**4. Conclusions**

In summary, we introduced a two-step protocol to synthesize double aza-oxa[7]helicene **3** using an electrochemical approach. This novel double hetero[7]helicene shows interesting structural features that were reflected in its excellent optical properties. We have studied the photophysical characteristics of this compound and correlated its absorption and fluorescence behavior based on DFT calculations. Further development for this two-step protocol towards the preparation of other multiple helicenes and PHAs and study their photophysical and chiroptical features are currently under investigation.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/molecules27249068/s1, Table S1: Optimization of the acid-mediated annulation step; Table S2: Optimization of the electrochemical sequential reaction; Table S3: Selected experimental and calculated structural parameters of double aza-oxa[7]helicene **3**; Tables S4–S6: Summary of the TD-DFT calculation results of **3**; Scheme S1: A plausible mechanism for the electrochemical domino reaction; Figure S1: Crystal measurements; Figure S2: Measurements of the optimized structures; Figure S3: Selected molecular orbitals of **3**; Figure S4: Simulated UV-vis absorption and CD spectra of (*P*,*M*)-**3**; Figure S5: Further NICS(0) calculations [57,63–68].

**Author Contributions:** Conceptualization, supervision, and project administration, H.S. and S.T.; methodology, DFT calculations, investigation, data analysis, and writing original draft, M.S.H.S.; methodology, investigation, validation, and formal analysis A.S. and M.I.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by JSPS KAKENHI Grant Numbers 22K06502 in Grant-in-Aid for Scientific Research (C), Transformative Research Areas (A) 21A204 Digitalization-driven Transformative Organic Synthesis (DigiTOS), 22KK0073 in Fund for the Promotion of Joint International Research (Fostering Joint International Research (B)) from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), and the Japan Society for the Promotion of Science (JSPS), JST CREST (No. JPMJCR20R1), and Hoansha Foundation.

**Informed Consent Statement:** The study does not involve humans.

**Data Availability Statement:** CIF of the crystal of **3** is available as Supplementary. The X-ray crystallographic coordinate for the structure reported in this study has been deposited at the Cambridge Crystallographic Data Centre (CCDC) under deposition numbers CCDC-2156335 (**3**). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data\_request/cif.

**Acknowledgments:** We thank Yoichi Hoshimoto for helping with chiral HPLC resolution. We acknowledge the technical staff of the Comprehensive Analysis Center of SANKEN.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of compound **3** are available from the corresponding author upon reasonable request.
