Pyrans oxidative rearrangement

A peroxy acid mediated oxidative rearrangement of 2-alkoxy-3,4-dihydro-2//- pyrans affords 5-alkoxytetrahydrofuran-2-carbaldehydes (79JCS(Pi)847>. This reaction pathway was used in developing a method for the synthesis of optically active monoalkylfurans. (S)-2-Ethoxy-5-s-butyl-3,4-dihydro-2//-pyran (319), obtained through a cycloaddition reaction of (S)-2-s-butylacrolein to ethyl vinyl ether, was converted to (S)-2-s-butyl-5-ethoxytetrahydrofuran-2-carbaldehyde (320) (Scheme 85). 

An alternative approach to the oxidative rearrangement uses chlorine as the oxidant in a one-pot synthesis (80JOC1109). Not only does this reagent convert the furan to the pyran-3-one, but it takes the latter to the 4-halogeno derivative via the labile 3,4-dihalogeno compound. The pyran-4-one is obtained by an in situ acid-catalyzed hydrolysis. The overall yield of maltol is over 60%, and ethylmaltol is obtained in a similar conversion from ethylfurfuryl alcohol. 

Oxidative rearrangement of furyl carbinols to 6-hydroxy-2/f-pyran-3(6//)-ones, a useful synthon for the preparation of a variety of heterocyclic compounds. A review , Georgiadis, M. R, Albizati, K. F. and Georgiadis, T. M., Org. Prep. Proc. Int., 1992, 24, 95. 

Mercuric salts induce oxidative rearrangement of cyclic alkenes and give cycloalkane carboxaldehydes. Cyclohexene, for example, gave cyclopentane carboxaldehyde (404) in 53% yield. Acyclic alkenes such as 2-butene can also be oxidized with mercuric sulfate and sulfuric acid, giving 2-butanone in this case.566 Thallium nitrate [T1(N03)3] is an important reagent for the oxidative rearrangement of cyclic alkenes.567 The reaction is not restricted to carbocyclic compounds, but can also be applied to heterocyclic compounds as shown by the oxidation of 3,4-dihydro-277-pyran (405) to the dimethyl acetal (406) in 65% yield.568... 

In particular, the key intermediate product 381 in the synthesis of wighteone was prepared by the oxidative rearrangement of 3 -iodotetraalkoxychalcone 380 [474] and the key step in the synthesis of ( )-indatraline involved the HTIB-promoted diastereoselective ring contraction of a 1,2-dihydronaphthalene 382 to construct the indane ring system 383 (Scheme 3.152) [475]. A similar oxidative rearrangement of 3-cinnamoyl-4-hydroxy-6-methyl-2/7-pyran-2-ones with HTIB in dichloromethane followed by cycliza-tion was used by Prakash and coworkers for the direct conversion of o-hydroxychalcones into isoflavone derivatives [476]. 

The bromination and oxidation of securinine have been reported (Scheme 87). Oxidative difunctionalization of 2-amino-4//-pyrans occurs with iodobenzene diacetate and A -chlorosuccinimide reagents. An oxidative rearrangement of [7]helicene by heating with iron(n) chloride and aluminium(III) chloride involves skeletal rearrangement with the loss of helicity (Scheme 88). ... 

Furfural — see Furan-2-oarbaldehyde, 532 Furfuryl acetate, o -(butoxycarbonyl)-anodic oxidation, 1, 424 Furfuryi acrylate polymerization, 1, 279 Furfuryl alcohol configuration, 4, 544 2-Furfuryl alcohol polyoondensation, 1, 278 reactions, 4, 70-71 Furfuryl alcohol, dihydro-pyran-4-one synthesis from, 3, 815 Furfuryl alcohol, tetrahydro-polymers, 1, 276 rearrangement, 3, 773 Furfuryl chloride reactions... 

Thiopyranones containing an alkyne group, upon treatment with OT-chloroperbenzoic acid (MCPBA), undergo a sulfoxide rearrangement to afford thieno[3,2-f]pyrans (Equation 63). In some cases, where the aryl group is substituted with a chlorine atom in the 2-position, or disubstituted with chlorine atoms in both 2- and 4-positions, oxidized products, 130, were formed <2002T10309>. 

When muscat juices were heated under similar conditions a significant enhancement in concentration of volatile monoterpenes was observed. With the notable exceptions of a-terpineol, llnalool, nerol, geraniol and the pyran ring llnalool oxides most of the heat-induced terpenoids of the juices could be attributed to rearrangement products of grape polyols (18). 

The oxidation of furyl-2-carbinols can prodnce 6-hydroxy-2//-pyran-3(6//)-ones (the Achmatowicz rearrangement), which have several synthetic nses, the most important in the heterocyclic context being for the formation therefrom of pyrylinm-3-olate species (11.1.7). The oxidation can be condncted with meto-chloroperbenzoic acid, vanadinm(lll) acetylacetonate with f-butyl peroxide, or with singlet oxygen (18.7). 

---