Mitsunobu reaction macrocyclic lactones

In a process resembling the Mitsunobu reaction (Chapter 17), alcohols and acids can be coupled to give esters, even macrocyclic lactones as shown below. In contrast to the Mitsunobu reaction, the reaction leads to retention of stereochemistry at the alcohol. Propose a mechanism that explains the stereochemistry. Why is sulfur necessary here ... 

A further reaction mechanistically similar to the Mitsunobu reaction as shown in Scheme 26, with the use of AT,iV-dimethylformamide dineopentylacetal (80), can also be employed for macrolactonization [47]. Takei and coworkers [48] applied it to the synthesis of the macrocyclic antibiotic A26771B (55). As shown in Scheme 27, treatment of the linear precursor 82 with 80 in refluxing dichloromethane for 7 h afforded the lactone 83 (39% yield). 

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