Highly stereocontrolled synthesis of some polyfunctionalized cyclohexenes. A short formal total synthesis of (.+-.)-chorismic acid

The [4 + 2] Cycloaddition of 2‐Pyrone in Total Synthesis

“…Further studies by Posner and his coworkers realized Lewis acid or high pressure could promote the Diels‐Alder reaction of the pyrone sulfoxide with vinyl ethers or thioethers . As shown in Scheme a, phenyl vinyl thioether 82 underwent cycloaddition smoothly with pyrone sulfoxide 23 under 6.8 kbar at room temperature, affording the endo ‐phenylthio adduct 83 with high diastereoselectivity.…”

Phenyl Vinyl Sulfide

“…Phenyl vinyl sulfide reacts with tropone regiospecifically to provide bicyclo[3.2.2]nonadienone systems (eq 4), 21 reacts with pyrone sulfoxide 22 with high diastereoselectivity under high pressure (eq 5), has trapped the transient [1,5]-sigmatropic shift isomer of isodicyclopentanediene, 3 and undergoes photosensitized electron transfer (PET) initiated cycloaddition 23 with dienes to give Diels-Alder products (eq 6). The resulting sulfides of these adducts have been cleaved to the corresponding alkanes, 3 oxidized to the sulfoxides followed by thermolysis to give the corresponding alkenes, 22 and oxidized to the sulfones followed by αhydroxylation to give the corresponding ketones. 23 Phenyl vinyl sulfide participates in 1,3-dipolar cycloadditions with 2-azaallyl anions, generated from N-(tributylstannyl) methanimines and n-BuLi, to provide substituted pyrrolidines (eq 7) 4 and highly stabilized isoquinolium methylides, 24 and also undergoes thiyl-radical catalyzed chain cyclizations to give methylenecyclopentanes.…”

Asymmetric [4+2] Cycloadditions Mediated by Sulfoxides

“…Subsequently, Posner published the completely regioselective and highly stereoselective cycloadditions of racemic 3-(p-tolylsulfinyl)-2-pyrone (141) (Scheme 70) with 1,1-dimethoxyethylene [133], vinylether, and vinylthioethers [134]. With the first dienophile, the best diastereoselectivity (an 88:12 ratio of the two endo-adducts) was achieved at room temperature in toluene or hexane as the solvent (48 h).A 10:1 endo/exo mixture of cycloadducts was obtained with vinylether in the presence of ZnBr 2 as the catalyst, whereas a total endo selectivity was observed in reactions of 141 with vinylthioethers [134] conducted under high pressures.…”

“…With the first dienophile, the best diastereoselectivity (an 88:12 ratio of the two endo-adducts) was achieved at room temperature in toluene or hexane as the solvent (48 h).A 10:1 endo/exo mixture of cycloadducts was obtained with vinylether in the presence of ZnBr 2 as the catalyst, whereas a total endo selectivity was observed in reactions of 141 with vinylthioethers [134] conducted under high pressures. The bridged bicyclic lactone cycloadducts 142 have been used as versatile synthons in the synthesis of shikimic acid derivatives.…”

“…The bridged bicyclic lactone cycloadducts 142 have been used as versatile synthons in the synthesis of shikimic acid derivatives. Although enantiomerically pure samples of compound 141 could be obtained [134] it was not used as a starting material for asymmetric Diels-Alder reactions (the low yield of (S)-141 precluded this).…”