1.3- Oxathiane metallated

The 1,3-oxathiane 8, derived from (5)-l,2,4-butanetriol, is lithiated to form the equatorial anion 9, which adds benzaldehyde with high induced but moderate simple diastereoselectivity (4 1) to form the alcohols 10 and 1117. The selectivity is enhanced to 7 1 by metal exchange by means of magnesium bromide. Deprotection affords (5)-2-hydroxy-l,2-diphenylethanone with 75% ee. It is expected that the method could be extended to aliphatic aldehydes. 

An asymmetric synthesis of mevalolactone in over 87% e.e. employs a 1,3-oxathiane as the chiral auxiliary (81TL2859). The reagent (818), easily prepared from (+)-pulegone (81TL2855), was metallated with u-butyllithium and the anion reacted with acetaldehyde. Oxidation of the diastereomeric mixture of alcohols to the ketone (819) and reaction of... 

Like the 1,3-dithianes, 1,3-oxathianes can be metallated with alkyllithium reagents and they hydrolyse about 10,000 times faster with protic acids. However, their value is diminished by the limited stability of the lithio derivatives and their inherent lack of symmetry which introduces the complications of diaster-eoisomerism. Eliel and co-workers have exploited the diastereoisomerism of... 

Calcium in ammonia offers some advantagesin that most cases of overreduction can be prevented by limiting the amount of active metal, although derivatives of benzaldehyde lose both C—S bonds. Oxathiolanes and oxathianes give clean cleavage of the C—S bond. In the t-butylcy-clohexylidene series, the isomeric pairs (71) and (72) give almost convergent results, with the trans... 

Several other acyl anions or potential acyl anion equivalents bearing at least one nonoxidized sulfur atom have since been proposed, and some of them have been alkylated successfully. This is effectively the case for the following metallated compounds (i) yV-methylthioformaldine (Scheme 73, entry c) (ii) 1,3-oxathianes, and a-trimethylsilylmethyl analogs (iii) a-methoxythioanisole (Scheme 74, entry a Scheme 75, entry the parent compound also allows the synthesis of acetals (on acid-... 

Whereas thioanisole, CH3SPh, is reported to give a mixture of ring-metallated and side-chain-metallated products upon interaction with butyllithium, the 0,S-acetal CH3OCH2SPh is lithiated only on the methylene carbon atom by BuLi in THF or sec-BuLi-TMEDA in THF [1-3]. Analogous metallations have been realized with 1,3-oxathiane [4]. The obtained lithium compounds have a limited thermal stability [5], so functionalizations with alkyl halides and epoxides, which are usually less fast than reactions with other electrophiles , give reduced yields. The 0,S-acetal PhSCH2OCH3 can be lithiated in a reasonable time with butyllithium in a THF-hexane mixture but the temperature has to be kept below — 40 °C to prevent decomposition of the lithiated intermediate into PhSLi and other (unidentified) products [5],... 

---