Sulfinate, menthyl sulfoxide synthesis

The Andersen synthesis of chiral sulfoxides has also been extended to diastereomerically or enantiomerically pure arenesulfinamides, which on treatment with methyllithium give optically active methyl aryl sulfoxides (83,85). The use of menthyl sulfinates in the synthesis of sulfoxides has been exploited in the preparation of optically active sulfoxides 47 and 48, which are chiral by virtue of isotopic substitution, H- D (86), and (87), respectively. More recent... 

In sulfinic acids, RS(0)0H, sulfur is in the same oxidation state as in sulfoxides, and many esters of sulfinic acid (sulfinates) are known in which separate enantiomers have been isolated. If the ester is composed of a chiral alcohol, e.g. menthol, the resultant sulfinate is diastereomeric. Menthyl / -toluenesulfinate (48) has been used in synthesis. 

An enantioselective synthesis of chiral QUINAP 234 was reported by Knochel et al. (07SL2655). The organolithium species obtained from l-(2-bromo-l-naphthyl)isoquinoline by treatment with f-BuLi reacted with (—)-menthyl (S)-p-toluene-sulfinate at-78 °C. The resulting diastereomers were separated via column chromatography. One pot sulfoxide lithium exchange at low temperature, Ph2PCl reaction, sulfur protection with Ss and a Raney-Ni desulfurization step afforded optically pure QUINAP (99% ee) in 60% yield. The s)mthetic route avoided the use of Pd complexes for the resolution. The ees were determined after resulfurization on Chiralcel OD-H. 

This reaction is more stereoselective than the corresponding synthesis of menthyl sulfinate diastereoisomers in the Andersen procedure, allowing for easier fractional crystallization. Optically active (/ )-(+)-methyl phenyl sulfoxide (13) is obtained on reaction of (lf ,2S)-(12) with methyllithium (Scheme 2.14). 

Montanari has also applied chiral sulfinamide methodology to the asymmetric synthesis of methyl aryl sulfoxides [25]. (5)-(-)-p-Toluene sulfinamides (14) were prepared from (5)-(-)-menthyl p-toluene sulfinate and the corresponding bromomagnesium dialkylamides in moderate yield and good enantioselectivity (Scheme 2.15). 

Asym. synthesis. Benzyl chloride and Mg allowed to react in ether, filtered, the filtrate added to a refluxing ethereal soln. of ( — )-menthyl ( —)-p-toluene-sulfinate, and refluxing continued 0.5 hr. ( + )-benzyl p-tolyl sulfoxide. Y ... 

---