Dienolates of Cycloalkenones and α,β‐Unsaturated Esters Form Diels–Alder Adducts by a Michael/Michael‐Tandem Reaction Rather Than in One Step

Abstract

α,β‐Unsaturated esters and lithium 1,3‐dien‐2‐olates are known to furnish bicyclic lithium enolates by anionic Diels–Alder reactions. However, in principle, the respective products might form not only in a single step but also in two consecutive – or “tandem” – Michael additions, the first of which occurs intermolecularly, the second intramolecularly. Three cyclic lithium dienolates and four esters with a stereogenic Cα=Cβ bond reacted to give Diels–Alder adducts (10 times) or failed to react (2 times). Seven of the reactive combinations furnished adducts wherein the configuration of the former ester moiety had in part inverted. This precludes concerted pathways as their origins. This was a surprise since donors at C‐2 of the 1,3‐diene accelerate normal electron‐demand Diels–Alder reactions in the order alkyl < aryl < alkoxy ≈ trialkylsiloxy < acylamino. With Li⊕O⊖ being a far better donor still, it is not obvious why the mechanism is non‐concerted rather than concerted (and still more asynchronous).

Re-examining the stereochemistry of polycyclic suffruticosine via TDDFT calculations, ECD spectroscopy, and chemical synthesis

“…Notably, we observed the formation of the C9′ epimer when the reaction was performed without DMPU, ruling out the anionic Diels–Alder reaction pathway. 17…”

“…Notably, we observed the formation of the C9′ epimer when the reaction was performed without DMPU, ruling out the anionic Diels-Alder reaction pathway. 17 With tetracyclic compound 20 in hand, we next investigated the formation of the five-membered hemiaminal ether ring.…”

Solvent‐Free and under Vacuum O‐Benzylation of Carboxylic Acids by Using a Rotary Evaporator

“…The successful benzylation of aryl carboxylic acids can be conducted using BnBr [18] as a starting material in basic conditions or through transesterification reactions. [19] Similarly, benzylation of α,β-unsaturated carboxylic acids can be accomplished using BnBr [20] in basic conditions or DCC [21] as an activating reagent. Benzylation of cholic acid, primary bile acid containing multiple hydroxyl groups, was successful with a 90.5 % isolated yield.…”