Lewis acid mediated 5/6/7-endo-dig hydroalkoxylation-reduction cascade on internal alkynols gave an expedient, stereoselective synthesis of cyclic ethers and 1,4-oxazepanes. The strategy has been extended to the first examples of hydroalkoxylation-alkyne Prins-type cyclization cascade of alkyne-tethered alkynols, giving access to oxa-bicyclic scaffolds. This method was used as the key step in the stereoselective total synthesis of calyxolane A-B, as well as (±)-centrolobine and its homologue.
A TMSOTf mediated 5/6-endo-dig reductive hydroamination cascade on internal alkynylamines gave expedient, stereoselective access to pyrrolidine and piperidine derivatives. We also demonstrate that a protecting group on nitrogen has a profound effect on the reactivity as well as diastereoselectivity.
Synthesis and isolation of highly unstable azirinobenzoxazole and benzoxazines in a chemodivergent fashion from aryl azido vinylogous carbonates by simple change in transition metal acetate is described. Thermal or rhodium(II) acetate‐mediated decomposition of these azides gave dihydroazirino benzoxazole. Their nickel(II) acetate‐promoted reaction gave 4‐dihydro‐2H‐benzoxazines, whereas copper(II) acetate led to the corresponding oxidized imine derivatives. Benzaoxazine derivative could be kinetically resolved using a proline‐catalyzed Mannich reaction. The benzoxazines were rapidly elaborated to angularly fused tetracyclic systems and coumarin‐fused derivatives in a “one pot” fashion.
A substrate dependent regio‐ and stereoselective domino hydroalkoxylation‐formal‐[4+2] cycloaddition is described for the facile synthesis of linear as well as spirocyclic 1,4‐heterocycle‐fused chromene ketals. Enantiospecific synthesis of oxazepino chromene derivatives was successfully carried out using chiral pool amino alkynols. The developed hydroalkoxylation cascade offered rapid access to the spirocyclic [6‐6‐7‐6] tetracyclic core of cytorhizhins B–D with correct relative configuration.
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