Synthesis of a Cyclic Tetrameric Purine by Successive Cross‐Coupling Reactions and Subsequent Pd‐Catalyzed Cyclization

The tetrameric N ‐benzyl‐protected purine (quaterpurine) 2 was synthesized and characterized as its palladium complex [ 2· Pd]. The synthesis commenced with the Pd‐catalyzed cross‐coupling of 8‐ z incated 9‐benzyl‐6‐chloro‐8‐iodopurine ( 9 ) and 9‐benzyl‐6‐iodopurine ( 11 ) establishing the first C‐6/C‐8 bond. The sequence was repeated twice after iodo‐de‐chlorination at C‐6′ (C‐6″) of the respective dimer 12 and trimer 15 . The final ring closure was achieved at the tetrameric 6″′‐chloro‐8‐iodoquaterpurine 3b by a reductive intramolecular cross‐coupling with hexamethylditin in the presence of Pd 2 (dba) 3 and P(2‐furyl) 3 . The overall yield in the eight step sequence was 17 % starting from 9‐benzyl‐6‐chloropurine ( 8 ), the immediate precursor of 11 . Other strategies to combine the purine fragments, i.e. by dimer/dimer bond formation or by regioselective cross‐coupling, were not successful. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)