The interactions of the nine most significant nucleobases (thymine, uracil, dihydrouracil, cytosine, adenine, guanine, diaminopurine, xanthine, hypoxanthine, in their deprotonated forms) with zinc and with themselves in pyrazolylborate zinc complexes Tp(Cum,Me)Zn-base and Tp(Ph,Me)Zn-base are described. Except for guanine, the complexes Tp*Zn-base could be isolated in all cases. Structure determinations could be performed for seven of the eight product types. Except for dihydrouracil and xanthine, the zinc ion is attached to that nitrogen of the base which in nucleosides bears the sugar moiety. In the solid state, all zinc-bound nucleobases are involved in hydrogen bonding interactions. Except for xanthine, this includes homo base pairing across a crystallographic inversion center.

Kovalent gebundene F‐Atome sind effektive Donoren Dies wird durch die F4O3N1‐Koordination des Cs+‐Ions im Komplex mit dem makropolycyclischen Liganden 1 belegt. Drei der vier kürzesten Bindungen zum Metallion sind Cs+‐F‐Bindungen, wobei eine mit einer Länge von nur 284.3(3) pm außergewöhnlich kurz ist.

In solution, the pyrazolylborate-zinc-nucleobase complexes show self-association and base pairing with external nucleobases. The self-association was studied quantitatively for Tp(Cum,Me)Zn-hypoxanthinate and Tp(Cum,Me)Zn-thyminate; the dimerization constants K(D) are 63 +/- 8 and 0.2 +/- 0.1 M(-1), respectively. Of the external nucleobases, 9-ethyladenine forms stable base pairs with the thyminate, uracilate, and xanthinate complexes, 9-isobutylguanine only with the cytosinate complex, 1-methylthymine with the adeninate and diaminopurinate complexes, and 1-methyluracil with the diaminopurinate complex. The association constant for the base pair Tp(Cum,Me)Zn-thyminate:9-ethyladenine was determined by NMR methods as K = 66 +/- 10 M(-1). Structure determinations of the crystalline adducts have confirmed the base pairing for Tp(Cum,Me)Zn-thyminate:9-ethyladenine, Tp(Cum,Me)Zn-cytosinate:9-isobutylguanine, and Tp(Cum,Me)Zn-xanthinate:9-ethyladenine. Both Watson-Crick and Hoogsteen base pairs have been observed. In the solid state, extended base pairing leads to quartet and polymer arrangements.

COMMUNICATIONS tion, one of the methoxy resonances was shifted upfield to b = 3.28 as a result of shielding of the methoxy group by the neighboring phenyl ring.r1b1 This group occupies a nearly orthogonal position, which would also explain the absence of dilactone formation after closure of the first lactone ring.In the final synthetic step a n intramolecular Pd-catalyzed Heck reaction was applied for the formation of the isoquinoline ring. This reaction afforded lamellarin G trimethyl ether (8) in 74 YO yield. To our knowledge, this is the first example of a Heck reaction in which after oxidative addition, the Pd" intermediate fragments by elimination of CO, . This method delivers the cyclization product 8 in higher yields than the conventional Heck reaction with the free pyrrole obtained by decarboxylation of 7.This short sequence (three steps) affords lamellarin G trimethyl ether (8) in 33% overall yield. Application of our method to the synthesis of other lamellarins, especially to those with unsymmetrically substituted aryl groups, is in progress. Since lamellarins of type 2 are easily dehydrogenated to the corresponding dihydroisoquinoline derivatives"'] by action of 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ), compounds of the latter type can also be obtained by our procedure. Experimental Section6: nBuLi (3.56 mL of a 2.5 M solution in hexane, 2.0 equiv) was added dropwise to a stirred solution 3-(3,4-dimethoxyphenyl)pyruvic acid (3) (1.0 g, 4.46 mmol) in dry T H F (80 mL) at -78 "C. After the mixture had been stirred for 20 min, a solution of iodine (0.56 g, 2.23 mmol, 0.5 equiv) in THF (20 mL) was added dropwise. The reaction mixture was allowed to warm up to room temperature, stirred for another hour, and then 2-(2-bromo-4,5-dimeth0xyphenyl)ethylammonium bromide (61 (5) (1.0 g, 2.93 mmol) was added and the resulting solution was stirred for 12 h over molecular sieves (4 A). The cloudy solution was quenched with 2 u NaOH. The layers were separated, the aqueous phase was washed with ethyl acetate (3 x 30 mL), acidified with concentrated HCI to pH 4. and extracted with ethyl acetate (3 x 30 cm3). The combined organic layers were dried (Na,SO,), filtered, and the organic solvent was removed in vacuo to give 1.9 g of crude material, which was recrystallized from methanol to yield 6 as a yellowish powder (1.39 g. 62%). M.p. 201 'C: UViVis (CHIOH): i.,,, (qualitative. nm) = 208, 288; ' H NMR (300.13 MHz,

Attachment of deprotonated nucleosides or of alkylated nucleobases, as their analogues, to zinc was achieved by condensation reactions between TpZn‐OH complexes and nucleobase derivatives. Pyrazolylboratozinc complexes with derivatives of uracil, thymine, guanine, xanthine and hypoxanthine were obtained and characterized by structure determination. In the uracil, thymine, guanine and hypoxanthine derivatives, zinc is bound to the nitrogen atom which bears the proton in the nucleoside. In the xanthine derivatives all three possible bonding modes are realized. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)