Question

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4. For each pair of structures below circle the most relatively stable one. Show work by drawing relevant chair structures and/or Newman projections as needed) to clearly illustrate why. It is best practice here that only the most stable conformations for each compound are being compared in each set. For clarity. provide a few words that explain your analysis when comparing the illustrations. (12pts total for this problem) (a) ♡ va ♡ 5. (a) (3pts) Look up trans-decalin and cis-decalin in your text book. Which of these is not able to undergo a chair-flip? Briefly explain The tetracyclic structure (shown below) is comprised of rings A,B,C, and D and it represent a typical steroid skeleton along with the numbering scheme used for this class of compounds. The methyl (Me) groups and hydrogens (H) atoms shown are at the bridgehead positions and stereochemistry has been shown for clarity. 12 Me (b) (4pts) Ring A and B look very similar to decalin. Draw only the A and B ring in the correct chair structure form as it would exist in this steroid structure. Include also the H at C-5 and the Me at C-10 in the A correct orientation (axial or equatorial). You do not need to draw the rest of the structure 1 9 H H (c) (Ipt) Are ring A and B cis or trans fused to one another? (d) (Ipt) Look up the structure for cholic acid in your text book. Does the A and B ring in Cholic acid represent a cis or trans-decalin like structure?

Answer 1

4.

above oriented below oriented above oriented above oriented More stable less stable In this, both methyl are above oriented. One methyl is axial and one is equatorial In this, one methyl is above oriented and one methyl is below oriented. both methyl are axial one 1,3-diaxial interaction = 0.9 kcal/mol (CH3-H) two 1,3-diaxial interaction - 2 x 0.9 (CH2-H) 1.8 kcal/mol above oriented above oriented above oriented above oriented below oriented above oriented More stable less stable In this, all methyl are at equatorial position In this, two methyl are equatorial and one methyl is at axial position no 1,3-diaxial interaction = 0.0 kcal/mol (CH3-H) one 1,3-diaxial interaction = 0.9 kcal/mol (CH3-H)