[curiouscat]

The Wikipedia article on Aldol Condensation says at one point:

The reaction between menthone and anisaldehyde is complicated due to steric shielding of the ketone group. The solution is use of a strong base such as potassium hydroxide and a very polar solvent such as DMSO in the reaction below

Why does a polar solvent help against steric shielding? 

More in general, how does one go about selecting a solvent for aldol condensation; any  guidelines? Especially curious to know if solvent selection could be made to reduce undesirable self condensation?

[orgopete]

Just an opinion. We don't know the credentials of the Wikipedia author, so we don't necessarily know what he or she meant. On the one hand, it is simple. The isopropyl group prevents dehydration of an aldol product on its side of the isopropyl group. In that sense, the isopropyl sterically shields the ketone.

This reaction presumably could give either addition product. However, the pKa differential is relatively small between the benzylic alkoxide and the enolate of the ketone, so any addition can reverse. Dehydration will stop(?) the reverse reaction. We know both enolates are forming as epimerization of the methyl group from S to R accompanies the condensation.

Enolization of 2-methylcyclohexanone can occur in two directions. The kinetic product is the less substituted enolate and the more substituted enolate is preferred. KOH only generates a limited amount of enolate as water is more acidic than the ketone. Therefore, the conditions result in an equilibrium of ketone and enolate(s). The dehydration reaction probably drives the equilibria to the product isolated.

In this case, a polar solvent is not necessarily the key as much as the equilibration that accompanies it. A kinetic enolate can be generated in THF, so in that sense a polar solvent would be better. An excess of ketone also equilibrates enolates. Presumably, the kinetic enolate could have been generated, condensed with the aldehyde, and dehydrated in a separate step (acid catalyzed). However, this is a one step reaction, therefore the equilibrium product is favored.

I'm sure discomolide can elaborate much further on solvents as he can explain about industrial production. Otherwise, I would guess an alcohol or THF are probably used the most frequently in laboratory preps. The selection is really guided by the chemistry and what one is attempting to accomplish.

[PhDoc]

When I was in graduate school, my research adviser used to take sadistic delight in asking his students questions regarding the references in the papers they presented at group meeting. He did this to force us into the habit of dotting all our "eyes" and crossing all our "teas."

Did you look up the reference indicated as number 18? I'd do it myself, however I don't have access to the electronic journals right now. ^ Vashchenko, V.; Kutulya, L.; Krivoshey, A. (2007). "Simple and Effective Protocol for Claisen-Schmidt Condensation of Hindered Cyclic Ketones with Aromatic Aldehydes". Synthesis 2007 (14): 2125–2134. doi:10.1055/s-2007-983746.

It's important to realize the Aldol Condensation is not as simple as it appears. The combined efforts of Profs. Herbert House, Clayton Heathcock and Howard Zimmerman helped elucidate the stereochemical outcome of Aldol Condensations taking place in non-polar media, i.e. diethyl ether or tetrahydrofuran. Enolates in such media don't necessarily exist as monomers. Furthermore, when ketone enolates add to aldehydes, then do so via a chelated 6-membered transition state. Please read the article Aldol Condensation - Stereochemistry for a basic understanding of this effect:

http://lennoxtutoring.com/2011/10/12/the-aldol-condensation-part-three-stereochemistry/

Analysis of the ketone in your example indicates it would prefer to exist in a twist-chair conformation to minimize potential energy. How this might behave in a kinetically controlled Aldol Condensation is unknown. If you have the time after reading the article, see if you can construct a transition state analysis for the ketone.

Some ketones (uncommon) fail to react with aldehydes under conditions of kinetic control due to unusual aggregations states of the corresponding enolates. Aggregation states of enolates are broken up in dipolar aprotic solvents such as DMSO, DMF or HMPA. It's not always necessary to run the reaction completely in the dipolar aprotic solvent as mixtures sometimes suffice. The combination of NaOH and DMSO will permit the reaction to proceed more smoothly under thermodynamic control if the energetics of the transition state are unfavorable in non-polar media.