Diastereoselective Singlet Oxygen Ene Reaction (Schenck Reaction) and Diastereoselective Epoxidations of Heteroatom-Substituted Acyclic Chiral Olefins: A Mechanistic Comparison

The directing propensity of allylic and homoallylic
heteroatom substituents on the diastereoselective singlet
oxygen ene reaction (Schenck reaction) of acyclic, chiral olefins has
been investigated. These extensive stereochemical
results offer evidence for our proposed mechanism of the
diastereoselective singlet oxygen ene reaction. Thus,
provided a substrate possesses sufficient 1,3-allylic strain to
populate preferentially the appropriate conformation,
the photooxygenation is directed threo-selectively by
intramolecular hydrogen bonding in the threo-A
exciplex of
allylic alcohols, while electron-accepting or bulky substituents result
in the erythro-configurated products. A
detailed
comparison of the diastereoselectivities for the singlet oxygen ene
reaction with those of the epoxidation by
m-chlorobenzoic acid (mCPBA) and dioxirane of several
judiciously chosen substrates showed that both oxygen
transfer processes are influenced similarly by allylic and homoallylic
substituents. Thus, it is shown that the acyclic,
allylic alcohol 1a is epoxidized highly
threo-selectively by
methyl(trifluoromethyl)dioxirane due to hydrogen
bonding
in the transition state and that bulky and/or electron-accepting
substituents direct all three oxidants
erythro-selectively.
These pronounced steering effects on the diastereoselectivity of
oxygen transfer processes (photooxygenation and
epoxidation) should be valuable for the rational design of
stereocontrolled oxyfunctionalizations by 1O2,
mCPBA,
and dioxiranes.