1. II. Electrocyclizations Only a single component (starting material) Involves the rearrangement of  bonds to form one new  bond. Important Questions: 1. Which conjugated  systems work or don’t work? 2. What is the basis for stereochemical control? 3. Why do heat and light give different (opposite) stereochemical results?

2. 1. Minimal structural criteria for electrocyclizations: A. To work, a  system must be conjugated without interruption. B. Conjugated system must be able to achieve planarity. C. Conjugated system must have a chromophore (molecular antenna that absorbs light) for photochemistry.

3. 2 & 3. Stereochemistry: Thermal vs. Photochemistry Can be understood by reliance on a model which utilizes molecular orbitals. Only 2 M.O.s are necessary to predict product: HOMO & LUMO The model that uses these orbitals has the following rules: 1. Thermal reactions utilize the HOMO of the conjugated system. Photochemical reactions utilize the LUMO of the conjugated system (the excited state of the system). 2. The lobes rotate (which leads to the stereochemistry of the the product) in such a way as to maintain maximal overlap (bonding). Lobes can rotate either disrotatory or conrotatory.

4. 2 & 3. Stereochemistry: Thermal vs. Photochemistry Summary: # of double bonds Participating Thermal (HOMO)Photochemical (LUMO) odd disrotatory conrotatory even conrotatory disrotatory Note: Thermal and photochemical conditions have opposite direction of rotation of lobes. Odd and even # of  bonds have opposite direction of rotation of lobes.

5. Carbohydrates - Sugars Polyfunctional compounds: alcohol, aldehyde Chemistry from the OH and CHO (you’ve already seen!) One major class of biologically organic molecules Major reservoir of organic carbon on earth: ≤ 0.03% of earth is carbon > 99% of that carbon is rock, as CO 3 2- Carbon gets incorporated into the biosphere as seen in photosynthesis: Most of the initial carbohydrate formed is n = 6, glucose C 6 H 12 O 6 Glucose

6. Classification of Carbohydrates

7. Complex Carbohydrates: Disaccharides & Polysaccharide: two or more carbohydrates Example: Sucrose - made up of 1 glucose and 1 fructose Example: Cellulose - made up of ~3000 glucose units

8. Depiction of Carbohydrates: Fischer Projections Glucose: 4 stereocenters; carbons are chiral! (R)-Glyceraldehyde - simplest monosaccharide

9. Depiction of Carbohydrates Since carbons are chiral - need to address the stereochemistry. Three (3) ways to depict/deal with stereochemistry: 1. R/S 2. +/- 3. D/L 1. R/S - Assigning configurations to stereocenters; structural information (a) Assign priorities as usual (b) Rotate such that the lowest priority group is back (need to hold one group steady and rotate the other 3 either clockwise or counter- clockwise) (c) Assign the configurations to each stereocenter Examples: glyceraldehyde & glucose (on board)

10. Depiction of Carbohydrates 2. +/- Indicates a physical property, i.e., the direction that the carbohydrate rotates a plan of polarized light. e.g. (+)-glucose and (-)-glucose; both enantiomers Contains no structural information

11. Depiction of Carbohydrates 3. D/L Gives structural information Historical labels, relates to the absolute stereochemistry of a carbohydrate to a reference compound, natural glyceraldehyde: Natural glyceraldehyde Rotates plane polarized light in the (+) or D (dextrorotatory) direction, thus is D-glyceraldehyde Example: glucose Compare “lowest” stereocenter to glyceraldehyde’s stereocenter Thus, this is D-glucose. L-glucose L for levorotatory