Structural studies of organoboron compounds. XLIX.' 4,6-Bis(l-phenyl-2-nitroethyl)-2-(4-methoxyphenyl)-l,3-dioxa-4,6-diaza-2-boracyclohexane

WOLFGANG KLIEGEL AND GOTTFRIED LUBKOWTZ Institut fur Pharmazeutische Chemie der Technischen Universitat Braunschweig, Beethovenstrasse 55, 3300 Braunschweig,

Bundesrepublik Deutschland

AND

STEVEN J. RETTIG AND JAMES TROTTER Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, B.C., Canada V6T IZI

Received November 2. 1990

WOLFGANG KLIEGEL, GOTTFFUED LUBKOWTZ, STEVEN J. RETTIG, and JAMES TROTTER. Can. J. Chem. 69, 1227 (1991). The preparation of N,N'-bis(l-phenyl-2-nitroethyl)-N,N'-dihydroxymethanediamine and its subsequent reaction with 4-

methoxyphenylboronic acid to yield 4,6-bis(l-phenyl-2-nitroethyl)-2-(4-methoxyphenyl)- 1,3-dioxa-4,6-diaza-2-boracyclohex- ane are described. Crystals of the latter compound are monoclinic, a = 8.4790(8), b = 16.2100(9), c = 9.4367(6) A, P = 109.342(5)", Z = 2, space group P2,. The structure was solved by direct methods and was refined by full-matrix least-squares procedures to R = 0.032 and R, = 0.039 for 1749 reflections with I 2 3u(4. The molecule has a six-membered BONCNO cycloboronate structure. Bond lengths involving the trigonal planar boron atom are (N)O-B = 1.362(4) and 1.364(5), (ary1)C-B = 1.548(4) A.

Key words: organoboron compound, crystal structure, cycloboronate, methanediamine.

WOLFGANG KLIEGEL, GOTTFFUED LUBKOWITZ, STEVEN J. RETTIG et JAMES TROTTER. Can. J. Chem. 69, 1227 (1991) On dCcrit la prkparation de laN,N'-bis(l-phCnyl-2-nitroCthyl)-N,N'-dihydroxymtthanediamine et sa rkaction substquente avec

l'acide 4-mtthoxyphCnylboronique qui conduit au 4,6-bis(l-phtnyl-2-nitroethyl)-2-(4-mtthoxyphCnyl)- 1,3-dioxa-4,6-diaza-2- boracyclohexane. Les cristaux de ce dernier composC sont monocliniques, avec a = 8,4790(8), b = 16,2100(9) et c = 9,4367(6) A, P = 109,342(5)", Z = 2 et groupe d'espace P2,. On a rCsolu la structure par des mCthodes directes et on l'a affinCe par la mithode directes et on 1'affinCe par la mCthode des moindres carrCs jusqu'h des valeurs respectives de R = 0,032 et R,, = 0,039 pour 1749 rtflexions avec I 2 3u(4. La molCcule contient une structure cvcloboronate BONCNO h six chainons. Les longeurs des liaisons impliquant I'atome de bore plan trigonal sont (N)O-B - 1,362(4) et 1,364(5), (ary1)C-B = 1,548(4) A.

Mots clPs : composk organobort, structure cristalline, cycloboronate, mkthanediamine. [Traduit par la rtdaction]

Introduction The condensation of a newly synthesized N-(P-nitroalkyl)

substituted dihydroxyaminal 2a (R = CHPhCH2N02) with p- methoxyphenylboronic acid (Ar = 4-MeOC6H4) did not lead to the 1:2 reaction product with the general structure 1 which is the usual product of the reaction of simple N-alkyl derivatives 2 with arylboronic acids (1, 2). The elemental analysis and spec- tral data for the isolated crystalline substance was consistent with a 1:l condensation product of 2a and the boronic acid, having a probable structure 3. As we have recently reported (3), a six-membered cyclic arylboronate 3b has been obtained by condensation of a weakly basic aminal2b (R = CMe2CN) with a sterically hindered arylboronic acid (Ar = 2,4,6-Me3C6H2). On the basis of this result, the formation of a cyclic condensate 3a would be plausible on the grounds of an attenuation of the basicity at the aminal nitrogen centres by the electron- withdrawing P-nitroalkyl residues. The arylboronic acid moiety in 3a, however, is not hindered sterically, but its Lewis acidity may perhaps be reduced by the inductive and mesomeric effects of the p-methoxy group (4).

