Pyran/furan isomers

Gas chromatographic analysis allows the determination of the monosaccharides present by comparing the retention times with those of reference compounds. It is noteworthy to mention that the chromatographic profile is quite simple, because the deuterated compounds are not separated from the non-deuterated, even if a small difference in retention time can be evidenced by comparison of specific ion chromatograms. Each monosaccharide can give a maximum of four peaks, corresponding to the cd(3 and the pyran/furan isomers. 

Zirconocene-catalyzed kinetic resolution of dihydrofurans is also possible, as illustrated in Scheme 6.8 [18]. Unlike their six-membered ring counterparts, both of the heterocycle enantiomers react readily, albeit through distinctly different reaction pathways, to afford — with high diastereomeric and enantiomeric purities — constitutional isomers that are readily separable (the first example of parallel kinetic resolution involving an organome-tallic agent). A plausible reason for the difference in the reactivity pattern of pyrans and furans is that, in the latter class of compounds, both olefmic carbons are adjacent to a C—O bond C—Zr bond formation can take place at either end of the C—C 7T-system. The furan substrate and the (ebthi)Zr-alkene complex (R)-3 interact such that unfavorable... 

The reversal of the well-known transformation of sugars into pyrans has been detailed as a method for assembling simple monosaccharides from simple furans (71T1973). A compound of the 2-furylcarbinol type was converted by the Br2/MeOH procedure into a mixture of the cis and trans isomers of the corresponding 2,5-dimethoxy-2,5-dihydrofuran derivative (129). Mild acid hydrolysis of (129) resulted in cleavage of the acetal bonds with formation of the dicarbonyl compound (130) which underwent immediate cyclization to 2,3-dideoxy-DL-alk-2-enopyranos-4-ulose (131 Scheme 29). 

Six furopyran structures are listed in Table 7. Each of the isomers contains a six-membered pyran ring attached to a five-membered furan ring. Not many studies have been carried out on these furopyran compounds. 

Fig. 4.5 Extent of hydrolysis of glycoside of different volatile compounds during MLF with four strains of malolactic bacteria (MLB). Values are calculated as a percentage ratio between the concentration of glycosides in MLF samples and in a non-MLF control. Sum of 1-hexanol, trans- and CM-3-hexenol, trans- and c/i-2-hexenol sum of isoamyl alcohols, heptanol, and 4-hydroxy-4-methyl-2-pentanol sum of benzyl alcohol and 2-phenylethanol sum of vanillin and benzaldehyde sum of 4-vinylphenol and 4-vinylguaiacol sum of Unalool and a-terpineol sum of nerol and geraniol sum of cis- and rrani-linalool oxides (pyranic and furanic) sum of 3,7-dimethyl-l,5-octadien-3,7-diol and the two 2,7-dimethyl-2,7-octadien-l,6-diol isomers (from Ugliano and Moio 2006, reproduced with permission)...

Dehydration Reactions. Detailed analysis of the pyrolysis tar as discussed previously (Figure 12 and Scheme 3) shows the presence of levoglucosan, its furanose isomer (1,6-anhydro-p-D-glucofuranose) and their transglycosylation products as the main components. In addition to these compounds, the pyrolyzate contains minor amounts of a variety of products formed from dehydration of the glucose units. The dehydration products detected include 3-deoxy-o-erythrohexo-sulose, 5-hydroxymethyl-2-furaldehyde, 2-furaldehyde (furfural), other furan derivatives, levoglucosenone (l,6-anhydro-3,4-dideoxy-P-D-glycerohex-3-enopyranos-2-ulose), l,5-anhydro-4-deoxy-D-hex-l-ene-3-ulose, and other pyran derivatives. The dehydration products are important as intermediate compounds in char formation. 

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... 

The reactivity was similar in the reactions of benzonitrile oxide (107) with furan and pyran e%o-glycals 101 and 104 and their Z-isomers. These cydoaddition reactions proceed at room temperature and give open-chain isoxazoles, for example 108, because of facile yS-elimination of the sugar ring oxygen on the intermediate isoxazoline ring system (Scheme 12.45, as a representative example of cydoaddi-tions to furan exo-glycals). 

Butenolide 49 can be treated with TBSOTf which results in pyran cleavage and formation of butenolide 52 as a mixture of geometric isomers (Scheme 11). Alternatively, reduction of the lactone carbonyl with DIB AL and dehydration affords C-glycosyl furan 53(20). 

Epoxidation of linalool occurs at the more substituted double bond. Ring closure of this epoxide gives a mixture of the cis [5989-33-3] and trans [34995-77-2] isomers of the furan (131), and also the cis and trans isomers of the pyran (133) [14049-11-7] as shown in Fig. 8.27. This mixture is known as linalool oxide, and sometimes erroneously as epox-ylinalool or epoxydihydrolinalool. Linalool oxide has a sweet woody, floral powerful, sweet, and penetrating odor with earthy undertones. It is used in perfumes and in essential oil reconstitutions in which it adds a natural note to linalool. 

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