Drawing the chair conformation of a pyranose ring

chemistry College MCAT organic chemistry

In this blog post, we will complete the following example problem:

Draw the chair conformation of α-d-galactopyranose 

First, we must draw the Haworth structure of α-d-galactopyranose from the Fischer projection of d-galactose.

For the MCAT, you need to memorize this structure, as well as those of D-glucose, D-mannose, and D-fructose.

Screen Shot 2021-06-09 at 2.40.42 PM

Note that we will not be covering the “long way” of converting a Fischer projection to a Haworth projection. For more on that topic, follow this link!

In our “shortcut” method, we will start with the skeleton of a pyranose ring and number the carbons as follows:

Screen Shot 2021-06-09 at 2.41.57 PM

Note that the accepted convention is to place the oxygen atom in the back right position. 

Now we must account for all of the carbon atoms. A pyranose ring contains only five carbon atoms, but D-galactose has six carbon atoms! Where do we put the sixth carbon? The sixth carbon (C6) in D-galactose is a CH2OH group, and it is connected to the C5 position. Therefore, we draw a CH2OH group connected to the C5 position in the pyranose ring. The “D” indicates that the CH2OH group must be in the UP position in the pyranose ring.

Screen Shot 2021-06-09 at 2.42.45 PM

That takes care of the configuration at C5. Now let’s focus on the remaining positions. The configuration at the anomeric carbon (C1) is conveyed in the name of the pyranose ring form. The term alpha means that the OH group at C1 must be DOWN. A mnemonic that I use is: fish (alpha) swim in the sea (down), and birds (beta) fly in the sky (up).

Screen Shot 2021-06-09 at 2.43.29 PM

The remaining chiral centers (C2, C3, C4) are drawn using the following rule: any OH groups on the RIGHT side of the Fischer projection will be pointing DOWN in the Haworth projection, while any OH groups on the LEFT will be pointing UP.

Screen Shot 2021-06-09 at 2.44.08 PM

Now that we have our Haworth projection, we will draw the chair formation

First, draw the skeleton of a chair and number the carbons as follows:

Screen Shot 2021-06-09 at 2.44.47 PM

Note how the oxygen is placed on the upper, rear-right corner.

The next step is to draw all substituents on the chair. Each substituent is labeled UP or DOWN and then placed in the appropriate position on the chair conformation:

Screen Shot 2021-06-09 at 2.45.29 PM

And there we have it!

For practice, try the following problems:

Draw the chair conformation for each of the following compounds:

(a) β-d-Galactopyranose

(b) α-d-Glucopyranose

(c) β-d-Glucopyranose 

To summarize:

  1. Draw the skeleton of a Haworth projection, with the ring oxygen in the back right corner. Number each position
  2. For D sugars, place the CH2OH group pointing up at C5
  3. Place the OH group at the anomeric position. Alpha = DOWN, and beta = UP
  4. Place the remaining OH groups at C2, C3, and C4. RIGHT = DOWN, LEFT = UP
  5. Draw the skeleton of a chair conformation, with the oxygen atom in the upper, rear-right corner.
  6. Label each substituent as either “UP” or “DOWN” and place each substituent on the chair accordingly.

For more practice, I recommend reading Organic Chemistry by David Klein and the companion series Organic Chemistry as a Second Language. 

Answers to practice problems:

Screen Shot 2021-06-09 at 2.50.16 PM  Screen Shot 2021-06-09 at 2.50.23 PM  Screen Shot 2021-06-09 at 2.50.30 PM

 

Comments

topicTopics
academics study skills MCAT medical school admissions SAT college admissions expository writing English strategy MD/PhD admissions writing LSAT GMAT physics GRE chemistry biology math graduate admissions academic advice law school admissions ACT interview prep test anxiety language learning career advice premed MBA admissions personal statements homework help AP exams creative writing MD test prep study schedules computer science Common Application summer activities mathematics history philosophy organic chemistry secondary applications economics supplements research grammar 1L PSAT admissions coaching law psychology statistics & probability dental admissions legal studies ESL CARS SSAT covid-19 logic games reading comprehension PhD admissions engineering USMLE calculus mentorship Spanish parents Latin biochemistry case coaching verbal reasoning DAT English literature STEM admissions advice excel medical school political science skills AMCAS French Linguistics MBA coursework Tutoring Approaches academic integrity astrophysics chinese gap year genetics letters of recommendation mechanical engineering Anki DO Social Advocacy algebra art history artificial intelligence business careers cell biology classics dental school diversity statement geometry kinematics linear algebra mental health presentations quantitative reasoning study abroad tech industry technical interviews time management work and activities 2L DMD IB exams ISEE MD/PhD programs Sentence Correction adjusting to college algorithms amino acids analysis essay athletics business skills cold emails data science finance first generation student functions graphing information sessions international students internships logic networking poetry proofs resume revising science social sciences software engineering trigonometry units writer's block 3L AAMC Academic Interest EMT FlexMed Fourier Series Greek Health Professional Shortage Area Italian JD/MBA admissions Lagrange multipliers London MD vs PhD MMI Montessori National Health Service Corps Pythagorean Theorem Python Shakespeare Step 2 TMDSAS Taylor Series Truss Analysis Zoom acids and bases active learning architecture argumentative writing art art and design schools art portfolios bacteriology bibliographies biomedicine brain teaser campus visits cantonese capacitors capital markets central limit theorem centrifugal force chemical engineering chess chromatography class participation climate change clinical experience community service constitutional law consulting cover letters curriculum dementia demonstrated interest dimensional analysis distance learning econometrics electric engineering electricity and magnetism escape velocity evolution executive function fellowships freewriting genomics harmonics health policy history of medicine history of science hybrid vehicles hydrophobic effect ideal gas law immunology induction infinite institutional actions integrated reasoning intermolecular forces intern investing investment banking lab reports letter of continued interest linear maps mandarin chinese matrices mba medical physics meiosis microeconomics mitosis mnemonics music music theory nervous system