Investigations on the mechanism of the Diels-Alder reaction and synthetic studies on the Prezizaene sesquiterpenes

Spiro(bicyclo[2.2.1 ]heptane-2,1’-3-cyclopentene-2.5-dione) (69) which was obtained from norcamphor in three steps underwent cycloaddition with cyclopentadiene to give all of four possible adducts in a 50 : 22 : 24 : 4 ratio. The π-facial stereoselectivity in the endo region (72 : 28) was consistent with that of the complementary diene spiro(bicyclo[2.2.1]heptane-2,1’-[2,4]-cyclopenadiene) (58) with Z-ethylenic dienophiles (70 : 30). Likewise, the π-facial stereoselectivity of the cycloaddition of symmetrical dienophile spiro(bicyclo[2.2.2]octane-2,1’-3-cyclopentene-2,5-dione) (70) with cyclopentadiene is similar to that of the corresponding diene sipro(bicyclo[2.2.2]octane-2.1’-[2,4]cyclopentadiene) (59) in the Diels-Alder reaction with Z-ethylenic dienophiles. These results suggest very strongly that the steric interactions are responsible for the observed π-facial stereoselectivity in these spiro-addends. -- The relatively large proportions of exo addition products in the cycloadditions of spiro(bicyclo[2.2.1]heptane-2,1’-3-cyclopentene-2,5-dione) (69) and spiro(bicycle[2.2.2]octane-2,1’-3-cyclopentene-2,5-dione) (70) with cyclopentadiene stimulated the reevaluation of the endo-exo selectivity observed with simple dienophiles 4-cyclopentene-1,3-dione (118), spiro[4.5]dec-2-ene-1,4-dione (104), and 2,2-dimethyl-4-cyclopentene-1,3-dione (107). The results are discussed based on frontier molecular orbital theory. -- Comparison experiments of dienes 2-(trimethylsiloxy)-1,3-cyclohexadiene (145a), 6,6-dimethyl-2-(trimethylsiloxy)-1,3-cyclohexadiene (145b), and 5,5-dimethyl-2-(trimethylsiloxy)-1,3-cyclohexadiene (145c) with both symmetrical and unsymmetrical dienophiles were conducted. The cycloadditions of diene 6,6-dimethyl-2-(trimethylsiloxy)-1,3-cyclohexadiene (145b) with symmetrical dienophiles proceeded at roughly the same rates as those of diene 5,5-dimelhyl-2-(trimethylsiloxy)-1,3-cyclohexadiene (145c), which suggested that the reaction was not merely concerted but also synchronous. -- 2-Methyltricyclo[6.2.1.0¹,⁵]undecan-7-one (278) could serve as a precursor to the prezizaene sesquiterpenes. The synthesis of 2-methyltricyclo[6.2.1.0¹,⁵]undecan-7-one (278) was started with 1,4-dioxaspiro[4.5]decane-8-one (294), which was treated with methyllithium to give 8-methyl-1,4-dioxaspiro[4.5]decan-8-ol (295). The spiro-annulation of this ketal alcohol with 1,2-bis(trimethylsiIoxy)cyclobutene (77) proceeded smoothly to produce 8-methylspiro[4.5]dec-7-ene-1,4-dione (281). Addition of methyl-lithium and ozonolysis, followed by intramolecular aldol cyclization afforded a 1 : 1 mixture of two double bond isomers 7-acetyl-4-methylenespiro[4.4]non-7-en-1-one (299) and 7-acetyl-4-methylspiro[4.4]nona-3,7-dien-1-one (280). Hydrogenation and base-induced aldol condensation of this mixture gave 2-methylspiro[6.2.1.0¹,⁵]undecan-7-one (278 and 304). -- During the synthetic studies of prezizaene sesquiterpenes we found that the addition of methyllithium to 8-methylspiro[4.5]dec-7-ene-1,4-dione (281) is stereoselective. Similar diketones were treated with both methyllithium and sodium borohydride. In all the cases the nucleophiles prefer to approach the spiro-diketones at the same face as the double bond of cyclohexene ring. The observed facial selectivity is consistent with the predictions based on the Cieplak model