Zanardi, Franca and Curti, Claudio and Sartori, Andrea and Rassu, Gloria Maria Rita and Roggio, Anna Maria and Battistini, Lucia and Burreddu, Paola and Pinna, Luigi and Pelosi, Giorgio and Casiraghi, Giovanni (2008) Further uses of pyrrole-based dienoxysilane synthons: a full aldol approach to azabicyclo[x.2.1]alkane systems. European Journal of Organic Chemistry, Vol. 2008 (13), p. 2273-2287. eISSN 1099-0690. Article.
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Two racemic 2-azabicyclo[2.2.1]heptane structures, 15 and 21, and two chiral non-racemic 6-azabicyclo[3.2.1]octane representatives, 28 and 36, have been synthesized starting from 1-(tert-butoxycarbonyl)-2-(tert-butyldimethylsilyloxy)pyrrole (TBSOP, 5) and suitable ketones, 9, 16, 22 and 29. 2-Azabicycle 15 was then elaborated to racemic cyclopentane amino acid 38, while 6-azabicycle 36 served to access the enantiomerically pure normorphan-type structure 40. For all substrates, a uniform synthetic scheme was implemented based on the combination of two diastereoselective aldol-type carbon-carbon bond-forming reactions, the efficiencies of which were secured by appropriate aldol-stabilizing steps. A mechanistic rationale accounting for the markedly diastereoselective character of the key Mukaiyama aldol reactions between TBSOP and the ketone acceptors has been postulated that involves hetero-Diels-Alder transition-state structures in which the preference for endo versus exo addition is governed by the electronic nature of the substituents in the ketone substrates.
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