Silver‐Catalyzed Asymmetric Synthesis of 2,3‐Dihydrobenzofurans: A New Chiral Synthesis of Pterocarpans

Abstract

2,3-Dihydrobenzofurans can be diastereoselectively prepared by condensation of aromatic aldehydes with 2,3-dihydrobenzoxasilepines under the catalysis of Ag(I) complexes, and in the presence of a source of fluoride ion. The application of this strategy by using chiral catalysts leads to a new enantioselective total synthesis of natural cis-pterocarpans and their trans isomers. Through this method, the first enantioselective total synthesis of the antifungal agent (-)-pterocarpin was achieved. In addition, a new entry into the heteroaromatic system of 2,5-dihydrobenzoxepine is also presented.

Recent Progress in the Synthesis of 2,3-Dihydrobenzofurans

“…In particular cases, the Mitsunobu cyclization is possible only if syn-diastereoisomeric hydroxy phenols are used, because the anti-diastereoisomers give a completely different reaction pathway leading to 2,5-dihydrobenzooxepines. 21 However, it should be noted that there are also some disadvantages associated with the Mitsunobu reaction. Diethyl azodicarboxylate (DEAD) is unstable and potentially explosive, and the by-products triphenylphosphine oxide and diethyl hydrazinedicarboxylate are of considerable mass, making the process non-ideal from the point of view of atom economy and not suitable for scale-up.…”

Palladium-catalyzed approach to the synthesis of five-membered O-heterocycles

“…Heterocyclic synthesis involving transition metal complexes has become of common use in the past decade because a metal-catalyzed reaction can directly build complicated molecules from readily accessible starting materials under mild conditions [15,16]. In this review, we focus on the use of palladium for the construction of various heterocyclic compounds with oxygen heteroatom.…”

Reaction of benzoxasilocines with aromatic aldehydes: Synthesis of homopterocarpans

“…[2,3] We have applied it to the stereoselective synthesis of dihydrobenzofurans by means of the condensation of benzoxasilepines with aromatic aldehydes in the presence of Lewis acids. [4,5] Using this methodology and through convergent synthetic routes, we have prepared pterocarpans [6] and neolignans. [7] These good results have encouraged us to undertake the extension of the method to the use of benzo [ g ][1,2]oxasilocines for the preparation of chromans.…”