In compound 3b both of the bulky N-cyanoalkyl substituents were found to occupy equatorial positions of a semiplanar ring ("sofa" conformation) 3 EE, most likely stabilized by the mini-

known to be the predominant form present in solutions of 3,5-dimethyl-I-oxa-3,5-diazacyclohexane ( 3 , the only form present in solutions of a related seven-membered cyclic aminal (6), and in equilibrium with other isomers in solutions of simple 1,3-diazacyclohexanes (7). The axial-axial conformer 3 AA is thought to be highly unfavorable due to 1,3-syn-diaxial steric interaction, inducing the inversion at nitrogen leading to 3 AE/EA or the ring inversion converting 3 AA directly to 3 EE. Other possible structures for the condensation product include the five-membered ring system 4 or its dimer, and the six- membered cycloboronate 5 , which could be considered as a rearrangement product of 4 resulting from an aminal-N-oxide rearrangement to an N,O-acetal(8).

Infrared and nmr spectroscopic data do not allow for an unambiguous assignment of one of the alternative structures 3-5. In the 'H nrnr spectrum, two different signals were found for the CH2 protons of the aminal methylene group, one pre- dominant signal at 3.68 ppm (65-80% of the total peak area for CH2) and another signal at 3.58 ppm (20-35%). It is not yet clear whether this phenomenon results from an equilibrium between two forms in solution, or from partial decomposition in the chosen ~ o l v e n t . ~ An X-ray crystallographic study has been

mization of steric interactions (3). FO; compound 3a, having 2~-(~-~itroalkyl)aminals of the type 2a seem to be quite unstable

less bulky N - ~ I ~ Y ~ substituents, the axial-equatorial isomer (3 (see Experimental section). Preliminary variable-tempera- AE Or EA) must be Axial-equatorial isomers are ture nmr studies of the cycloboronate 3a show broadening and splitting

'previous paper in this series: ref. 18. of the dominant methyline signal into an AB pattern at lower tempera- tures, the minor signal remaining unaffected.

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CAN. J. CHEM. VOL. 69, 1991

undertaken in order to determine the structure and conformation of this material in the solid state.

Experimental N,N1-Bis(1 -phenyl-2-nitroethyl)-N,N'-dihydromethanediamine, 2a N-(I-Phenyl-2-nitroethyl)-hydroxylamine (9) (5.46 g, 30 mmol) is

dissolved, with slight heating, in 30 mL ethanol (96%). The solution is cooled down to room temperature and mixed with aqueous formalde- hyde (15 mmol of a 40% solution). Stirring is continued until crystal- lization commences. The solution is then cooled down to - 18°C and, after 6 h, filtered. The crystalline precipitate is washed with ethan01.~ Yield: 3.27 g (58%). Melting point 116-1 17°C (from ethanol/petroleum ether). Anal. calcd. for C17H20N406: C 54.25, H 5.36, N 14.89; found: C54.21, H5.39, N 14.82. Infrared(KBr): 3180 (0-H), 1600 ( C X , weak), 1550 cm-' (NO2). 'H nmr (90 MHz, CDCI3/TMS): 6 (ppm) = 3.50 (s, N-CH2-N), 4.22-5.02 (ABX pattern, 2 CH2CH), 7.08-7.52 (m, 2 C6H5), 9.35 (s, 2 OH, exchange- able). Mass spectrum (70 eV, 60°C): m/z = 150 (25%,

4,6-Bis(1 -phenyl-2-nitroethyl)-2-(4-methopheny1)- ,3-dioxa-4,6- diaza-2-boracyclohexane, 3a

2a (0.75 g, 2 mmol) and 4-methoxyphenylboronic acid (0.30 g, 2 rnrnol) are suspended in 20 mL of benzene and refluxed for 1 h with continuous removal of water using a Dean Stark trap. After partial evaporation of the solvent in vacuo, petroleum ether (bp 30-75°C) is added. Colorless crystals form upon slow cooling. Yield: 0.85 g

3 ~ h e product thus obtained is analytically pure. During recrystalliza- tion from ethanol, decomposition to N,N-bis(1-phenyl-2-nitroethyl)- hydroxylamine, mp 103- 105'C, occurs (10). The thermal instability of 2a can also be seen in the EI mass spectrum which does not display a molecular ion peak.

