191. Diastereo- and enantioselective synthesis of heterocyclic natural products
     Oppolzer, W.
     Gazz. Chim. Ital. 1995, 125, 207-213

Enantiomerically pure syn- or anti-β-acyloxy ketones, readily available by asymmetric syn- or anti-aldolizations of chiral N-acylsultams, undergo efficient, titanium-mediated cyclocondensations giving cis- or trans-2.3-dihydro-4H-pyran-4-ones such as the cigarette beetle pheromone (-)-serricorole, 6. Enantiomerically pure cyclic nitrones F, easily accessible via electrophilic α-hydroxyaminations of N-acylsultarns, serve as pivotal key intermediates in the highly stereoselective synthesis of various chiral piperidine and pyrrolidine alkaloids such as (-)-allosedamine, 27, and (-)-xenovenine, 44. Thermal cyclizations of 4-alkenylhydroxylamines give N-hydroxypyrrolidines in a suprafacial manner, consistent with a retro-Cope elimination mechanism. For example, the stereospecific cyclization of alkenylhydroxylamine 64 serves as a key step in a short and efficient synthesis of the enantiomerically pure Amaryllidaceae alkaloid (+)-trianthine, 66, thus obtained in 24% overall yield from the chlorobenzene-derived enone 60.

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