Is the para-position of anisoles more reactive than ortho in electrophilic substitutions?

In my group’s seminar last Friday, we discussed the total synthesis of 6-demethyl-6-deoxytetracycline **1** by Woodward *et al.*[1] One intermediate of their synthesis is *meta*-substituted anisole **2** that they subsequently wish to cyclise to create the first two rings of **1** by a Friedel-Crafts acylation. Since the Friedel-Crafts acylation can proceed on both the *ortho* and *para* carbons (when viewed from anisole’s oxygen substituent), they protected the *para*-position using a chlorine atom (**3**) — introduced by electrophilic aromatic substitution of **2** with chlorine gas. They reported $97\:\%$ yield.


![chlorination of **2**; structure of **1**](https://i.stack.imgur.com/9r54G.png)


This confused me. I remember listening to a *Concepts in Organic Chemistry* lecture given at my university where the electrophilic addition to an electron-rich aryl was discussed. Our professor gave us numbers that approximately equalled a $2\::\:1$ ratio of *ortho* versus *para* substitution. The reasoning was, that the HOMO has almost equal contributions on both *ortho* carbons and the *para* carbon (while having close to no *meta* contributions), meaning that none of the three be favoured. This is approximately supported by the [molecular orbitals of a calculated benzyl anion](http://www.cup.lmu.de/oc/zipse/benzyl-anion-c7h7.html) that the professor has uploaded to his site. (The benzyl anion was used as a model system because it gladly delocates its anionic electron pair into the ring much like oxygens would their unbonded electron pair.)


I do see that **2** has a further substituent in *meta* position. This substituent (alkyl) displays a weak $+\:\mathrm{I}$ effect meaning that it enhances substitution on the same atoms as the oxygen would. One of these is now *ortho* to two substituents (position 2) and likely reacts less — I understand that. But I would still have expected a $1\::\:1$ ratio of *ortho*-oxygen and *para*-oxygen substituted products rather than $97\:\%$ yield of the *para*.


How can we rationalise Woodward’s $97\:\%$? Is it wrong to say that the contributions of *ortho* and *para* carbons are approximately equal, and if so, why is the *para* contribution so much stronger that it is almost solely chlorinated?


[1]: [J. J. Korst, J. D. Johnston, K. Butler, E. J. Bianco, L. H. Conover, R. B. Woodward, *J. Am. Chem. Soc.* **1968**, *90*, 439.](http://dx.doi.org/10.1021/ja01004a041)


