Regioselectivity of the Diels‐Alder Additions of 2‐Substituted 5,6 Dimethylidenenorbornanes and‐bicyclo[2.2.2]octanes

Abstract

SummaryThe cycloadditions of methyl propynoate and methyl vinyl ketone to 5,6-dimethylidene-2-norbornanone (6) are 'para 'regio~elective~). A smaller 'para Lregioselectivity is observed for the addition of methyl propynoate to 5,6-dimethylidene-2-bicyclo[2.2.2]octanone (10). No regioselectivity is observed with 5,6-dimethylidene2exo-norbornyl alcohol (3), acetate (5) and 5,6-dimethylidene-2 exo-bicyclo[2.2.2]0~-tanol(9). PMO arguments based on the shape of the HOMO'S and subHOMO's of the dienes allow to rationalize these observations. Unpredictable 'para-or 'metat regioselectivities are found for the Diels-Alder additions of 5,6-dimethylidene-2en-do-norbornyl alcohol (2), acetate (4) and 5,6-dirnethylidene-2endo-bicyclo[2.2.2]0~-tanol(8). The carbonyl group ofP,y-unsaturated ketones such as 6 and 10 can act as an electron donating homoconjugated substituent. The n(CO)++o[C( l),C(2)]t+x [C(5),C(6)] hyperconjugative interaction can override the usual electron-withdrawing effect of this function. ~~Introduction. -The Diels-Alder reactivity of an exocyclic s-cis-butadiene6) grafted onto norbornane [2] [3] and bicyclo[2.2.2]octane [ 11 [4] skeletons (s. 1 and 7, resp.) can be affected by remote substitution of the bicyclic systems. The carbonyl group in 5,6-dimethylidene-2-norbornanone (6) and in 5,6-dimethylidene-2-bicycIo[2.2.2]0~-tanone (10) causes a significant rate retardation effect on the cycloadditions of the diene to strong dienophiles. We report now on the regioselectivity of the Diels-Alder additions of the homoconjugated dienones 6 and 10, and of the related endo-and exo-alcohols 2,8, 3 and 9, and acetates 4 and 5.Since our exocyclic dienes are grafted onto rigid bicyclic skeletons, geometry and steric factors should not play a dominant role in determining the regioselectivity

Regioselective Electrophilic Additions of Bicyclo[2.2.n]alk‐2‐enes Controlled by Remote Epoxide Functions

“…We attribute it to the electrostatic effect of the endo 0-atom in the endo-epoxides 13 and 15 which repels attack of chloride ion onto the endo face of the epi-sulfonium intermediate 37, thus favouring attack at C(5) instead of C (6). There is a correspondence between electrophilic additions to the endocyclic double bond in bicyclo[2.2.n]alk-5-en-2-ones 41 [ 181 [ 193, giving exclusively adducts 42 under conditions of kinetic control, and the Diels-Alder regioselectivity of the exocyclic dienes 43 toward an electron-poor dienophile such as methyl propynoate, which gave preferen-tially cycloadducts 44 [20]. The regioselectivity of both types of reactions was attributed to the electron-donating ability of the homoconjugated carbonyl group because of a favourable hyperconjugative interaction of the type n(CO),…”

“…All the adducts 22-33 were formed under conditions of kinetic control, i.e. they were not isomerized under the conditions of their formation (CHCl,,20") or upon heating to 50" for several hours.…”

1H‐NMR Spectra of Cyclohexa‐1,4‐dienes and Cyclohexenes Annellated to Bicyclo[2.2.1]hept‐2‐enes. The Inter‐Ring Homoallylic H,H Coupling Constants as Stereochemical Probes

“…1.02~7]undeca-2(7) .4-diene-4-carhoxylute (7). See [30]. Methyl Il-Oxrrtricycl0~6.2.1.0~~~~undecu-2(7j ,4-diene-4-carhoxyla1e (8).…”

Synthesis and Diels‐Alder Reactivity of 5,6,7,8‐Tetramethylidene‐2‐bicyclo[2.2.2]octanol and ‐octanone. Selective Oxidations of the Corresponding Bis(irontricarbonyl) Complexes

“…Methyl propynoate added to the anti-Fe (CO)3-monocomplexed tetraenol 26 and furnished a 1:l mixture of the adducts 37 and 38. The absence of regioselectivity was not a surprise as the cycloadditions of methyl propynoate to 5,6dimethylidene-2exo-norbornanol and dienol30 were not found to be regioselective either [7]. More interesting, however, was the Diels-Alder addition of methyl propynoate to the anti-Fe (C0)3-monocomplexed tetraenone 28 which led to a 3 : 1 mixture of the adducts 39 and 40.…”

“…The Diels-Alder reactivity of an exocyclic s-cis-butadiene moiety grafted onto norbornane and bicyclo [2.2.2]octene systems can be affected by remote substitution of the bicyclic skeleton [7] [813). For instance, the addition of the 5,6-dimethylidene-2-norbornanone (4) to methyl propynoate gives preferentially the para'-adduct 5, whereas 5,6-dimethylidene-2endo-norbornanol (7) yields preferentially the 'meta'adduct 8 [7] (minor products: 6 and 9, resp.). These findings have encouraged us (minor) to design new double exocyclic dienes 1 in which the bridge Z would be substituted in a way to control the stereo-and regioselectivity of the two successive cycloadditions with k l and kz, respectively.…”