Page 253 - Advanced Organic Chemistry Part A - Structure and Mechanisms, 5th ed (2007) - Carey _ Sundberg

Anomeric effects are also observed for elements in the heavier rows of the 233
periodic table. For example, trans-2,3-dichloro-1,4-dithiane exists in the diaxial confor-
mation. 255 TOPIC 2.3
The Anomeric Effect in
Cyclic Compounds
Cl
S
S
Cl

Tetravalent phosphorus groups prefer axial orientations in 1,3-dithianes and 1,3,5-
trithianes. NMR studies have indicated a preference of about 1 kcal for the
diphenylphosphoryl group. When combined with the conformational energy in a cyclo-
hexane ring, this suggests an anomeric effect of around 3 kcal. 256

O O
)
PPh 2 P(OCH 3 2
S S
S S
S
Ref. 257 Ref. 258

The anomeric effect is not limited to six-membered ring compounds. In five-
membered rings the anomeric effect can affect both ring and substituent conformation.
The 1,3-dioxole ring adopts a puckered conformation as a result of an anomeric
effect. 259 Similar effects are observed, although attenuated, in 1,3-benzodioxoles. 260

O O

The anomeric effect is also believed to be an important factor in the conformation
of ribonucleosides. The anomeric effect with the exocyclic nitrogen is stronger when
the heterocyclic base is in the pseudoaxial position. Steric factors, on the other hand,
favor the pseudoequatorial conformation. By analysis of the pH-dependent conforma-
tional equilibria, a contribution from the anomeric effect of as much as 5.6 kcal for
adenosine and 9.0 kcal for guanosine has been suggested. 261

255
H. T. Kalff and C. Romers, Acta Cryst., 18, 164 (1965).
256 E. Juaristi, N. A. Lopez-Nunez, R. S. Glass, A. Petsom, R. O. Hutchins, and J. P. Stercho, J. Org.
Chem, 51, 1357 (1986).
257 E. Juaristi, L. Valle, C. Mora-Uzeta, B. A. Valenzuela, P. Joseph-Nathan, and M. Fredrich, J. Org.
Chem., 47, 5038 (1982).
258
M. Mikolajczyk, M. Balczewski, K. Wroblewski, J. Karolak-Wojciechowska, A.J. Miller, M. W.
Wieczorek, M. Y. Antipin, and Y. T. Struchkov, Tetrahedron, 40, 4885 (1984).
259 D. Suarez, T. L. Sordo, and J. A. Sordo, J. Am. Chem. Soc., 118, 9850 (1996).
260 S. Sakurai, N. Meinander, K. Morris, and J. Laane, J. Am. Chem. Soc., 121, 5056 (1999).
261
J. Plavec, C. Thibaudeau, and J. Chattopadhyaya, Pure Appl. Chem., 68, 2137 (1996); I. Luyten, C.
Thibaudeau, and J. Chattopadhyaya, J. Org. Chem., 62, 8800 (1997).