Stereoselective Lewis Acid Mediated [1,3] Ring Contraction of 2,5‐Dihydrooxepins as a Route to Polysubstituted Cyclopentenes

Abstract

A room‐temperature diastereoselective [1,3] rearrangement results from treatment of 2,5‐dihydrooxepins with EtAlCl2 (see scheme). A modular synthesis of dihydrooxepins allows substituents to be incorporated at any position on the ring, which means that various polysubstituted cyclopentenes can be prepared through this Lewis acid mediated [1,3] ring contraction.

Catalytic Enantioselective Synthesis of 2,5‐Dihydrooxepines

“…Although cis ‐FVCs 1 and 2,5‐dihydrooxepines 2 are known to be in equilibrium that generally favors 1 , the equilibrium could be controlled to favor 2 by using π‐stabilizing substituents such as the COR 4 or R 1 groups (Scheme ) –. To the best of our knowledge, a catalytic asymmetric synthetic method to prepare dihydrooxepine derivatives has not been reported to date.…”

“…A similar transformation with cyclopropane derivatives where one of the vinyl groups is replaced with a C=O group, known as the retro‐Claisen rearrangement, leads to 2,5‐dihydrooxepine derivatives through an analogous mechanism . While retro‐Claisen rearrangements of this type are quite common for 2,5‐dihydrooxepines, asymmetric examples of this reaction are rare . Indeed, using this approach for the formation of oxepine derivatives is discouraged due to the multiple steps required to prepare chiral cis ‐1‐formyl‐2‐vinylcyclopropanes (FVCs) for such a rearrangement reactions…”

Bond Rotation in an Aromatic Carbaporphyrin: Allyliporphyrin

“…The syntheses of carbaporphyrin (2)a nd its Pd-2 werea ccomplished as summarized in Scheme1.R etrosynthetic analysis of the carbaporphyrin revealed that the condensation reaction of diol(8)a nd dipyrromethane affords the target compound. To obtain diol(8), the oxidation reactiono f( Z)-3-iodo-3-phenylprop-2-en-1-ol [11] with MnO 2 afforded acrylaldehyde (3) in an 80 %y ield. The Horner-Wadsworth-Emmons reactiono f acrylaldehyde (3)f ollowed by DIBAL reduction and MnO 2 oxidationafforded dienylaldehyde (6).…”

Syntheses of (+)-30-epi-, (−)-6-epi-, (±)-6,30-epi-13,14-Didehydroxyisogarcinol and (±)-6,30-epi-Garcimultiflorone A Utilizing Highly Diastereoselective, Lewis Acid-Controlled Cyclizations

“…Surprisingly, we discovered that 1:1 H 2 O/TFA induced unexpected cyclization at C2 of compound 27 cleanly furnishing type A PPAP core 28 in 79% yield (Scheme 2B). As one possibility, the cyclization of 27 may proceed through a cationic rearrangement wherein acid-induced pyran opening leads to cyclization at C2 and a formal 1,3-shift 25 followed by an oxycyclization/demethylation sequence. Pleasingly, olefin metathesis proceeded efficiently for all stereoisomers providing type B PPAPs (−)-6- epi - 6 , (+)-30- epi - 6 , (±)-6,30- epi - 6 , and type A PPAP (±)-6,30- epi - 5 in excellent yields (Scheme 2A,B).…”