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Porantherine (molecule M) is the main alkaloid of the low woody shrub Poranthera corymbosa distributed in the southwest. The following questions pertain to the attached synthetic scheme (J. Am. Chem. Soc. 1974, 96, 6516). The are key steps that utilize an aldol-like mechanism to close three rings (A) What kind of transformation is the D→ E step; show NO mechanism, but what was added to D? Hint: molecule E clearly has a terminal alkene and ketone, but what is the third functional group (it's not another alkene)? You've done the mechanism for making this functional group in chapter 20. (B) The E F transformation is rather elegant and it is promoted by an acid (toluenesulfonic acid). It involves formation of an enol (tautomerization) followed by protonation on the C=C bond in the ring to form a resonance- stabilized carbocation. Show the mechanism of this cyclization starting from the enol (i.e. double bond between c and d). Recall your acid-catalyzed aldol mechanism in conference. The squiggly line in F means both configurational stereoisomers are formed and use H3O+ as the acid catalyst. (C) How do you think the H➜ I transformation was accomplished? Show no mechanism but what is added to H?

Porantherine (molecule M) is the main alkaloid of the low woody shrub Poranthera corymbosa distributed in the southwest. The following questions pertain to the attached synthetic scheme (J. Am. Chem. Soc. 1974, 96, 6516). The are key steps that utilize an aldol-like mechanism to close three rings (A) What kind of transformation is the D→ E step; show NO mechanism, but what was added to D? Hint: molecule E clearly has a terminal alkene and ketone, but what is the third functional group (it's not another alkene)? You've done the mechanism for making this functional group in chapter 20. (B) The E F transformation is rather elegant and it is promoted by an acid (toluenesulfonic acid). It involves formation of an enol (tautomerization) followed by protonation on the C=C bond in the ring to form a resonance- stabilized carbocation. Show the mechanism of this cyclization starting from the enol (i.e. double bond between c and d). Recall your acid-catalyzed aldol mechanism in conference. The squiggly line in F means both configurational stereoisomers are formed and use H3O+ as the acid catalyst. (C) How do you think the H➜ I transformation was accomplished? Show no mechanism but what is added to H?

Organic Chemistry
Organic Chemistry
8th Edition
ISBN:9781305580350
Author:William H. Brown, Brent L. Iverson, Eric Anslyn, Christopher S. Foote
Publisher:William H. Brown, Brent L. Iverson, Eric Anslyn, Christopher S. Foote
Chapter21: Benzene And The Concept Of Aromaticity
Section: Chapter Questions
Problem 21.58P
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1. You have the tools to accomplish one of the early synthesis of a major alkaloid. Porantherine (molecule M) is the main alkaloid of the low woody shrub Poranthera corymbosa distributed in the southwest. The following questions pertain to the attached synthetic scheme (J. Am. Chem. Soc. 1974, 96, 6516). The are key steps that utilize an aldol-like mechanism to close three rings (A) What kind of transformation is the D → E step; show NO mechanism, but what was added to D? Hint: molecule E clearly has a terminal alkene and ketone, but what is the third functional group (it's not another alkene)? You've done the mechanism for making this functional group in chapter 20. (B) The E F transformation is rather elegant and it is promoted by an acid (toluenesulfonic acid). It involves formation of an enol (tautomerization) followed by protonation on the C=C bond in the ring to form a resonance- stabilized carbocation. Show the mechanism of this cyclization starting from the enol (i.e. double bond between c and d). Recall your acid-catalyzed aldol mechanism in conference. The squiggly line in F means both configurational stereoisomers are formed and use H3O+ as the acid catalyst. (C) How do you think the H➜ I transformation was accomplished? Show no mechanism but what is added to H? (D) In the JK transformation, dilute HCl (10%) was used to first deprotect/ unmask the acetal and ketal. Show the mechanism (use H3O+) for formation of the northern cyclohexene ring starting from the aldehyde and secondary amine. No need to show the mechanism of the acetal / ketal deprotection to the respective aldehyde and ketone. (E) Finally, in the K → L transformation, acid (toluenesulfonic acid) was added. It proceeds in the same manner as question (B). Show the mechanism for the ring closure to form L.
Transcribed Image Text:1. You have the tools to accomplish one of the early synthesis of a major alkaloid. Porantherine (molecule M) is the main alkaloid of the low woody shrub Poranthera corymbosa distributed in the southwest. The following questions pertain to the attached synthetic scheme (J. Am. Chem. Soc. 1974, 96, 6516). The are key steps that utilize an aldol-like mechanism to close three rings (A) What kind of transformation is the D → E step; show NO mechanism, but what was added to D? Hint: molecule E clearly has a terminal alkene and ketone, but what is the third functional group (it's not another alkene)? You've done the mechanism for making this functional group in chapter 20. (B) The E F transformation is rather elegant and it is promoted by an acid (toluenesulfonic acid). It involves formation of an enol (tautomerization) followed by protonation on the C=C bond in the ring to form a resonance- stabilized carbocation. Show the mechanism of this cyclization starting from the enol (i.e. double bond between c and d). Recall your acid-catalyzed aldol mechanism in conference. The squiggly line in F means both configurational stereoisomers are formed and use H3O+ as the acid catalyst. (C) How do you think the H➜ I transformation was accomplished? Show no mechanism but what is added to H? (D) In the JK transformation, dilute HCl (10%) was used to first deprotect/ unmask the acetal and ketal. Show the mechanism (use H3O+) for formation of the northern cyclohexene ring starting from the aldehyde and secondary amine. No need to show the mechanism of the acetal / ketal deprotection to the respective aldehyde and ketone. (E) Finally, in the K → L transformation, acid (toluenesulfonic acid) was added. It proceeds in the same manner as question (B). Show the mechanism for the ring closure to form L.
CH3 CH3 CH3. CH3 N-H HN CH3 CH3. CH3 CH3- FO CH3 E MgCl CH3 HCO₂Et CH3-N- CH3 CH3. CHO—N CH3 CH3. H ·N CH3 b CH3 B H OH CH3 CHO—N- CH3 G H CH3 CH3. CH3 CHO—N CH3 CH3 H CH3 M Porantherine
Transcribed Image Text:CH3 CH3 CH3. CH3 N-H HN CH3 CH3. CH3 CH3- FO CH3 E MgCl CH3 HCO₂Et CH3-N- CH3 CH3. CHO—N CH3 CH3. H ·N CH3 b CH3 B H OH CH3 CHO—N- CH3 G H CH3 CH3. CH3 CHO—N CH3 CH3 H CH3 M Porantherine
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