Scherer. .So/-Gr,I-Sr wni I r/lc P/iy.ws r i n d Cheniis 1ri' of Sol-Ge/-Pror.ersinp. Academic Prcss. Boston. 1990. A. M. Br/o/ouski. U . A r~h i l e~r i i r r -S~n l l~i , / i c C'iin/ro/ iii 7 ' h i i Filnn and Solirly (Ed.: T. Bein) (ACS Symp. Sw. 1992, 499).Thc Iipase was dissolved in water or buffer. centrifuged to remove undissolved solid\. and mixed with aqueous solutions of polyvinyl alcohol and sodium fluoride. The ailanes were then added in the order of increasing reactivity. The reaction mixture was thoroughly mixed and shaken until gelation occurred (ca. 1 miii). The gela thus obtained were allowed to stand in sealed vessels for 24 h. tlricd foi-3 d;iys at 37'C. washed with water, acetone. and pentane. dried and ground into il powder. We thank Novo Nordisk AIS (Denmark) for a sample of SP 523 (NOVO) and Amano En/yme Europe Ltd. (England) for PS lipase. Whereas thc commercial enzyme powder combined with the water produced l'rmi tlic rcnction and formed a viscous residue which was difficult to separate, the sol gel immobilized lipase could be recycled without any difficulty. We thank Dr. B. Tesche (Fritz Haber Institut. Berlin) for the scanning electron micrographs. D. Bianchi. P Cesti, E. Battistel. J. Org. Chrm 1988. 53, 5531. .4ccording IO C'esri ct.al [15] the reaction resulted in 95% er. However. it is known th;it the enantioselectivity of enzyme-catalyzed reactions can vary for different batchesm of enzyme. The influence of other parameters. for example the use of additives, the stoichiometrq water:silane. the amount of catalyst, and the use of other silane monoinerc :ire presently tinder investigation in our group.p. 405.

ZUSCHRIFTEN matischen Charakter zeigen. Die Aromatizitatreihe Pyrrol > Thiophen > Furan wird deutlich beschrieben, und das Cyclopentadienyl-Anion ist das ,,aromatischste" funfgliedrige Ringsystem. Eingegangen am 11. August 1994 [Z 72311 Stichworte: Ab-initio-Rechnungen . Aromatizitat . Heterocyclen [l] a) Die Literatur ist umfangreich; doch es gibt eine ausgezeichnete neue und umfassende Ubersicht: V. J. Minkin, M. N. Glukhovtsev, B. Y. Simkin, Aromaticity and Antiaromaticity, Electronic and Structural Aspects,