Artificial urushi

As "japan" implies the meaning of "a lacquer or varnish giving a hard, glossy finish" and/or "objects decorated and lacquered in the Japanese style,"205 urushi wares are regarded as one of the most typical symbols of Japanese art. Oriental lacquer (urushi) of Japan and China is a natural resinous sap of the Rhus vernicifera tree. Urushi coating is hard enough to give a brilliant polish and is highly durable (for more than a thousand years under appropriate conditions) and solvent-resistant in comparison with the synthetic coatings. Majima's pioneering work in the early days of the 20th century revealed that main components of urushi are "urushiols," whose structure is a catechol derivative with unsaturated hydrocarbon chains consisting of a mixture of monoenes, dienes, and trienes at 3- or 4-position of catechol.206 Crosslinking of the urushiol is supposed to be accomplished mainly by a laccase-catalyzed oxidative coupling of the phenol moiety of the urushiol and a subsequent autoxidation of unsaturated alkyl chains in air. Urushi can be regarded as the only example of practical natural paints utilizing in vitro enzymatic catalysis for hardening. Urushi is curable under air at room temperature without organic solvents, and hence, urushi seems very desirable for coating system from the environmental standpoint. However, modeling study of urushi has been limited, mainly due to the difficulty in preparation of the urushiol.

The reason why synthesis of natural urushiols involves multistep, tedious procedures is that the reactive unsaturated group cannot be directly introduced on the catechol moiety; protection and deprotection of the catechol moiety are

"OH

+ rcooh

"OH

+ rcooh

-h2O

Lipase

02cr ch2

Lipase

Lacease

02cr ch2

"OH

Lacease

'Artificial Urushi'

"OH

Scheme 28

Scheme 28

required. We designed new urushiol analogues, in which the unsaturated group is connected with the phenolic group through an ester linkage.207-210 The analogs were synthesized by a lipase-catalyzed esterification of phenols having a primary alcohol with unsaturated fatty acids of different numbers of double bonds (Scheme 28). The primary aliphatic hydroxy group in the phenolic precursors was regioselectively acylated by lipase BS to produce oily urushiol analogs. In this new approach, the analogs were obtained by one or two reaction steps via facile procedures from commercially available reagents. Furthermore, it is to be noted that the urushiol analogs showed no dermatitis-causing activity.

The curing was performed using laccase (PCL) as catalyst in the presence of acetone powder, a third component of the sap acting as emulsifier of oily urushiol and aqueous laccase solution. The urushiol analogs were crosslinked under mild reaction conditions without use of organic solvents to produce the polymeric film with high gloss surface and good elastic properties.

Cardanol, a main component obtained by thermal treatment of cashew nut shell liquid (CNSL), is a phenol derivative having mainly the meta substituent of a C15 unsaturated hydrocarbon chain with one to three double bonds as the major. Since CNSL is nearly one-third of the total nut weight, a great amount of CNSL is obtained as byproducts from mechanical processes for the edible use of the cashew kernel. Only a small part of cardanol obtained in the production of cashew kernel is used in industrial fields, though it has various potential industrial utilizations such as resins, friction-lining materials, and surface coatings. Therefore, development of new applications for cardanol is very attractive.

A new crosslinkable polymer was synthesized by the SBP-catalyzed polymerization of cardanol.211 When HRP was used as catalyst for the cardanol polymerization, the reaction took place in the presence of a redox mediator (phe-nothiazine derivative) to give the polymer.212 Fe-salen efficiently catalyzed the polymerization of cardanol in organic solvents (Scheme 29).213214 The polymerization proceeded in 1,4-dioxane to give the soluble polymer with molecular weight of several thousands in good yields. The curing of the polymer took place in the presence of cobalt naphthenate catalyst at room temperature or thermal treatment (150°C for 30 min) to form yellowish transparent films ("artificial urushi"

Fe-Salen or peroxidase/H202

Co or A

Crosslinked polymer ("Artificial Urushi")

Crosslinked polymer ("Artificial Urushi")

Scheme 29

Scheme 29

in a broad sense). The resulting crosslinked film exhibited good elastic properties comparable with natural urushi. FT-IR monitoring of the curing showed that the crosslinking mechanism is similar to that of the oil autoxidation.

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