Phosphates, and chirality

Mikami and Yoshida extended the scope of this method considerably by using propargyl phosphates and chiral proton sources [94], The propargylic phosphates thereby have been found to be advantageous owing to their high reactivity towards palladium and the extremely low nudeophilicity of the phosphate group [95]. In some cases, it was even possible to obtain allenes from primary substrates, e.g. ester 194 (Scheme 2.60) [96]. A notable application of this transformation is the synthesis of the allenic isocarbacydin derivative 197 from its precursor 196 [97]. 

Pseudorotations involving oxyligands are generally fast equilibria. Rapid rate-accelerated processes, such as the exocyclic cleavage observed in the hydrolysis of methyl ethylene phosphate, necessarily involve pseudorotation of TBP intermediates (Kluger et al., 1969 Boyd, 1969 Kluger and Thatcher, 1985, 1986). In spite of this, no pseudorotation has ever been demonstrated in an enzyme system in which nucleophilic substitution occurs at phosphorus. The evidence for this assertion is based on stereochemical analysis, employing chiral ( 0, 0-labelled) phosphates and chiral thiophos-phates as substrates. 

---