(86%). Melting point (decamp.) 148- 149°C (from benzene). Anal. calcd. for C24H25BN407: C 58.56, H 5.12, B 2.20, N 11.38; found: C 58.53, H 5.17, B 2.48, N 11.38. Infrared (KBr): 1600 ( C X ) , 1555 cm-' (NO2). 'H nmr (90 MHZ, CDC13/TMS): 6 (ppm) = 3.58 and 3.68 (s and s, 20-35% and 65-80%,4 N--CH2-N), 3.78 (s, 0CH3), 4.35-5.33 (overlapping ABX systems, 2CH2CH), 6.85 and 7.50 (dd, J = 9 Hz, B-C6H4), 7.00-7.35 (m, 2 C2H5). Mass spectrum (70 eV, 150°C): m/z = 492 (9%, M'), 432 (4%, M - CH2N02), 372 (6%, M - 2(CH2N02)), 328 (11%, M - N =CH(C6H5)CH2N02), 281 (6%), 268 (13%, (CH30C6H4BO)2), 197 (6%), 150 (13%, C6H5CHCH2N02), 134 (1 6%, CH30C6H4BO), 12 1 (20%), 104 (I@)%, C6H5CHCH2), 91 (39%, C7H7), 77 (36%, C6Hs), 65 (15%).

X-ray crystallographic analysis of3a A crystal ca. 0.10 X 0.20 X 0.25 rnrn in size was mounted on a glass

fiber. Unit-cell parameters were refined by least-squares on setting angles for 25 reflections (20 = 69.6-94.0") measured on a diffracto- meter with Cu-K, radiation (A = 1.54178 A). Crystal data at 21°C are as follows

Monoclinic, a = 8.4790(8), b = 16.2100(9), c = 9.4367(6) A, P = 109.342(5)", V = 1223.8(1) A3, Z = 2, p, = 1.336Mg m-3, F(000) = 516, ~(CU-K,) = 7.83 cm-'. Absent reflections: OM), k odd, space group P2, (No. 4) from structure analysis.

Intensities were measured with graphite-monochromated Cu-K, radiation on a Rigaku AFC6S diffractometer. An w-20 scan at 16' min-I over a range of (1.00 + 0.20 tan 0)' in w (with up to eight rescans, background/scan time ratio = 0.5) was employed. Data were measured to 20 = 155". The intensities of three check reflections, measured every 200 reflections throughout the data collection, showed

4~ariable for successive experiments at room temperature over a period of several hours.

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KLIEGEL ET AL.: 4 1229

TABLE 1. Final atomic coordinates (fractional) and Beg (A2)*

Atom x Y z Be,

only small random variations. After data r e d ~ c t i o n , ~ an empirical absorption correction, based on azimuthal scans for three reflections, was applied. Transmission factors range from 0.89 to 1.00. Of the 2785 reflections measured, 2606 were unique (Rin, = 0.021) and 1749 (67.1%) had intensities greater than or equal to 3u(F2) above back- ground where u2(F2) = [s2(C + 4B) + (0.025F2)']/Lp2 with S = scan speed, C = scan count, B = total background count, and Lp = Lorentz-polarization factor.

The structure analysis was initiated in the noncentrosymmetric space group on the basis of the E-statistics, this choice being confirmed by the subsequent successful solution and refinement of the structure. The structure was solved by direct methods, the coordinates of all non- hydrogen atoms being determined from an E-map. The non-hydrogen atoms were refined with anisotropic thermal parameters and the hy- drogen atoms were fixed in idealized positions (C-H = 0.98 A , BH =

1 .2Bbondcd atom). A correction for secondary extinction was applied, the final value of the extinction coefficient being 5.46 X lo4. Scattering factors for all atoms and anomalous dispersion corrections for the non-hydrogen atoms were taken from ref. 11. The weighting scheme w = 4F,2/a2(F,2) gave uniform average values of w(lFoI - IF,^)' over ranges of both IF0I and sin 0 / h and was employed in the final stages of full-matrix least-squares refinement of 325 variables. Reflections with I < 3 4 4 were not included in the refinement. Convergence was reached at R = 0.032 and R, = 0.039 for 1749 reflections with I ? 3 4 4 . The function minimized was Xw(lF 4 - IF,~)~, R = E F ~ - IFcI(/XIFol, R,, = (Cw(lFoI - I F ~ ) ~ / c ~ ~ F ~ I ~ ) ~ ~ . A parallel refinement of the mirror- image structure resulted in marginally higher residuals, the R and R,, factor ratios being 1.003 and 1.002, respectively.

On the final cycle of refinement the maximum parameter shift corres- ponded to 0 . 0 6 ~ . The mean error in an observation of unit weight was 1.45. The final difference map showed maximum fluctuations of k0.12 e A-3. The final positional and (equivalent) isotropic thermal parameters for the non-hydrogen atoms appear in Table 1. Bond lengths and angles are given in Tables 2 and 3 and intra-annular torsion angles in Table 4. Hydrogen atom parameters, anisotropic thermal

5 ~ ~ ~ ~ ~ ~ / ~ ~ ~ ~ ~ ~ structure analysis package which includes versions of the following: MITHRIL, integrated direct methods, by C. J . Gilmore; DIRDIF, direct methods for difference structures, by P. T. Beurskens; ORFLS, full-matrix least-squares, and ORFFE, func- tion and errors, by W. R. Busing, K. 0. Martin, and H. A. Levy; ORTEP 11, illustrations, by C. K. Johnson.

