Epoxides, reaction with camphorsulfonic acid

Sulfur ylides, derived from benzyl bromides and an optically active alkyl sulfide, undergo base-promoted reactions with aryl aldehydes to produce optically active 1,2-diaryl epoxides. The reaction is illustrated by equation (17) and produces epoxides with optical purities in the range of 28-47% ee. The bicyclic sulfide shown in equation (17) was derived from (-i-)-camphorsulfonic acid and produces the () j )-enantiomer of the epoxide in excess. 

Acid-catalysed addition of primary, secondary, and tertiary alcohols to 3,4-dihy-dro-2//-pyran in dichloromethane at room temperature is the only general method currently in use for preparing THP ethers and the variations cited below concern the choice of acid. The reaction proceeds by protonation of the enol ether carbon to generate a highly electrophilic oxonium ion which is then attacked by the alcohol. Yields are generally good. Favoured acid catalysts include p-toluenesulfonic acid or camphorsulfonic acid. To protect tertiary allylic alcohols and sensitive functional groups such as epoxides, the milder acid pyridinium p-toluenesulfonate has been employed (Scheme 4.316]. A variety of other acid catalysts have been used such as phosphorus oxychloride, iodotrimethylsilane- and bis(trimethylsilyl)sulfate. but one cannot help but suspect that in all of these cases, the real catalyst is a proton derived from reaction of the putative catalysts with adventitious water. Scheme 4.317 illustrates the use of bis(trimethylsilyl)sulfate in circumstances where other traditional methods failed. - For the protection of tertiary benzylic alcohols, a transition metal catalyst, [Ru(MeCN)2(triphos)](OTf)2 (0.05 mol%) in dichloromethane at room temperature is effective. ... 

Fumkawa and coworkers realized the first enantioselective substoichiometric epoxidation reaction of aldehydes via the sulfide alkylation/deprotonation route, based on their one-pot procedure for the synthesis of oxiranes using sulfides directly rather than the preformed sulfonium salts [6]. In the presence of 50mol% sulfide 1, which was prepared in three steps starting from (-l-)-camphorsulfonic acid, a 50% yield and 47% ee were obtained in the reaction of benzaldehyde with benzyl bromide under solid-liquid two-phase conditions (solid KOH/CH3CN). A catalyst turnover (TON = 2.3) was also observed when 10mol% catalyst was used in the reaction of 4-chlorobenzaldehyde (Scheme 20.3). The 0-methylated derivative of sulfide 1 gave the trows epoxides with an opposite configuration. 

---