Furfural disulfide

Any oxidisable substance, such as ethanol, methanol, glacial acetic acid, acetic anhydride, benzaldehyde. carbon disulfide, glycerol, ethylene glycol, ethyl acetate, methyl acetate and furfural. 

Methylfuran-3-thiol is responsible for a boiled note in milk, chicken and beef flavours and this chemical as well as its disulfide are widely used in flavours. Both molecules can be prepared by heating xylose and hydrogen sulfide in an autoclave (Scheme 13.13). Alternatively, these sulfur compounds can also be synthesised from 2-methylfuran made from furfural derived from oat hulls. 

Furfural comes from pentose sugars in cereal straws and brans. Furfural is the precursor of furfuryl mercaptan and its disulfide, difurfuryl disulfide, which are both important chemicals for coffee, meat and roasted flavours. They are prepared by the reaction of furfural and hydrogen sulfide (Scheme 13.15). 

Scheme 13.15 Formation of furfirrylthiol and its disulfide from furfural...

Furfuryl mercaptan cannot be prepared according to the classical method using furfuryl chloride and potassium sulfide.2 It has been prepared by reduction of 2-furfuryl disulfide, obtained from furfural and ammonium hydrosulfide.3 The mercaptan has also been obtained in 33% yield 2 by the reaction of furfuryl chloride with thiourea and subsequent decomposition of the intermediate S-2-furfurylisothiourea according to the general method described in Organic Syntheses.4 In the present method, which has been published previously, the use of the very unstable and difficultly available furfuryl halides is avoided.6... 

Shibamoto (30) formed a-thiophenecarboxaldehyde by reacting furfural and hydrogen disulfide. This means that there probably was exchange between the S and 0 in the furan ring during heating. 

Condensation product of furfural with tetramethyl diuram disulfide and polyethylenepolyamine... 

Properties Colorless or yellowish syrupy liquid odorless hygroscopic. Bp 122C (15 mm Hg), mp 18C, d 1.2. Miscible with water, alcohol, glycerol, and furfural insoluble in chloroform, petroleum ether, carbon disulfide. Cannot be distilled at atmospheric pressure without decomposition when concentrated above 50% it is partially converted to lactic anhydride. It has one asymmetric carbon and two enantiomorphic isomers. The commercial form is a racemic mixture. 

If the Hydrogen Sulfide and Furfural are allowed to react in a buffered ethanolic solution at -35°C, substantial yields of Difurfuryl Disulfide (FEMA 3146) are obtained. This is a powerful flavor material witii a roasted coffee and roasted meaty taste. It is widely used to obtain roasted notes in meat flavors (equation 8). 

Some of these reaction flavors occur in nature. The oxidation of mercaptans to form disulfides and other products is in part responsible for the loss of flavor from fresh roasted coffee only a short time after brewing. Furfiiryl Mercaptan is one of the most potent flavor components in coffee and is responsible for a fresh brewed coffee flavor. While it is present at only 1.55 ppm versus Furfuryl Alcohol (515 ppm) and Furfural (157 ppm), its flavor contribution is 310,014 times its flavor threshold (0.005 ppb). If there were a 100% conversion of Furfuryl Mercaptan to Diflirfuryl Disulfide, the coffee would appear to be 10 times weaker due to Difurfliryl Disulfide s flavor threshold being 0.05 ppb, or ten times weaker than Furfuryl Mercaptan (equation 10). The overall effect of this oxidation is that the coffee has lost its flavor. 

The synthesis of furan-2-methanethiol was described by Reichstein and Staudinger (1928). Furfural (1.63) is converted into furfuryl disulfide (difurfuryl disulfide, 1.136) by treatment with the sulfide or hydrosulfide of an alkaline-earth metal or of ammonia. The disulfide in an alcoholic solution can be reduced, for example, by addition of zinc dust and a small quantity of acetic acid. A method has been described by Kofod (1955). 

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