Esters as a protective group

A facile synthetic route to a-santalol (105) from the previously known aldehyde (106) has been reported by Corey et a/. in which a stereospecific modified Wittig reaction [(i) ethylidenetriphenylphosphorane, (ii) n-butyl-lithium, and (iii) paraformaldehyde] yielded the desired cis-isomer. Erman and co-workers, in a continuation of their synthetic work on the santalols, have reported the preparation of -santalol (107) from 3-methylnorcamphor. The route also provided a sample of the tmns-isomer which had previously been considered to be the natural isomer. The use of a borate ester as a protecting group for a hydroxy-function facilitated the synthesis of dihydro- -santalol (108) as outlined in Scheme 3. (These borate esters are stable to anhydrous acid or base but are readily hydrolysed in aqueous media.)... 

Scheme 11 Thiol ester as a protecting group for thiol...

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