Comments on: Electrocyclic Ring Opening And Closure (2) – Six (or Eight) Pi Electrons https://www.masterorganicchemistry.com/2020/06/26/electrocyclic-ring-opening-and-closure-2-six-or-eight-pi-electrons/ Wed, 22 Apr 2026 15:13:42 +0000 hourly 1 https://wordpress.org/?v=6.9.4 By: Yanping Nora Soong https://www.masterorganicchemistry.com/2020/06/26/electrocyclic-ring-opening-and-closure-2-six-or-eight-pi-electrons/#comment-719243 Tue, 10 Dec 2024 22:01:26 +0000 https://www.masterorganicchemistry.com/?p=20764#comment-719243 C9 and C10 of 7-dehydrocholesterol are likely partially sp2-hybridized due to forced planarity of the sterol backbone, making it different from a generic 4pi => 6pi electrocyclic ring opening. How does this affect the HOMO calculations for the diene prior to electrocyclic ring opening? What is the relationship between the HOMO of the diene reactant (7-dehydrocholesterol) vs. the initially occupied C9-C10 sigma bonding orbital? Is there significant HOMO contributions to electron density between C9 and C10 prior to ring opening?

In the generic inverse reaction (ring closure), the old HOMO (of the 6pi system) becomes the new sigma orbital, which presumably drops in energy below the new HOMO (of the 4pi system). Does this hold true for the hypothetical ring closure of pre-vitamin D3 to 7-dehydrocholesterol, which presumably would be thermal and disrotary?

I’m running some calculations in ORCA, but wow, ORCA is really picky about syntax,

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