TABLE 2. Bond lengths (A) with estimated standards deviations

Atom Atom Distance Atom Atom Distance

O(1) N(1) 1.460(3) C(5) C(6) 1.384(5) o(1) B(1) 1.364(5) C(6) C(7) 1.367(?) o(2) N(2) 1.463(3) c(7) c(8) 1.358(7) o(2) B(1) 1.362(4) C(8) C(9) 1 .384(5) o(3) (321) 1.362(4) c ( 10) c(11) 1.517(4) o(3) (324) 1.418(5) c ( 10) (312) 1.5 17(4) o(4) N(3) 1.214(4) c(12) '313) 1.387(4) o(5) N(3) 1.213(4) (312) c ( 17) 1.374(4) O(6) N(4) 1.198(4) (313) C( 14) 1.392(5) o(7) N(4) 1 ,205 (4) '214) c( 15) 1 .366(6) N(1) c(1) 1.443(4) c(15) c ( 16) 1.370(5) N(1) C(2) 1.480(4) C( 16) ~ ( 1 7 ) 1.383(4) N(2) c(1) 1.465(4) c(18) c ( 19) 1.384(4) N(2) c(10) 1.478(4) (318) C(23) 1.389(5) N(3) (33) 1.501(4) c(18) B(1) 1.548(4) N(4) c(11) 1.494(4) c ( 19) 0) 1.390(4) c(2) c(3) 1. 500(5) c(20) c(21) 1.370(5) c(2) c(4) 1.521(4) c ( 2 1) '222) 1.374(5) c(4) c(5) 1.377(5) c(22) c(23) 1.369(5) c(4) c(9) 1.3?7(5)

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CAN. J . CHEM. VOL. 69, 1991

TABLE 3. Bond angles (deg) with estimated standard deviations

Atom Atom Atom Angle Atom Atom Atom Angle

TABLE 4. Intra-annular torsion angles (deg) with estimated standard deviations in parentheses

Atoms Torsion angle

parameters, torsion angles, intermolecular contacts, least-squares planes, bond lengths and angles involving hydrogen, and structure factors have been deposited.6

Results and discussion The X-ray analysis establishes the cycloboronate structure 3a

for the condensation product of 2a with p-methoxyphenyl- boronic acid (Ar = 4-CH30C6H4). This is the second reported example of the new BONCNO heterocycle 3 , the only other fully characterized member of this class being 3b (3). In striking contrast to the N-(a-cyanoalkyl) substituted 3b, which has both of the N-substituents in equatorial positions of the semiplanar C(1)-envelope ring (form 3 EE), the N-(P-nitroalkyl) deriva- tive 3a exists in the 3 AE form in the solid state (Fig. 1). This difference could result from a less sterically hindered axial N-alkyl substituent in 3a, as well as from dipolar and stereoelec-

6Supplementary material mentioned in the text may be purchased from the Depository of Unpublished Data, Document Delivery, CISTI, National Research Council of Canada, Ottawa, Canada KIA 0S2.

tronic effects would could favor the axial position of one of the N-substituents. In such an arrangement the antiperiplanar orientation of the non-bonding electron pair at N(1) with respect to the C ( l t N ( 2 ) bond could contribute to the stabilization of the molecular geometry by a generalized anomeric effect via n-u* overlap (12) and/or minimization of the destablilizaing, through space, n-n interaction between the two nitrogen lone pairs (13). The presence of an n-u*(.rr) effect is suggested by the relatively short N(1)-<(I) bond (1.442(4) A) compared to the adjacent C ( l t N ( 2 ) bond (1.465(4) A). In addition, the axially positioned substituent at N(l) does not interfere with the substituent at B( l ) because of the essentially planar arrangement at the sp2 boron, the nearly coplanar orientation of the B-aryl ring (dihedral angle between normals to the mean planes is 17.3"), and the near planarity of the N - - B - 0 - N moiety of 3a (planar to within 0.066(3) A).

