This compound is an antifungal agent from the fermentation culture MF6020
found by the Merck group in 1997. Interesting characteristics are its ability to inhibit the biosynthesis of inositol phosphorylceramide (IPC) and its antifungal activity.
This group first looked at two smaller mimic compounds to analyze the chiral centers separately.
They then compared the data for the chiral centers and arrived at an estimation of what the stereochemistry of khafrefungin would look like.
Finally, they completed the total synthesis of 4 possible stereoisomers of khafrefungin and found one that had antifungal activity and matched known data of khafrefungin.
Stereochemical Assignments
The stereochemistry at the C10,C11,C12 region and the C2',C3',C4' region needed to be elucidated. The following shows the
relative numbering pattern for khafrefungin:
| Mimic 1 | Mimic 2 |
|---|---|
 |
 |
For each mimic there were 4 possible combinations at the chirality centers.
Mimic 1
| 1a | 1b | 1c | 1d |
|---|---|---|---|
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 |
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| H11: 4.0, 7.3 Hz C11: 80.6 ppm |
5.8 Hz 79.8 ppm |
5.4, 5.9 Hz 81.0 ppm |
2.7, 8.5 Hz 79.0 ppm |
Mimic 1 was a construct of the 11 position of khafrefungin. At the 11 position khafrefungin had H11 = 5.7 Hz and C11 = 79.8 ppm. Mimic 1b matched this data and therefore
the probable stereochemistry of the 11 position was 10,11-anti and 11,12-syn.
Mimic 2
| 2a | 2b | 2c | 2d |
|---|---|---|---|
 |
 |
 |
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| 1H NMR: J2',3' = 3.7, J3',4' = 7.8, J4',5' = 3.2, 4.6 Hz 13C NMR: 61.6, 72.4, 73.9, 74.9 ppm |
J2',3' = 2.0, J3',4' = 9.0, J4',5' = 2.8, 3.9 Hz 61.4, 71.2, 72.2, 74.9 ppm |
J2',3' = 2.8, J3',4' = 6.5, J4',5' = 3.7, 4.9 Hz 61.6, 71.2, 72.2, 76.8 ppm |
J2',3' = 7.5, J3',4' = 3.1, J4',5' = 5.8, 5.8 Hz 61.4, 72.2, 72.8, 74.9 ppm |
Mimic 2 like mimic 1 only consisted of a portion of khafrefungin. The spectral data for khafrefungin was 1H NMR: J2',3' = 1.9, J3',4' = 9.1, J4',5' = 2.8, 4.2 Hz
13C NMR: 61.5, 71.3, 71.8, 75.5 ppm. This data was closest matched to Mimic 2b, with 2',3'-syn and 3',4'-syn stereochemistry. Now, there was an idea of what possible stereochemistry existed for khafrefungin. The next
step was to complete a total synthesis that would result in a good yield of a compound with the 1b and 2b stereochemistry.
Total Synthesis Overview
The following is a synopsis of the synthesis with key steps noted.
This aldol condensation was done with tin(II)-chiral diamine complex. This proved to be an important way of accomplishing aldol condensations throughout the synthesis.
However, the yield of the proper stereoisomer was low by this method, so a different method using the Kishi protocol was utilized.
Step 1
This is the result of the Kishi protocol after a few modification steps to result in product 5.
Product 5 was then utilized in scheme 4 to obtain 7b.
Step 2
A series of steps were utilized to obtain 7b
Numerous steps similar to the aldol condensation and modification steps were utilized to form compound 24ab.
Step 4
The result of step 3 (24ab) was combined with 15e, a similar precursor of the mimic 2 series:
DMAP-HCL gave a high yield of the combination product. A few modifications yielded 25a:
Step 5
Finally, all the protecting groups that had been added along the way were removed with BCL3 to
to give a 57% yield of product 18a. This matched the data of khafrefungin for 1H, 13C NMR, Rf values on TLC, retention time of RP HPLC and optical rotation. Furthermore,
product 18a had antifungal activity. Other similar stereoisomers were made, but none matched the data and none had antifungal activity. The conclusion is that khafrefungin
exists in a single active stereoisomer in nature and is with the following configuration: