, the rings are less flattened and more twisted. When the series I-IVc is studied, the changes are in general the same as they are in I-IVb, but smaller in magnitude. The most striking difference is the polarities of the 111~conformers, where conformer (A) is more polar than conformer (B ). In contrast, for IVc, conformer (B ) is more polar than conformer (A). This behavior is in accordance with the experimental data given for similar compounds by Zefirov et al. [ 41. Another general feature observed is that the equatorial conformers (B ) have a greater tendency to be more twisted. The remaining geometric parameters also changed but to a lesser degree and not so systematically. Data in Table 4 reveal that types of interactions which contribute to the total energy are almost of the same magnitude for different molecules. In general, interactions seem to be of the same kind in these molecules. In Table 6, the total energies for all the conformers are summarized. For 2substituted I and III, the predominance of the equatorial conformer is a straightforward consequence of the two equatorial conformers being more stable than the two axial conformers. For 2-substituted II and IV, according to Table 6, the lowest enthalpy is found for the equatorial (anti) conformer, and the equatorial (syn) conformer is highly unstable compared to the equatorial (anti). The orientation of the second ring in the axial conformers is of minor significance, so that the energies of the axial (syn) and (anti) conformers are similar. As a result, the two conformers with axial X and the one with equa-
190
torial X are relatively stable. Thus, the axial conformation should have greater entropy (mixing entropy), i.e. the content of the axial conformer in the equilibrium mixture should increase with increasing temperature. This is in accordance with experimental results [ 11. For 111~ and IVc, polarity of the solvent is expected to play an important role in conformational equilibria. Compound 111~ is predicted to display an axial shift in polar solvents, whereas IVc is predicted to show an equatorial shift. These predictions are in parallel with experimental results reported for similar compounds [ 11. ACKNOWLEDGMENTS
I thank Prof. Hadi &bal for encouragement, advice and most valuable discussions, and also for critically reading the final manuscript prior to publication. This work was supported by Bogazici University Research Funds, grant number 88B0545.
REFERENCES 1 I.G. Mursakulov, E.A. Ramasanov, M.M. Guseinov, N.S. Zefirov, V.V. Samoshin and E.L. Eliel, Tetrahedron, 36 (1980) 1885. 2 I.G. Mursakulov, M.M. Guseinov, N.K. Kasumov, N.S. Zefirov, V.V. Samoshin and E.G. Chalenko, Tetrahedron, 38 (1982) 2213. 3 N.S. Zefirov, E.G. Chalenko, I.G. Mursakulov, M.M. Guseinov, N.N. Kasumov and E.L. Ramazonov, Zh. Org. Khim., 14 (1978) 1560. 4 I.G. Mursakulov, V.V. Samoshin, R.V. Binnatov, N.K. Kasumov, M.I. Povolotskii and N.S. Zefirov, Zh. Org. Khim. 19 (1983) 2527. 5 H. Gzbal, T. Varnali and A.E. Getin, Can. J. Chem., submitted for publication. 6 N.L. Allinger, MM2 (85) -PC, converted and modified by A.B. Buda. Dept. of Chem. Princeton University, Princeton NJ, USA. 7 U. Burkert and N.L. Allinger, Molecular Mechanics, American Chemical Society, Washington, DC, 1982, p. 96. 8 U. Burke& Tetrahedron, 35 (1979) 691.