Leaving group requirements

The closure of the lactone must be done with inversion of configuration at the alcohol centre. We should normally use a Mitsunobu reaction for this but mesylation is a good alternative. Mesylatior occurs via the sulfene with retention and then displacement of this excellent leaving group require, inversion and the cis lactone is formed. 

Energetics Poorer leaving groups require a higher temperature. Selenoxides eliminate easiest, sulfoxides next, and amine oxides require the highest temperatures. 

Direct in-line substitution occurs with inversion of configuration at phosphorus, whereas attack adjacent to the leaving group requires pseudo-rotation leading to retention of configuration at phosphorus. Phosphoryl transfer via free metaphosphate species is predicted to occur with racemiza-tion. These stereochemical outcomes are detectable by the chiral phosphate... 

Leaving groups require partial charges greater than 0. 

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