Bredt's rule

[2] There are a few instances where the anti-Bredt phenomenon is mentioned, but the isolation of these molecules is difficult, so they are typically trapped in situ.

[5] Bredt's rule results from geometric strain: a double bond at a bridgehead atom necessarily must be trans in at least one ring.

Bredt's rule also applies to carbocations and, to a lesser degree, free radicals, because these intermediates also prefer a planar geometry with 120° angles and sp2 hybridization.

[6] There has been an active research program to seek anti-Bredt molecules,[7][8] with success quantified in S, the non-bridgehead atom count.

[14] Although exceptions to the rule have long been known, in 2024 chemists from University of California, Los Angeles demonstrated a general method to access Anti-Bredt olefins with S ≤ 7.