Abstract
[n.3.0]Bicycles (n = 3–6) can be synthesized using palladium-catalyzed asymmetric allylic alkylation followed by ruthenium-catalyzed cycloisomerization. New types of triarylphosphino-1,2-diaminooxazoline ligands show the same high levels of enantioselectivity observed with Trost ligand when employed in Pd-catalyzed allylic alkylation reactions. The enyne products of these allylic alkylation reactions were further elaborated using a Ru-catalyzed redox isomerization process, for which a mechanism is proposed.
Highlights
The development of novel and effective cyclization methods constitutes a continuing challenge in organic synthesis, as five- and six-membered cycles are essential structural units in pharmaceuticals, agrochemicals, and other biologically active molecules [1]
The utility of ruthenium catalysis has been studied in intramolecular cycloisomerizations, providing an efficient, atom-economical method to access an array of 1,3- or 1,4-dienes [31] and other types of bicyclic compounds [36,37]
In summary, a novel method was developed successfully to synthesize asymmetrically a series of [n.3.0] bicyclic molecules (n = 3–6) bicyclodienes of various ring sizes in three steps starting from dimethyl 2-(4-(trityloxy)but-2-ynyl) malonate 3a[65] or symmetrically in two steps starting from dimethyl 2-(4-hydroxybut-2-ynyl)malonate 2a via palladium-catalyzed allylic alkylation and Ru-catalyzed cycloisomerization
Summary
The development of novel and effective cyclization methods constitutes a continuing challenge in organic synthesis, as five- and six-membered cycles are essential structural units in pharmaceuticals, agrochemicals, and other biologically active molecules [1]. Palladium-catalyzed asymmetric allylic alkylation (AAA), known as the Tsuji-Trost reaction, is one of the most efficient methods for forming allylic C-C, C-O, C-N, C-S, and C-P bonds It tolerates a broad range of olefins that contain a leaving group in the allylic position, employs mild reaction conditions, and forms the desired products in very high enantioselectivities and yields [6,46,47]. Starting from malonate protected by trityl group and cycloalk-2-enyl carbonyl derivatives, enyne compounds can be synthesized with high enantioselectivities and yields. These compounds were deprotected using BiCl3 and treated with [IndRu(PPh3)2Cl] 1 [48] for cyclized end products (Figure 1)
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