Heteroatom Influence on the π-Facial Selectivity of Diels−Alder Cycloadditions to 1-Oxa-4-thia-6-vinylspiro[4.5]dec-6-ene, 3-Methoxy-3-methyl-2-vinylcyclohexene, and 3-Methoxy-2-vinylcyclohexene^,

Abstract

The facial selectivities of the Diels-Alder cycloadditions of several dienophiles to the title substrates were studied. The observed selectivities are interpreted as a consequence of the relative steric interactions offered by the substituents. The addition of dimethylacetylene dicarboxylate (DMAD) is influenced by the electrostatic repulsion arising from the interaction of an electron pair orbital on the acetal oxygen and the orthogonal pi-orbital of the acetylene unit in DMAD in the syn-to-oxygen addition of the latter. This repulsion is offset on coordination of Li+ to the said oxygen electron pair orbital, and the addition thus proceeds syn to oxygen. The enhanced and accelerated syn-to-oxygen addition in lithium perchlorate in nitromethane is interpreted as a consequence of the coordination of Li+ to both the acetal oxygen and a heteroatom in the dienophile that brings them in close proximity to facilitate a reaction. The Li+-oxygen combination, however, also exerts some steric effect that results in reduced syn-to-oxygen addition of dienophiles having large substituents such as N-phenylmaleimide.

Total Synthesis of Malyngamides K, L, and 5′′-epi-C and Absolute Configuration of Malyngamide L

“…The preparation of stannane 20 and boronic acid 6 both began with 2-cyclohexen-1-one ( 11 ). Thus, bromination of enone 11 with bromine in the presence of triethylamine generated the bromoenone, followed by protection of the carbonyl group with ethylene glycol (HOCH 2 CH 2 OH) in the presence of p -toluenesulfonic acid ( p -TsOH) in refluxing benzene to afford the ketal 19 in 54% yield in two steps . Ketal 19 was easily transformed to viable intermediates stannane 20 and boronic acid 6 by treatment with tri- n -butyltin chloride ( n -Bu 3 SnCl) in the presence of tert -butyllithium ( t -BuLi), or trimethyl borate [B(OMe) 3 ] in the presence of n -butyllithium ( n -BuLi), and then treatment with hydrogen chloride, in 81% and 61% yield, respectively.…”

Recent Advances in the Baylis−Hillman Reaction and Applications

“…During their work on π-facial selectivities Yadav et al studied the Diels−Alder cycloaddition reactions of various dienophiles with diene ( 253 ) [obtained from the Baylis−Hillman adduct, that is, 2-(hydroxymethyl)cyclohex-2-enone]. One representative example is shown in Scheme 187 …”

Structure-Dependent Oxidative Bromination of Unsaturated C−C Bonds Mediated by Selectfluor

“…For a system having a double bond conjugated with a carbonyl group, α-bromo-conjugated products were obtained in good yields (Table ). Such compounds are normally synthesized by a two-step procedure, i.e., dibromination followed by elimination in the presence of base . Few one-pot syntheses have been reported directly involving oxidative bromination of corresponding conjugated carbonyl compounds except DMP/TBAB. , It is likely that the byproduct of Selectfluor, compound 2 , plays an important role as base to trigger the elimination reaction in our case.…”