Facile Synthesis of S-Fused Multi-Membered Polycyclic Heterocycles: A Construction Strategy towards Thermally Stable Thiepine Derivatives

S-fused heterocycles have become popular building blocks to construct functional polycyclic compounds. In contrast to the abundant synthetic methodologies for thiophene-fused aromatics, the synthesis of S-heterocycles containing six-membered thiopyran and seven-membered thiepine rings is much less reported owing to their unfavorable synthetic protocols and the thermal instabilities. Herein, a series of thiepine-containing polycyclic S-heterocycles have been successfully synthesized via different synthetic routes which involve initial construction of sulfur bridges and final ring-closure reactions. Therefore, the dilithium intermediates are excluded, which facilitates the fusion on the thiepine ring with different S-heterocycles, including thiophene and thiopyran derivatives. Typically, a S-fused multi-membered polycyclic compound simultaneously involving thiophen, thiopyran, and thiepine rings has been successfully prepared. Interestingly, nucleus-independent chemical shift calculations reveal that the incorporated thiopyran and thiepine rings demonstrate aromatic and nonaromatic characteristics, respectively. Moreover, the thermal stabilities of the thiepine derivatives have been tremendously improved after the fusion on the three vinyl groups in the thiepine unit, which is attributed to the enhancements of the activation energies for the S-extrusion reactions, as revealed by density functional theory calculations. Therefore, our findings not only provide a facile synthetic methodology for S-fused multi-membered polycyclic heterocycles, but also furnish a novel construction strategy towards thermally stable thiepine derivatives.