What are the roles of pyridine and DCM in the acylation of an alcohol?

What role does pyridine and $\ce{CH2Cl2}$ play in this reaction?


[![Acylation of an alcohol](https://i.stack.imgur.com/R0Skr.png)](https://i.stack.imgur.com/R0Skr.png)


(Prior to this step, the corresponding carboxylic acid was converted to the acyl chloride using DMF, oxalyl chloride, and DCM.)


This reaction is taken from Bear, R. B.; Shea, K. J. Tandem Hetero Diels−Alder Reaction: Synthesis of Oxygenated Macrocycles. *Org. Lett.* **2001,** *3* (5), 723–726. [DOI:10.1021/ol007040s.](http://doi.org/10.1021/ol007040s).




Dichloromethane (DCM) is a common organic solvent and this is its role in both reactions (and in most cases where it is used). The first reaction is indeed the conversion of a carboxylic acid to an acyl chloride. The DMF is a catalyst and the reactions proceeds through an imidoyl chloride intermediate, $\ce{Me2N+=CHCl}$.


Pyridine is a weak base, with a $\mathrm{p}K\_\mathrm{aH}$ of 5.21 and it is often used to remove extra protons during the course of a reaction. It is also a good nucleophile for attacking carbonyl groups to activate them towards attack by other, less good nucleophiles (thanks to @orthocresol and @DGS for pointing this out). In this reaction it serves both these purposes.


[![Nucleophilic catalysis by pyridine in acylation of an alcohol](https://i.stack.imgur.com/KyrwI.jpg)](https://i.stack.imgur.com/KyrwI.jpg)


An acid anhydride can also be used as the acylating agent. The principle is the same: pyridine acts as a nucleophilic catalyst and generates the same intermediate as above.


 4