A New Insight into the Stereoelectronic Control of the Pd⁰‐Catalyzed Allylic Substitution: Application for the Synthesis of Multisubstituted Pyran‐2‐ones via an Unusual 1,3‐Transposition

Abstract

Pyran‐2‐ones 3 undergo a novel Pd0‐catalyzed 1,3‐rearrangement to afford isomers 6. The reaction proceeds via an η2‐Pd complex, the pyramidalization of which (confirmed by quantum chemistry calculations) offers a favorable antiperiplanar alignment of the Pd−C and allylic C−O bonds (C), thus allowing the formation of an η3‐Pd intermediate. Subsequent rotation and rate‐limiting recombination with the carboxylate arm then gives isomeric pyran‐2‐ones 6. The calculated free energies reproduce the observed kinetics semi‐quantitatively.

Synthesis of highly polarized [3]dendralenes and their Diels–Alder reactions

“…The lowest yields were obtained for the vinyl ester 9s (30%), and vinyl ethers 9v (34%) and 9w (10%), presumably as a result of the limited stability of the starting (ethoxyvinyl)tributylstannanes at 70 °C. Lactone 9x was prepared previously by us 18 in the same way from 6 and ( E )-2-tributylstannylbut-2-en-1,4-diol. While this attractive heterogeneous protocol worked well for the coupling of β-iodoacrylate 6 , all attempts to prepare the previous series 9a–9n using this procedure either failed or gave inferior yields.…”

“…Using this variant, acrylate derivatives 9o and 9p were Scheme 6 Preparation of dendralenes 9o-w; 9x was synthesized previously. 18 obtained in considerably higher yields than their regio-and stereoisomers 9m and 9n. Triesters 9q and 9r were also formed in good yields (78 and 57%, respectively).…”

“…In addition, an interesting pentacyclic lactone (12x) was obtained from lactone 9x. 18 In sharp contrast to the reactions of 9a,c,i, and 9t (vide supra), modification of the reaction conditions for 9e and 9g (temperature decrease from 100 to 70 °C, dendralene : maleimide ratio 2 : 1, extended reaction time) enabled us to obtain monoadducts 11e and 11g in high yields (Scheme 8). Hence, the addition of another, different dienophile became a distinct possibility.…”