Furans and Pyrans

As with many other furans, it is formed simply by heating glucose (Heyns et al., 1966a Fagerson, 1969), but many other complex pathways can explain its presence in roasted coffee. 

It is characterized by a peculiar spicy-smokey, slightly cinnamon-like odor (Arctander, 1967). It has a threshold value of 4.5 ppm in water (Mulders, 1973b). 

It was identified in the products of thermal degradation of glucose (Heyns et al., 1966a Walter and Fagerson, 1968), in the model reaction cysteine xylose (Ledl and Severin, 1973), and in model reactions of serine and threonine with sucrose (as well as in coffee) by Baltes and Bochmann 

The odor is ethereal, sickly. The odor threshold in water is 3.5 ppm (Mulders, 1973b) and 27 ppm in a bland edible (cottonseed) oil (Evans et al., 1971). 

It is formed by thermal degradation of glucose (Heyns et al, 1966a Fagerson, 1969). 

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The 3,4-di-O-benzylated D-fructopyran derivative (Scheme 26) would limit the possibilities for spiro-furan and -pyran derivatives. Application of the usual two-step process afforded with a 60% overall yield a mixture of OZTs in which the spiro-furan forms predominated (2 1 ratio) over the spiro-pyran forms. After column chromatography separation, acetylation allowed isolation of each epimer either a-furan (37%) and (3-furan (35%) forms as well as a-pyran (41%) and (3-pyran (27%) forms. 

Numerous furan and pyran derivatives, many of which originate from heat treatment of carbohydrates, largely determine the odor of processed foods. Of this group, 2,5-dimethyl-4-hydroxy-277-furan-3-one and maltols are used in fairly large quantities in flavors. The following compounds are used in relatively small amounts in flavor compositions ... 

Related substituted bicyclic ketones of both furan- and pyran-fused derivatives were prepared in an analogous fashion in 60 90% yield.8... 

The earliest studies centered on the cyclization of oi-hydroxyalkenes to furan and pyran ring systems (equations 4 and 5).10 The regiochemistry (attack at the more substituted position) was that expected and the yields were only modest. However, modem advances in catalysis should permit much more efficient processes and many older systems probably warrant re-examination. 

The terms furanose and pyranose arise from attachment of carbohydrate endings to the names of the cyclic compounds, furan and pyran. 

Furlan Mendes AN, da Rosa RG, Gregorio JR (2005) Furans and pyrans derivatives from the hydroformylation of ethyl ricinoleate and castor oil. Catal Commun 6(6) 379-384. doi 10.1016/j. catcom.2005.03.001... 

Ti, Al-MCM-41 Ti-BEA Linalool Furan and pyran hydroxy esters Epoxidation + ring closure 80 >99 MeCN solvent TBHP/linalool = 1.1 1 bar 353 K 24 h glass batch reactor [24]... 

Table 1 gives an up-to-date compilation of selected natural products in the lapachol, furan and pyran-naphthoquinones classes, together with their occurrence and the biological activities investigated for each. 

Oxacycles of the furan- and pyran-type from unstabilized oo-oxacarbonyl ylides... 

Cyclization of hydroxyalkenes to furanes and pyranes Alkenes suitably substituted by a hydroxyl group undergo cyclization and carbonylation in the presence of PdCl,/ CuCI to pyranes and/or furanes. The preference for five- or six-membered rings depends mainly on the geometry of the alkenc (E)-alkcnes cyclize mainly to pyranes and (Z)-alkenes cyclize mainly to furanes. 

A similar synthesis of 3 alken-l-ols from 2,3 dichlorotetrahydrofuran has been devised, and the stereochemical relationships of both syntheses have been investigated. Many cleavages of furan and pyran rings have been reviewed. ... 

The addition of [2- " C] but-2-ene to reacting n-butane + oxygen mixtures at 315 °C [56] showed that the reverse isomerization reaction (— 14a) does not occur to any appreciable extent, since the 2-methyloxetan found in the products was inactive. It can be safely concluded, therefore, that the formation of derivatives of oxetans, furans and pyrans is diagnostic of alkylperoxy radical isomerization and subsequent decomposition, reactions (14) + (16). Table 1 thus presents a considerable volume of evidence for the wide occurrence of this chain-propagation step. 

Corma et aL [239] took advantage of both redox and acidic properties of Ti/Al-MCM-41 (Si/Ti = 68 and Si/Al = 196) aluminosOicate to carry out the multistep oxidation of linalool to cyclic furan and pyran hydroxy ethers (Scheme 6). 

The opening of halocyclopropanes to allyl systems according to equation 124 can happen thermally or with the assistance of electrophiles and nucleophiles . Some recent examples include an efficient cyclopentenone synthesis (equation 125) ", an electrocyclic opening/cyclization sequence giving functionalized furan and pyran derivatives (equation 126), an elegant total synthesis of the very fast death factor alkaloid ( ) anatoxin and a nice application of the well known nucleophilic opening to the preparation of crystalline methylene aziridines (equation 127) . 

The free volatiles consist mainly of linalool, geraniol, nerol, furan and pyran forms of the linalool oxides (17, 18, 19a, 20a respectively), a-terpineol, hotrienol 3 and citronellol 4a, which are also the major terpene aroma compounds of the fruit. 

Of a series of tetrahydro-furan- and -pyran-fused pregnanes (211—215) only... 

Heterocycles such as furan and pyran derivatives can easily be constructed from Knoevenagel products either after double bond isomerization or functionalization at the y- or 8-position. Hence, acid-catalyzed ring closure of (287) affords the 4,S-dihydrofurans (289), presumably via the double bond isomer (288 Scheme 56). ° Knoevenagel condensation of acrolein and malonic acid in the presence of... 

Heterocyclization on alkylation of phenols with terpenoids leading to partially hydrogenated polycyclic fused furans and pyrans 00KPS198. 

E Acids and anhydrides F Esters G Lactones H Phenols I Furans and pyrans J Thiophenes K Pyrroles L Oxazoles M Thiazoles N Pyridines O Pyrazines... 

Other compounds containing alcohol function(s) are present In section 5.D (ketones), 5.E (acids and anhydrides), 5.F (esters), 5.G (lactones), 5.1 (furans and pyrans), 5.J (thiophenes), 5.K (pyrroles), 5.N (pyridines) and 5.Q (sulfur compounds). ... 

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