Domino Cyclodimerization of Indole‐Derived Donor–Acceptor Cyclopropanes: One‐Step Construction of the Pentaleno[1,6‐a,b]indole Skeleton

A Straightforward Approach to Tetrahydroindolo[3,2‐b]carbazoles and 1‐Indolyltetrahydrocarbazoles through [3+3] Cyclodimerization of Indole‐Derived Cyclopropanes

“…Our previous study revealed that the application of the conditions that are appropriate for [3+3] cyclodimerization of arylcyclopropane‐1,1‐diesters to N ‐alkyl‐indolyl‐cyclopropanes induces an unusual domino process, affording pentaleno[1,6a‐ b ]indoles, whereas the simple replacement of the N ‐alkyl group by tosyl tunes [3+2] dimerization towards cyclopenta[ b ]indoles . Further efforts were directed to design new routes to bisindoles, that is, fine‐tuning the reaction towards novel chemoselective [3+3] cyclodimerization of indole‐derived cyclopropanes.…”

Donor‐Acceptor Cyclopropanes in the Synthesis of Carbocycles

“…[48] The related transformation of indole-derived D-A cyclopropanes 70 produced pentaleno [1,6a-b]indoles 71, the first examples of molecules with this tetracyclic skeleton (Scheme 48). [59] Moreover, other dimerizations were reported by Tomilov's group [47] and others. [60] Some of them are similar to processes discussed here, other ones include isomerization of D-A cyclopropanes into alkenes 54 followed by their reaction with the second molecule of cyclopropane.…”

“…The related transformation of indole‐derived D‐A cyclopropanes 70 produced pentaleno[1,6a‐ b ]indoles 71 , the first examples of molecules with this tetracyclic skeleton (Scheme ) …”

Indole‐derived Donor‐acceptor Cyclopropanes

“…Oppositely, treatment of cyclopropanes 95 with SnCl 4 in nitromethane only leads to the formation of lactones (see Section 3.6) if the reaction is performed at room temperature, but affords pentaleno[1,6a‐ b ]indoles 124 at 60 °C . This unusual dimerization proceeds via SnCl 4 ‐induced cyclopropane ring opening to zwitterion 121 , a combination of two zwitterions forming a new zwitterionic species 122 and an ipso ‐attack of the electrophilic center in this species on the C3 atom in the indole ring, producing intermediate 123 .…”

“…Among other reactions of IDDA cyclopropanes, ring enlargement processes are important. The simplest example is lactone formation, which is usually considered an undesirable side‐process ,,. However, Kerr and coworkers called attention to the fact that this lactonization can be used for targeted synthesis of substituted butanolides.…”