Synthetic Application of Hetero-substituted 1,3-Dienes Generated in Situ from Enone Cyclic Acetals and Dithioacetals

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Recently, we have demonstrated that 2-cyclopent-1-ene ethylene acetal 1 is capable of undergoing the ring-cleavage to afford cyclopenta-1,3-dienol ether 2, which in turn is trapped by dienophiles to give 2-norbornanone ethylene acetals 4. In contrast to the corresponding keto-enol tautomerism, the thermal equilibration between cyclic acetal and its ring-opened enol ether form has been little exploited synthetically. In this study, we examined generality, selectivity, scope, and limitation of this Diels-Alder strategy. The intermediate 2 was detected by UV spectroscopy, and its content relative to 1 at a stationary state at 70ﾟC in acetonitrile was estimated to be about 0.2%. Of the several solvents examined, acetonitrile was most satisfactory. The addition reactions were highly regioselective and the 1,3-cyclopentadien-2-yl intermediates selectively derived from the corresponding acetal precursors via 1,2-elimination. Dithioacetals of 2-cyclopentenones, which are readily available from the corresponding enones, also react with dienophiles in a similar manner. The reation of dithioacetals were catalyzed with Zn (OTf) _2. The addition of ketene equivalent to (dithio) acetals allowed the convinient preparation of singly acetalized 2,5-norbornadienes.