The Diels--Alder reaction in total synthesis

Intramolecular Diels-Alder reactions of 5-vinyl-1,3-cyclohexadienes

[reaction: see text] The intramolecular Diels-Alder reaction of 5-vinyl-1,3-cyclohexadienes affording substituted tricyclo[3.2.1.0(2,7)]oct-3-enes is studied.

Total synthesis of Elisabethin A: intramolecular Diels-Alder reaction under biomimetic conditions

We describe the first total synthesis of the marine diterpenoid elisabethin A. The synthesis uses (S)-hydroxy-2-methyl-propionate as the chiral starting material, which is elaborated into a dienyl-iodide and added to an aryl acetic acid ester via enolate alkylation. The hydroquinoid system is oxidized to the quinone which serves as the dienophile in a highly stereocontrolled intramolecular Diels-Alder addition. This IMDA reaction, which to our knowledge is the first one to employ a terminal (Z)-diene, proceeds under biomimetic conditions (water, ferrichloride as the oxidant, room temperature) with high yield and stereoselectivity. The Diels-Alder adduct is transformed into the natural product via a three-step sequence including selective hydrogenation, base-catalyzed epimerization of the cis- into the trans-decalin system and O-demethylation.

Highly stereoselective hydroxy-directed Diels-Alder reaction

The successful stereocontrol of the Diels-Alder reaction of semicyclic dienes possessing a secondary and tertiary allylic magnesium alkoxide alcohol functionality and activated dienophiles such as methyl acrylate, methacrolein, acrolein, and N-phenylmaleimide is described.

A DFT study of the mechanisms and regio- and stereochemistry of the lewis acid-catalyzed reactions of 5-alkoxyoxazoles with aldehydes: aryl substitution at the 2-position of 5-alkoxyoxazole is critical to the formation of 4-alkoxycarbonyl-2-oxazoline

Density functional theory at the B3LYP/6-31G level has been used to study the mechanisms and regio- and stereochemistry of the Lewis acid-catalyzed reactions of aldehydes with 5-alkoxyoxazoles. Similar to the uncatalyzed reaction between aldehyde and 5-methoxyoxazole, which has an activation energy of 30.5 kcal/mol and intrinsically favors production of 2-alkoxycarbonyl-3-oxazoline, the Lewis acid-catalyzed reaction also prefers to generate 2-alkoxycarbonyl-3-oxazoline in a more efficient way with an activation energy of about 3 kcal/mol (with respect to separated acetaldehyde-AlCl3 complex and 5-methoxyoxazole) in the gas phase. Only when an aryl group is introduced to the 2 position of 5-alkoxyoxazoles can the Lewis acid-catalyzed reactions furnish 4-alkoxycarbonyl-2-oxazolines. The reasons for this switch of regiochemistry and the factors affecting the stereochemistry are discussed.

Sulfoxide-directed thermal intramolecular [4 + 2] cycloadditions between 2-sulfinyl butadienes and unactivated alkynes

2-Sulfinyl butadienes tethered to unactivated alkynes undergo a facile thermal intramolecular Diels-Alder cycloaddition, often at room temperature, to produce cyclohexa-1,4-dienes with good selectivities, in high yields, and preserving the valuable vinyl sulfoxide functionality.