# Synthesis of m,n‑Diaza[n]helicenes via Skeletal Editing of Indeno[2,1‑c]fluorene-5,8-diols

## TL;DR

This paper introduces a new chemical method to create complex ring structures with two pyridine rings using a double Schmidt reaction, offering control over the final product's structure.

## Contribution

The first example of a double Schmidt reaction to synthesize m,n-diaza[n]helicenes from indeno[2,1-c]fluorene-5,8-diols.

## Key findings

- The double Schmidt reaction converts 5,8-disubstituted indeno[2,1-c]fluorene-5,8-diols into m,n-diaza[5]helicenes in high yields.
- Regioisomeric ratios can be controlled by adjusting reaction conditions.
- Enantioenriched diols were successfully transformed into enantioenriched m,n-diaza[7]helicenes without significant loss of enantiopurity.

## Abstract

Skeletal editing of 5-membered carbocycles to 6-membered
heteroaromatic
compounds represents an attractive concept that would enable late-stage
heteroarene construction. However, the current state of the art does
not offer many synthetic strategies on how to achieve such a goal.
One of those is based on a reaction of aromatic tertiary alcohols
with sodium azide under acidic conditions, i.e., the Schmidt reaction.
Herein, we present a hitherto unexplored double Schmidt reaction toward
aromatic compounds possessing two pyridine rings. It is the first
example of a conversion of 5,8-disubstituted indeno­[2,1-c]­fluorene-5,8-diols to m,n-diaza­[5]­helicenes
in high yields. We also demonstrate that the regioisomeric ratio can
be controlled, to a certain extent, by tuning conditions. Mechanistic
investigation encompassing experimental as well as DFT calculations
sheds light on the course of the reaction and provides a rationale
for the observed regioselectivity and its control. In addition, structures
of several diaza[5]­helicenes and intermediates were unequivocally
confirmed by single crystal X-ray diffraction analyses. The double
Schmidt rearrangement strategy was also applied for the transformation
of enantioenriched [7]­helical indeno­[2,1-c]­fluorene-5,8-diols
into the corresponding enantioenriched m,n-diaza­[7]­helicenes without significant loss of enantiopurity.
Their structures were also unequivocally confirmed by single crystal
X-ray diffraction analyses.