Sulfoxide, chiral vinyl asymmetric induction

The addition of electrophilic reagents to chiral a,/3-unsaturated sulfoxides is also accompanied by asymmetric induction. Stirling and Abbott (318,322) found that the addition of bromine to the optically active (.R)-vinyl-p-tolyl sulfoxide 319 yields a mixture of diastereo-meric a,/3-dibromosulfoxides 320. Oxidation of this mixture gives the optically active sulfone 321, with a center of chirality at the a-carbon atom only. The optical purity (32%) of this sulfone was estimated by comparing its specific rotation with that obtained as a result of oxidation of diastereomerically pure sulfoxide (/ )-320. The assignment of configuration at the a-carbon atom was based on the analysis of the polarizabilities of substituents. 

Addition to allylic mesylates.6 Conjugate addition of organocyanocuprates to acyclic allylic mesylates substituted at the P-position with a chiral sulfoxide group involves an SN2 -substitution with high Z/E stereoselectivity and high asymmetric induction. This reaction provides a route to chiral trisubstituted vinyl sulfoxides. 

With regard to isomerizations of double bonds, sulfinyl enynes gave unexpected results when submitted to PKR. These chiral substrates were thought to give high asymmetric inductions due to the proximity of the chiral sulfur atoms to the reaction centers. Surprisingly, both cis and trans ferf-butyl vinyl sulfoxides (237-238) were transformed into the same PK diastereoisomer 239 with high ee and moderate yields (Scheme 67) [103,104]. 

The asymmetric induction on the 1,3-dipolar cycloaddition reaction of carbonyl ylides has also been studied using chiral dipolarophile. The Rh2(OAc)4-catalyzed reactions of o-(methoxycarbonyl)diazoacetophenone 89 with enantiomerically pure vinyl sulfoxides 103 afforded 4,10-epoxybenzo-[4,5]cyclohepta[l,2-c]furan-3,9-dione 105, in good or moderate yield with complete regioselectivity [113]. The endo stereoisomer 105a is favored with respect to the exo isomer 105b and interestingly, high diastereoselectivity and complete enantioselectivity have been achieved (Scheme 32). 

The plausible mechanism of asymmetric induction is proposed (Scheme 2). The palladium catalyst reacts from the sterically less crowded downward direchon of the lone pair side of the chiral sulfinyl group in the conformationally most stable form of (5s)-l with syn-coplanality between the sulfinyl-oxygen bond and the carbon-carbon double bond of the chiral vinyl sulfoxide, forming 3. Based on the stereochemistry of the product, the... 

Ene Reactions. In the presence of Lewis acids, isobutene adds to various enophiles to yield ene adducts. Enophiles such as alkoxyaldehyde (eq 13), dialkyl aminoaldehyde (eq 14), haloaldehyde (eq 15), and vinyl sulfoxides (eq 16) have been utilized. Chiral organoaluminum (eq 17) and organotitanium reagents (eq 18) have been reported to give high levels of asymmetric induction in the ene reaction of isobutene with activated aldehydes. 

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