3.9: Study Questions
- Page ID
- 285635
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Turner’s synthesis of PGF2α uses endo dicyclopentadiene (2) as starting material and generates an intermediate 1.
Use one or more of the following terms to answer each of the following questions: thermodynamic control, stereoelectronic control, steric approach control, or temporary bridge. How is stereocontrol achieved:
(a) at the 11 position relative to the 9 position in 1?
(b) at the 8-position relative to the 9-position in 1?
(c) at the 12-position relative to the 8-position in 1?
2. Corey’s second synthesis of PGF2α exploited a lactone intermediate 3 that contains all of the cyclopentane ring stereochemical information present in PGF2α.
Use one or more of the following terms to answer each of the following questions: thermodynamic control, stereoelectronic control, steric approach control, or temporary bridge.
(a) Two factors favor the correct relative stereochemistry for the aldehyde substituent in 3 during its synthesis from 6 by way of 5 and 4. What are these two stereocontrolling factors?
(b) How is stereocontrol achieved at the 9-position relative to the 8-position in 3?
(c) How is stereocontrol achieved at the 11-position relative to the 8-position in 3?
3. In Corey’s first synthesis of PGE1, he uses a substituted cyclohexene precursor that is suggested by a polar disconnection of the subtarget 7 and by his intention to use target-related functionality at the 15-position to stabilize a carbanion at that carbon during assembly of the lower side chain.
(a) What is the structure of Corey’s cyclohexene intermediate in his synthesis of 7?
(b) Why does Corey choose an \(\ce{-NO2}\) group in his cyclohexene intermediate to serve as a precursor of the \(\ce{NHCHO}\) group in 7?
4. Two syntheses of the PGF2α precursor 8 were described as outlined in the following retrosynthetic analysis:
(a) Why was the 10 to 9 conversion not achieved enantioselectively?
(b) How was the 12 to 11 conversion accomplished enantioselectively?
(c) What stereocontrolling factor is responsible for the configuration of the epoxy group relative to the other stereocenters in 8 when this epoxide is prepared from the alkene 11?
5. Woodward’s PGF2α synthesis generates the key intermediate 13, that is similar to Corey’s lactone 3, by a ring contraction of 14.
(a) In an attempt at preparing the precursor 15 of 14 by intramolecular alkylation of enolate 16, the desired ketone 17 was not obtained. Why?
(b) Woodward achieved the synthesis of 15 from 18 by a multistep sequence that began with a polar process closely related to the 16 to 17 reaction. How did he accomplish the 18 to 15 conversion?
