Steric and conformational control of the regioselectivities in the ene reaction with trisubstituted cycloalkenes: comparison of the enophiles singlet oxygen, triazolinedione, and nitrosoarene
The nitrosoarene ene reaction with the cycloalkenes 1-3 and E-4 proceeds in high twix regioselectivity to afford the hydroxylamine ene products 1a-4a (twix) and 1b-4b (twin, except far E-4 twix). Steric interactions in the enophile attack are responsible for the skew trajectory of the nitrosoarene enophile. For Z-1-methylcyclooctene (Z-4), twin abstraction dominates, caused by conformational constraints (transannular interactions) in the hydrgogen-atom abstraction. The balance between these steric and conformational factors dictates the regioselectivity in the ene reaction
Iminonitroso ene reactions: experimental studies on reactivity, regioselectivity and enantioselectivity.
Ene reactions of iminonitroso agents with olefins were investigated in both solution and solid phase. Reactions afforded allyl hydroxylamine products in up to 99% yield and with high regioselectivity. A Cu(I)-mediated enantioselective nitroso ene reaction gave an ene product with up to 40% ee.
Hydroxy-directed regio-diastereoselective ene reaction of singlet oxygen with chiral allylic alcohols
The photooxygenation of chiral allylic alcohols 1a, (Z)-1f-k, ethers 1b-d, and acetate 1e gave the corresponding hydroperoxy homoallylic alcohols and derivatives 2 through the ene reaction with singlet oxygen. While the reaction of the acetate 1e proceeded erythro-diastereoselectively as a result of the classical cis effect, for the allylic alcohols 1a and (Z)-1f-k, in which an alkyl group is located cis to the hydroxy-bearing substituent, high threo selectivity was observed. This finding is explained in terms of coordination of the nucleophilic hydroxy functionality of the stereogenic center with the incoming electrophilic singlet oxygen enophile