Japanese lacquer (Urushi in Japanese) is a natural paint that has been used since ancient times and — Organic Chemistry Chemistry Question
Structure studies of urushiol
Japanese lacquer (Urushi in Japanese) is a natural paint that has been used since ancient times and is made from the sap of the lacquer tree. Japanese lacquer is glossy and beautiful and is used for lacquer ware and traditional craft objects. The major component of the sap of the lacquer tree is urushiol. Oxidation and polymerization of urushiol catalyzed by the enzyme (laccase) contained in a lacquer tree solidify the sap of the lacquer.
The chemical structure of urushiol has been studied from the beginning of 20th century and was determined by Professor Rikou Majima in 1918. Urushiol was a mixture of compounds with similar chemical structures, and even determination of its molecular formula is very difficult because it was very unstable substance that polymerized easily. Since destructive distillation of urushiol gave a mixture of catechol (1) and unsaturated hydrocarbons, urushiol seemed to be catechol derivatives with unsaturated alkyl chains. In addition, treatment of urushiol with dimethyl sulfate afforded dimethylurushiol. Furthermore, catalytic hydrogenation of urushiol under ordinary hydrogen pressure just developed by Willstätter at that time provided pure hydrourushiol as crystals, and the molecular formula of hydrourushiol was determined as C21H36O2. This advanced structure studies of urushiol greatly.
[VISUAL]
If the saturated alkyl chain of urushiol is one kind and is not branched, there are two possible structures for hydrourushiol. Draw each structure. Show the alkyl group such as (CH2)n-CH3.
Model Answer
Since hydrourushiol has the molecular formula C21H36O2 and is a catechol derivative, it consists of a benzene ring with two adjacent hydroxyl groups (catechol core) and a saturated 15-carbon side chain (pentadecyl group, -CH2(CH2)13CH3 or -(CH2)14CH3). If the side chain is unbranched, there are two possible positional isomers:
1. 3-pentadecylcatechol (1,2-dihydroxy-3-pentadecylbenzene):
[VISUAL]
2. 4-pentadecylcatechol (1,2-dihydroxy-4-pentadecylbenzene):
[VISUAL]
To determine the structure of hydrourushiol, the following two syntheses (I) and (II) were performed. Among synthesized compounds B and D, compound D was identical to dimethylhydrourushiol derived from natural urushiol. Thus, the location of the unsaturated alkyl group of urushiol was determined. Draw the structure of the synthetic intermediates A and C.
[VISUAL]
Model Answer
In synthesis (I), Friedel-Crafts acylation of 1,2-dimethoxybenzene with CH3(CH2)13COCl in the presence of AlCl3 occurs at the 4-position to yield the intermediate ketone A, which is then reduced via Clemmensen reduction to yield compound B (4-pentadecyl-1,2-dimethoxybenzene).
In synthesis (II), acylation of 1,2-dimethoxybenzene derivative with (CH2)2COCl side chain by sodium acetylide CH3(CH2)9C≡CNa yields the alkynone intermediate C. Subsequent Clemmensen reduction and catalytic hydrogenation yield compound D, which is identical to dimethylhydrourushiol.
Structures of the intermediates:
- A: 1-(3,4-dimethoxyphenyl)pentadecan-1-one (or 4-tetradecanoyl-1,2-dimethoxybenzene)
[VISUAL] (structure shows a 3,4-dimethoxybenzene ring with a -CO(CH2)13CH3 substituent at the 4-position)
- C: 1-(3,4-dimethoxyphenyl)pentadec-4-yne
[VISUAL] (structure shows a 3,4-dimethoxybenzene ring with a -(CH2)2C≡C(CH2)9CH3 substituent at the 4-position)
To determine a number and a position of the double bonds of the unsaturated alkyl chain of urushiol, ozonolysis of dimethylurushiol was carried out. Because urushiol was a mixture, various carbonyl compounds were provided by this experiment (question 4, see below). However, carbonyl compounds with three carbons (OHC-CH2-COOH and HOOC-CH2-COOH) are not detected by this experiment. This is because both carbonyl compounds decompose as in the following equations. Show the chemical formula of E, F, and G.
OHC-CH2-COOH → E + F
HOOC-CH2-COOH → G + F
Model Answer
These three-carbon compounds readily undergo thermal decarboxylation:
- OHC-CH2-COOH decarboxylates to give acetaldehyde (E) and carbon dioxide (F).
- HOOC-CH2-COOH (malonic acid) decarboxylates to give acetic acid (G) and carbon dioxide (F).
Chemical formulas:
- E: CH3CHO
- F: CO2
- G: CH3COOH
Carbonyl compounds obtained by ozonolysis of dimethylurushiol are shown below. You may assume that all of the double bonds and benzene rings were cleaved. Based on the experiment results, draw three structures of urushiol among possible urushiols. Show the unsaturated alkyl chain to determine the position of the double bond as in (CH2)nCH=CH(CH2)nCH3. You do not need to consider the geometry of the double bond (cis or trans).
[VISUAL]
Products:
dimethylurushiol + O3 → CH3CHO, CH3(CH2)5CHO, CH3(CH2)5COOH, HOOC(CH2)7COOH, HCHO, HCOOH, CH2(COOH)2, (COOH)2, CO2, HOOC(CH2)4COOH
Model Answer
Based on the ozonolysis cleavage products and the fact that the side chain is attached at the 4-position, we can reconstruct the double-bond positions. Representative structures of possible urushiols (showing the unsaturated chains attached to the 4-position of catechol) include:
1. 4-[(8Z,13Z)-pentadeca-8,13-dien-1-yl]benzene-1,2-diol:
- Structure: Catechol ring with a -(CH2)7CH=CH(CH2)4CH=CH2 chain at the 4-position.
2. 4-[(8Z,11Z,13E)-pentadeca-8,11,13-trien-1-yl]benzene-1,2-diol:
- Structure: Catechol ring with a -(CH2)7CH=CHCH2CH=CHCH=CHCH3 chain at the 4-position.
3. 4-[(8Z,11Z,14)-pentadeca-8,11,14-trien-1-yl]benzene-1,2-diol:
- Structure: Catechol ring with a -(CH2)7CH=CHCH2CH=CHCH2CH=CH2 chain at the 4-position.
4. 4-[(8Z,10E,13Z)-pentadeca-8,10,13-trien-1-yl]benzene-1,2-diol:
- Structure: Catechol ring with a -(CH2)7CH=CHCH=CHCH2CH=CHCH3 chain at the 4-position.