Rules of the Day 4-27-07

1. The Guanidine group is extremely basic, because the protonated form is triply resonance stabilized. This group is very common in biology. Llike amines, the guanidinium group is protonated and positively charged at neutral pH in water.

2. Pericyclic reactions are an entirely new class of reactions in which pi bonds are exchanged simultaneously for sigma bonds via an "aromatic" transition state. The Diels-Alder reaction is a great example of this kind of reaction involving a diene in the s-cis conformation reacting with a dienophile (alkene) to give a six-membered ring product. You can see this reaction on the movie page.

3. Because the Diels-Alder reaction is concerted: 1) the orientation of groups originating on the diene are determined by the s-cis conformation and 2) the configuration of the alkene (dienophile), either cis or trans, is maintained in the product.In addition (excuse the pun), carbonyl groups react in the so-called endo position "underneath" the double bonds of the diene.

4. THE KEY issue with carbohydrates is stereochemistry. The stereochemistry of carbohydrates are based on (+) D and (-) L glyceradehyde. A D-sugar has the same configuration as (+)-D-glyceraldehyde at the chiral center farthest from the carbonyl group. An L-sugar has the same configuration as (-) -L-glyceraldehyde at the chiral center farthest from the carbonyl group. The best way to see this is with Fisher projections. Click here for a molecule of the day that discusses much of this.

5. With Fisher projections, horizontal is toward you, vertical is away from you. Think of it like a teddy bear hugging you. Aaawwwwe.

6. Different stereoisomers of similar carbohydrates can be aldehydes (aldose) ketones (ketose) have different names and generally have 5 (pentose) or 6 (hexose) carbon atoms.

Homework: Begin the 10th and final homework set. This one is focussed on the material we have covered since the midterm. It is due on Friday May 5th, the last day of class. Read: 23.1-23.8, 24.6, 24.7, 25.-25.3(just the forward, not section A or B Problems: 23.1-23.6, 23.43-23.44, 24.25, 24.29, 24.32, 24.36, 25.1-25.3.