Although the nitro group of the axial N(1) substituent neigh- bors the boronate function, the 0(4)...B(l) non-bonding dis- tance (3.196 A) approaching the normal van der Waals distance of 3.07 A (14), it is unlikely to contribute to the stabilization of the conformation 3 AE. Any significant O...B interaction should result in some degree of pyrarnidalization above the 0 ( 1 j O ( 2 ) - C ( 1 8 ) plane at the B(1) atom due to partial rehy- bridization of the sp2 boron atom. The boron atom is actually displaced from the plane of its substituents by a considerable 0.109(4) A, but in the opposite direction from 0(4), probably a result of packing forces (see Fig. 2) in view of the otherwise normal geometry for a trigonal planar boron atom at B(1). An example of true partial pyramidalization, with a similar dis- placement of the boron atom, has recently been reported for an eight-membered heterocycle containing a weak transannular N...B interaction (1 5).

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KLIEGEL ET AL.: 4

FIG. 1. Stereoview of the 4,6-bis(l-phenyl-2-nitroethyl)-2-(4-methoxyphenyl)-l,3-dioxa-4,6-diaza-2-boracyclohexane molecule; 50% prob- ability thermal ellipsoids are shown for the non-hydrogen atoms.

FIG. 2. Stereoview of the packing arrangement of 4,6-bis(I-phenyl-2-nitroethyl)-2-(4-methoxyphenyl)-1,3-dioxa-4,6-diaza-2-boracyclohexane; 32% probability thermal ellipsoids are shown for the non-hydrogen atoms.

The size of the N-(P-nitroalkyl) substituents might account for the fact that bisaxial conformer 3 AA is not observed in the solid state. The 3 AA conformer would undergo severe 1,3-syn- diaxial steric interactions, thus 3 AE represents the preferred (and perhaps exclusive) form of compound 3a. Similar ex- perimental findings for 33-dimethyl-1-oxa-3,5-diazacyclohex- ane in solution explained the most stable axial-equatorial form in terms of the anomeric effect and reduced 1,3-diaxial interac- tions (5). Ab initio studies of the basic 1-oxa-3,5- diazacyclohexane system, however, revealed the diaxial form as the most stable geometry of this cyclic aminal, interpreted as a consequence of anomeric interactions (16). Another indication of an anomeric effect involving one of the nitrogen lone pairs might be the widening of the N(l)-C(l)-N(2) angle in the 3 AE form of 3a (1 11.5(2)") relative to the corresponding angle of 106.4" in the 3 EE form of 3b (3). Similar tendencies have been

calculated for the AE and EE forms of N,N1-dimethyl-l-oxa- 33-diazacyclohexane: 109.09 and 107.25", respectively (16); the N-C-N angle for the AA form is larger still: 113.05" (16).

The N 4 bond lengths in 3a (mean 1.462 A) are very similar to those observed in 3b (mean 1.461 A) (3) and in the simple aminal 2 (R = CH,) (1.458 A) (17). As in 3b (3), the partial double bond character (corresponding to a T-bond order of -0.5) of the &B bonds in the approximately planar N - O - B--0-N ring fragment is reflected by short &B distances (mean 1.363 A) resulting from p p ( ~ ) back donation. The B ( l w ( 1 ) distance of 1.548(4) A is slight1 shorter than the- corresponding B-C(ary1) bond of 1.560 1 in 3b, the latter value being typical for six-membered phenylboronates (ref. 3 and references therein). Inductive effects resulting from the presence of the p-methoxy group in 3a probably contribute to the slight shortening of the B-C(ary1) bond. The orientation of

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1232 CAN. 1. CHEM. VOL. 69, 1991

the B-aryl substituent with respect to the boron coordination plane (see above) is similar to that reported for most arylboron- ates except the mesityl derivative 3 b which has a nearly perpen- dicular orientation of these groups.

Because of the axial and equatorial positioning of the N-alkyl groups in 3 a ( A E form) the compound displays the same configuration at both amine centres (S,S), in contrast to the opposite configurations (R,S) observed at the amine nitrogen atoms in the EE form of 3 b (3). The chiral centres at C(2) and C(10) also both have S configurations, thus the stereoisomer shown in Fig. 1 is the S,S,S,S isomer. Since the material crystallizes in a chiral space group (P21), each crystal consists of only one stereoisomer. presumably the bulk sample contains equal numbers of crystals composed of S,S,S,S and R,R,R,R isomers (assuming that the aminal2a, synthesized from racemic starting materials, is a racemate as well). It has not yet been determined whether other stereoisomers such as that derived by inversion at both nitrogen atoms (R,R,S,S, diastereoisomer - results from ring inversion) are also formed.

Acknowledgements

W e thank the Natural Sciences and Engineering Research Council of Canada and the Fonds der Chemischen Industrie, Frankfurt a m Main, for financial support.

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