New Protocols for the Assembly of the Tetracyclic Framework Associated with the Aromatic Erythrina Alkaloids

Abstract

[reaction: see text] Treatment of the anion derived from the ring-fused gem-dichlorocyclopropane 4c with silver tetrafluoroborate afforded the spirocyclic compound 17 in 74% yield. Product 17 was readily converted, over three steps, into the beta-iodoethyl derivative 20 and treatment of this latter compound with n-Bu(3)SnH then afforded, in 93% yield and via a radical addition/elimination sequence, compound 2 incorporating the ABCD framework of the aromatic erythrina alkaloids.

Facile synthesis of 4,5,6a,7-tetrahydrodibenzo[de,g]chromene heterocycles and their transformation to phenanthrene alkaloids

“…Thus, commercially available phenylacetic acid 17 was dibenzylated to yield 18 . LAH reduction of 18 afforded alcohol 19 which was then condensed with aldehyde 20 40,41 in an oxa-Pictet-Spengler reaction 42 to give isochroman 21 . At this stage, we were set to attempt the key microwave-assisted C-H activation step to form the isochroman tetracycle.…”

A [4+4] annulation strategy for the synthesis of eight-membered carbocycles based on intramolecular cycloadditions of conjugated enynes

“…26 t -Butyllithium was titrated in pentane with menthol at 0 °C using 1,10-phenanthroline as the indicator. 3-Ethoxycyclobutanone, 12 cyclobutenecarboxylic acid, 20 cyclopentenyl trifluoromethanesulfonate, 27 and 2,4,6-tri( iso -propyl)benzenesulfonic acid (TIPPSO 3 H) 21 were prepared according to the previously reported procedures. Cyclohexenyl trifluoromethanesulfonate was prepared via a minor modification of the published procedure 28 in which LiHMDS was substituted for LDA.…”

Electrocyclic Ring-Opening Reactions of gem-Dibromocyclopropanes in the Synthesis of Natural Products and Related Compounds

“…This gives a spirocyclic product, which in the case at hand, has been sequentially transformed into 133 that contains the ABCD framework of the aromatic erythrina alkaloids. 86,134 Just as cyclopropyl-allyl opening can be performed to trap external pyrrole or indole (Section 3.3.4), assisted opening by an intramolecular pyrrole that also results in aromatic substitution has now been recorded for the tethered pyrrole of 134; again very impressive results are obtained (Scheme 66). 86 Other intramolecular electrophilic aromatic substitution reactions are known to occur from 1,1-dibromocyclopropanes as discussed by Tanabe and Nishii, 11 …”