Stilbene crowded

Steric crowding in the cis isomer of Mini-3 (16), a chain shortened triene analog of -carotene, and hexakis (2,2/,4,4/,6,6/-trifluoromethyl)stilbene (17) forces the polyene chromophores to adopt a spiral conformation. Some of the associated unusual spectroscopic properties (UV-VIS and NMR) of these compounds and a rare 1,7-H shift process were described. 

Deprotonation of N-methylpiperidine Al-oxide with LDA in THF at — 78°C takes place regioselectively on the least crowded methyl carbon to generate azomethine ylide 122 (8SJOC2910). Ylide 122 is again highly reactive toward nonactivated olefins such as ethene, cyclopentene, styrene, (E)- and (Z)-stilbene, and ( )-)S-methoxystyrene. 

Kaupp180e has rationalized the orientational selectivity observed in the ringopening of phenylcyclobutanes in terms of relief of steric strain present in the molecules as a result of phenyl-phenyl crowding. In accord with this, in a study of stilbene dimers it was shown that the cis-anti-cis-dimer (262) decomposed... 

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