A concise and diastereoselective total synthesis of cis and trans-pterocarpans

Abstract

A new strategy for the diastereoselective and convergent synthesis of pterocarpans which is able to control the relative stereochemistry of the molecule through allylation of aromatic aldehydes with cyclic allylsiloxanes is described.

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

“…The relative stereochemistry of (AE )-3 was assigned from NMR properties, the NOE effect observed between H-2 and H-1'' atoms being especially significant. The shielding of the signal of H-2 in the trans isomer compared to the cis isomer (formed in the BF 3 ·Et 2 O-promoted reaction [17] ) is also significant. This behavior was reported previously for similar compounds.…”

“…[17] The desirable enantioselection for this process could be derived from the use of Lewis acids that integrate chiral ligands into their structure. Firstly, we examined the TiF 4 /BINOL system that had proved to be useful for the catalytic allylsilylation of aldehydes by using allyltrimethylsilane, [18] but in our case, we just recovered the starting materials.…”

“…The 2,3-dihydrobenzofuran derivatives reported in entries 5-8 of the table can be transformed by following a previously reported methodology [17] into the natural pterocarpans pterocarpin, homopterocarpin, medicarpin, and maackianin, respectively. The behavior of benzo[f]A C H T U N G T R E N N U N G [1,2]oxasilepines 5 b and 5 c is remarkable and is in contrast with their low reactivity upon using BF 3 ·Et 2 O as Lewis acid.…”

“…[12] Not many syntheses of enantiomerically pure pterocarpans have been reported so far, and most of these are based on racemic resolutions. [13][14][15][16] Recently, we described a new strategy for a total synthesis of pterocarpans [17] in which one of the key reactions is the diastereoselective condensation of a 2,3-dihydrobenzoxasilepine with an aromatic aldehyde in the presence of a Lewis acid (Sakura-Hosomi modified reaction). In this paper, we investigated the possibility of using a chiral Lewis acid to perform an asymmetric condensation.…”

Diastereoselective Synthesis of 2‐Aryl‐3‐vinyl‐2,3‐dihydrobenzo[b]furans through a Sakurai Reaction: A Mechanistic Proposal

“…[22,23] In a previous communication, we have reported a novel, two-component methodology, which allows the synthesis of diastereomerically pure dihydrobenzofurans in an easy way. [24] A similar procedure, used for the preparation of trisubstituted tetrahydrofurans, was previously described, independently, by Marsden [25] and Cossy. [26] Our procedure involves the formation of benzoxasilepins by ring-closing metathesis (RCM) followed by a Sakurai-Hosomi-modified reaction.…”