Meso-2,3-butanediol

Only three not four stereoisomeric 2 3 butanediols are possible These three are shown m Eigure 7 10 The (2R 3R) and (2S 3S) forms are enantiomers of each other and have equal and opposite optical rotations A third combination of chirality centers (2R 3S) however gives an achiral structure that is superimposable on its (2S 3R) minor image Because it is achiral this third stereoisomer is optically inactive We call achiral mole cules that have chnahty centers meso forms The meso form m Eigure 7 10 is known as meso 2 3 butanediol... 

One way to demonstrate that meso 2 3 butanediol is achiral is to recognize that its eclipsed conformation has a plane of symmetry that passes through and is perpendicular to the C 2-C 3 bond as illustrated m Eigure 7 11a The anti conformation is achiral as... 

FIGURE 7 10 Stereo isomeric 2 3 butanediols shown in their eclipsed con formations for convenience Stereoisomers (a) and (b) are enantiomers of each other Structure (c) is a diastereo mer of (a) and (b) and is achiral It is called meso 2 3 butanediol... 

In the same way that a Fis cher formula is a projection of the eclipsed conformation onto the page the line drawn through its center is a projection of the plane of symmetry that is present in the eclipsed conformation of meso 2 3 butanediol... 

Figure 20. Simultaneous enantiomer separation of various classes of compounds ( Schurig test mixture 184) on CP-Cyclodextrin-/3-2,3,6-M-19 (permethylatcd /3-cyclodextrin in OV-1701) [25 m x 0.25 mm (i.d.) column, 70°C for 5 min followed by 3cC/miu, 0.65 bar hydrogen]143. 1+2 2,6,6-trimethylbicy-clo[3.1.1]hept-2-ene (x-pinene), 3 ( + )-(lJR)-//ms-2,6,6-trimethylbicyclo[3.1. l]heptane (pinane), 4 (-)-(lS )-fra/M-pinanc. 5 (-)-(lS)-fw-pinane, 6 ( + )-(l/J)-cw-pinane, 7 + 8 2,3-butancdiol. 9 meso-2,3-butanediol, 10 + 11 tetrahydro-5-methyl-2-furanone (y-valerolactone). 12 + 13 1-phenylethanaminc. 14 + 15 1-phenylethanol, 16 + 17 2-ethylhexanoic acid.

Which alkene, c/s-2-butene or trans-2-butene, would you choose in order to prepare meso-2,3-butanediol by epoxidation followed by acid-catalyzed hydrolysis Which alkene would yield /rjeso-2,3,-butanediol by osmium tetraoxide hydroxylation ... 

Exercise 15-35 How would you synthesize (a) meso-2,3-butanediol and (b) d,l-2,3-butanediol from c/s-2-butene ... 

Begin by drawing meso-2,3-butanediol, recalling that a meso form is achiral. The eclipsed conformation has a plane of symmetry. 

Let s try another example. This time our assignment is to prepare meso-2,3-butanediol from propyne ... 

The initial product of hydroxylation of a double bond is a cyclic osmate, which is reduced on treatment with NMO to cleave the osmium-oxygen bonds. Since no carbon-oxygen bonds are broken in the cleavage step, the final stereochemistry is the same as that of the initial adduct. The product is meso-2,3-butanediol. 

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