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Synthesis and conformational analysis of macrocyclic esters of 1,8-naphthalenediboronic anhydride

Junji Tanaka, Hirofumi Tagawa and Shuji Kanemasa

Institute of Advanced Material study, Kyushu University, Kasugakoen, Kasuga 816, Japan
Multinuclear Lewis acid catalysis which is expected to act with a new type of activity and selectivity has attracted much attention. Nozaki et al. found that ortho-bis[(4R,5R)-4,5-diphenyl-1,3-dioxa-2-borolane-2-yl]benzene forms 1:2 complex with chiral amines. [1] Also, Katz showed that 1,8-naphthalenediylbis(dimethylborane) forms stable, bridged complex with H-, F-, OH-.[2]

We planned to synthesiss 2 as a chelate Lewis acid by the reaction of 1 with ethane-1,2-diol. In general, the boronic anhyride moiety is easily reacted with alcohols to give esters.[3] Compound 1 and ethane-1,2-diol were expected to react at the boronic anhyride moiety to give 1,3,2-dioxaborolane groups (Scheme 1). The reaction with ethane-1,2-diol in toluene under reflux gave surprisingly the stable 14-membered cyclic ester which is barely soluble in most organic solvents. The structure of 3 was detemined by X-ray single cystallolgraphy 1H and 13C NMR spectra. The cystal structure of 3 is shown below. The ethylene bridges rotate freely in solvent (seen by low temperature 1H NMR in toluene). In this reaction boronic anhyride was very stable to diols because the geometry of the two boron atoms was fixed to the naphthalene ring.



Model of 3

Reaction with a variety of a,omega-diols gave macrocyclic boronic anhydride esters consisting of two moieties of the boronic anhydride and the diol (Table 1). On the other hand, reaction with hexane-1,6-diol gave an 11-membered cyclic ester 7 including one diol and boronic anhyderide moiety in a single molecure (Scheme 3).


We carried out the reaction with symmetric 1,2-diols by three methods (Table 2).

  1. Reaction in toluene wiht relux (Method A)
  2. Reaction in toluene with a catalytic quantity of p-toluenesulfonic acid (Method B)
  3. Reaction in acetone at room temp. (Method C)
These methods were chosen on the basis of the reactivity of diols. Methods A and C were used for relatively active diols and less reactive ones require method B. In method C, the product was precipitaed within 10 min.

Reactions with dl-, (R,R)-, mesobutane-2,3-diol afford the corresponding cyclic esters as single stereoisomers in each case. The fact that different isomers have been formed from the dl- and (R,R)-alcohols indicates that the product formed from dl-alcohol contains the (R,R)- and (S,S)-unit in the same molecure.

Conformational analysis of these butane-2,3-diol products are based on the X-ray single crystallography, 1H- and 13C NMR spectra, and semiempirical molecular orbital calculations. The confomation of (R,R)-butane-2,3-diol product was detemined by X-ray single crystallography (model 8). This conformation was in contrast to 3 because of the demand for arranging all methyl groups outside the ethylene bridges. The dl-isomer product xx must be the same as 3 for the same reason. The structure of meso isomer could not be determined by this rule. The most stable confomer was found by semiempirical molecular obital calculations.


Model of 8
Model of 10

Conclusion

We have found that reaction of 1,8-naphthalenediboronic anhydride with diols gives macrocyclic boronic anhydride esters consisting of two moieties of the boronic anhydride and the diol. Their conformations are determined by the arrangement of side chains.

References

  1. K. Nozaki, M. Yoshida, and H. Takaya. Angew. Chem., Int. Ed. Engl., 33, 2452 (1994).
  2. H. E. Katz, J. Org. Chem., 50, 5027 (1985).
  3. P. B. Brindley, W. Gerrard, and M. F. Lappert, J. Chem. Soc., 1955, 2956.