Endo Preference Research Articles

Tandem [4+2]/[3+2]-cycloadditions. 2. Asymmetric induction with a chiral vinyl ether

A tandem [4+2]/[3+2]-cycloaddition of nitroalkenes tethered to α,β-unsaturated ester dipolarophiles can be effectively triggered with a vinyl ether. Using the chiral vinyl ether 6 as the dienophile, nitroalkenes 1 and 2 undergo highly selective tandem cycloaddition. Hydrogenolytic cleavage of the resulting nitroso acetals produce α-hydroxy lactams (-)- 14 and (-)- 21 in >98% e.e. with excellent recovery of the chiral auxiliary. The high diastereoselectivity is seen to arise from a strong endo preference for the vinyl ether together with an intrinsic facial bias in the auxiliary.

(6S, 7S, 10R)- and (6R, 7S, 10R)-7-isopropyl-10-methyl-4-oxo-1,5-dioxaspiro[5.5]undec-2-enes having an electron-withdrawing substituent at the 2-position: synthesis and use in asymmetric Diels–Alder reactions

Chiral spirocyclic dioxinones (S)-(6) and (S)-(7) have been synthesized from (–)-menthone and used in Diels–Alder reactions with cyclopentadiene; remarkable diastereofacial selectivity (isopropyl side) and endo preference observed in these reactions have offered a new methodology for asymmetric Diels–Alder reactions.

Quantitative measurement of endo preference in homodienyl hydrogen shifts

From the kinetics of the gas-phase pyrolysis of a series of cis -1,1-dimethyl-2-alkenylcyclopropanes, the preference for the endo transition state in the homodienyl hydrogen shift (retro-ene reaction) is estimated to be at least 12 kcal/mol.

Base catalysed rearrangements involving ylide intermediates. Part 9. The rearrangement reactions of cyclic allylic ammonium and sulphonium ylides

The allylic ammonium ylides (22) and (47) and sulphonium ylide (53) are isolable, usually as crystalline solids, because their [3,2] sigmatropic rearrangements are inhibited by the strain associated with the bicyclic transition state of a concerted reaction mechanism. The tetrahydropyridinium (22) and dihydrothiopyranium (29) ylides undergo a thermal [3,2] rearrangement, but, the pyrrolinium ylide (47) rearranges by a [1,2], rather than a [3,2], sigmatropic pathway. The dihydrothiophenium ylide (53) does not undergo either a [1,2] or [3,2] sigmatropic rearrangement but instead reacts in a bimolecular fashion to give eventually buta-1,3-diene, 2,5-dihydrothiophen, and the heterocycle (55). The [3,2] rearrangements of the ylides (22) and (29) follow predominantly an endo-pathway leading to a single diastereoisomer of the products (23) and (31); this strong endo preference is not shown by analogous acyclic ammonium ylides.

Steric Control in the Diels-Alder Reaction

Abstract The Diels-Alder reactions of polychlorocyclopentadienes with substituted ethylenes were investigated, and the rates of the reaction and the endo : exo adduct distributions were determined. The introduction of a chlorine atom at the C5 position of cyclopentadiene resulted in an enhanced endo preference for substituents in the adducts, despite the depressed reactivity of the diene. This is contrary to what is to be expected from the so-called “selectivity-reactivity relationship in the Diels-Alder reaction.” The results in this work were explained in terms of steric interaction between the C5-chlorine atom of the diene and the substituent of dienophiles.

2-Alkenyl anions and their surprising endo preference. Facile and extreme stereocontrol over carbon-carbon linking reactions with organometallics of the allyl type

On structural equilibration, butenyl sodium, potassium and cesium strongly favor the cis (endo) configuration over the extended form. The cis-trans ratios vary from 93:7 to 96:4 and 99.9:0.1 in the series Na, K, and Cs; the corresponding value for 2-butene is 23:77. Homologous alkenylmetal compds. exhibit less extreme stereoselectivities, but still follow the same trend. The metal effect on the endo-preference is due to the electropositivity and bulk of the metal as well as intramol. H bonding between the alkyl group attached to the inner side of the allyl moiety with the electron excess accumulated at the other allylic terminus. [on SciFinder (R)]