GENERATION AND INTERCEPTION OF ISODICYCLOPENTADIENE TAUTOMERS BY DIENE TRANSMISSIVE DIELS-ALDER REACTION OF A 1,5-DIHYDROPENTALENE WITH A VARIETY OF DIENOPHILES

The cycloaddition of N-methyl-1,2,4-triazoline-3,5-dione (MTAD), dimethyl acetylenedicarboxylate (DMAD), maleic anhydride (MA), and p-benzoquinone (BQ) with 6-methyl-4-phenyl-1,5-dihydropentalene (1) afforded the mono- and bis-adducts which depended on the dienophilic strength and reaction conditions (temperature, concentration). Thus, for the highly reactive dienophiles MTAD and DMAD, exclusively the bis-adducts (2, 3) were isolated in high yields as products of diene transmissive Diels-Alder reactions; the corresponding mono-adducts (substituted isodicyclopentadienes) were not detected even as transients. The pi-facial selectivity was high (>95% anti selectivity for 2 and 3) for the second addition step, but it decreased to 75% when high substrate concentrations were used (formation of 25% of the syn product 3b). In the case of the less reactive dienophiles MA or BQ, the corresponding intermediary mono-adducts could be detected by H-1 NMR spectroscopy and their conversion to the more stable isodicyclopentadiene tautomers (e.g. 4a to 4b) monitored directly in the reaction mixture. Both tautomers could be trapped intermolecularly for the MA case by addition of MTAD (5, 7) or another equivalent of MA (6, 8). The intramolecular cycloaddition of the isodicyclopentadiene tautomer derived from BQ and 1, namely the mono-adduct 10b, led to the cage compound 11. The double cycloaddition of dienophiles to 1,5-dihydropentalenes provides an efficient and convenient methodology for the synthesis of complex polycyclic ring skeletons.