Dihydrofurans, pyrolysis

Via Elimination of A kohol or W alter from Dihydrofurans Pyrolysis of 2,5-dialkoxy- or -diacetoxy-2,5-dihydrofurans results in a smooth reaction with the formation of 2-substituted furans.39 40... 

The original preparation of y-crotonolactone by Lespieau involved a five-step sequence from epichlorohydrin and sodium cyanide. A recent detailed study of this procedure reported an overall yield of 25% for the lactone. Glattfeld used a shorter route from glycerol chlorohydrin and sodium cyanide hydrolysis and distillation of the intermediate dihydroxy acid yielded y-cro-tonolactone in 23% yield and -hydroxy-y-butyrolactone in 28% yield. The formation of y-crotonolactone in 15% yield has also been reported from pyrolysis of 2,5-diacetoxy-2,5-dihydrofuran at 480-500 . ... 

Hudlicky and Barbieri (178) developed a simple two-step strategy to generate vinyl oxiranes, and then rearrange the vinyl oxirane to form functionalized dihydrofurans in a formal [2- -3]-annulation process. Both pyrolysis and ring opening with TMSI have been used to initiate the rearrangement process (Scheme 4.90). 

Furanmethyl benzoate (381) on flash vacuum pyrolysis affords the cyclobutenone (382 40%). Deuterium labelling established the mechanism shown in Scheme 104. The 5-methyl analogue (383), however, gives the dihydrofuran (30) (74JOC1448). 

We studied the Diels-Alder reaction of levoglucosenone (2) with the very reactive diene 2,3-dimethylene-2,3-dihydrofuran (3),10 the fiiran-based orf/io-quinodimethane, which is readily available by the Flash Vacuum Pyrolysis (FVP) of ester 4.16 Diene 3 dimerizes very rapidly16... 

The control of furan versus oxepin manifolds has been addressed recently. Vinyloxiranes of type (139) yielded oxepins (140) at lower temperatures, whereas higher temperatures gave dihydrofurans (141 Scheme 29), indicating that the Cope-type rearrangement of cis-divinyloxirane may be controll by precisely defining the temperature profile of the flash vacuum pyrolysis. 

A [2 + 3] dihydrofuran aimulation methodology was recently developed based on this rearrangement. Divinyloxiranes (142) were generated by the stereospecific addition of the lithium dienolate of ethyl a-bromocrotonate to al hydes, and the subsequent pyrolysis to dihydrofurans (143 Scheme 30) occurred in excellent isolated yields (except for R = Bu", Pr ). ... 

One other example of a pyrogram of a copolymer is that for poly(propylene-gra/f-maIeic anhydride) or poIypropylene-gra/f-poly[3-(1-methyl-1-propenyl)-3,4-dihydrofuran-2,5-dione, CAS 107001-49-0. The polypropylene polymer was isotactic and had a low content of maleic anhydride (MA) and a M = 9,100. The pyrolysis was performed in conditions similar to other examples, at 600° C in He with separation of a Carbowax column. The results are shown in Figure 6.1.24. 

Alkyl-substituted methyl 2-(trimethylsiloxy)cyclopropanecarboxylates 24 undergo the same type of reaction when reacted with aldehydes or ketones in the presence of titanium(IV) chloride.The homoaldol adducts 25 were formed with high stereoselectivity and could subsequently be transformed to the highly substituted dihydrofuran derivatives 27 by treatment with boron trifluoride and/or by pyrolysis. 

Ohloff and Giersch [53] accomplished the synthesis of the drimanediol (41) from the norambreinolide (69). The latter was reduced into the semiacetal (74), whose acetate (75) on pyrolysis gave the dihydrofuran (76). Its ozonolysis and subsequent reduction of the ozonolysis products with NaBFU afforded the diol (41). Unfortunately, the yields of the products in [53] are not given. As it was described above, the hydroxy acetate (42) was transformed into the drimenol (2) [39]. It should be mentioned that the reverse conversion of the drimenol (2) into the diol (41) [54] also takes place. For this purpose, the drimenyl acetate (12) was... 

A recent synthesis of this building block has been published by Eastham et al. in 2006 (48). Their key step is a Dotz benzannulation reaction and is shown in Scheme 2.10. The bromohydrin 66 was formed from dihydrofuran (50). Cobalt-mediated cyclization, followed by ozonolysis with reductive work-up yielded 68 after hydrazine formation. Reductive removal of the hydrazine function, followed by chromium-carbonyl formation gave the Dotz reaction precursor 69. This reacted with an alkyne in the Dotz reaction, and was then oxidized and hydrogenated 70). Pyrolysis gave the protected alcohol and the remaining free alcohol was protected as a triflate (—> 71). Reductive removal of the triflate and deprotection of the silyl ether yielded the desired 33 in 1.2% overall yield. 

R = D) has now been shown to be formed. Hence, the carbene mechanism is ruled out, and a proposed alternative involving two sigmatropic rearrangements followed by elimination of benzoic acid (Scheme 30) finds support in the formation of 2,5-bismethylene-2,5-dihydrofuran on pyrolysis of (329). ... 

The carbomethoxypivaloylketene 467 was formed as a neat hquid by decarbonylation upon flash vacuum pyrolysis of 2,3-dihydrofuran-2,3-dione 466, and [4 + 2] cycloaddition took place across the C=N bond of the diphenyl-Af-(4-methylphenyl)imine involving reaction of the less hindered but normally less reactive ester carbonyl, giving raise to 468 (Scheme 145) (2001T6757, 1997M381). 

To test for the intermediacy of free carbenes in the decomposition of metal-carbene complexes, the thermolysis of the 2-oxacyclopentylidene complex XVII was studied (Anderson and Casey, 1975). 2-Oxacyclopentylidene has been generated by pyrolysis of the sodium salt of butyrolactone tosylhy-drazone and shown to rearrange to give both dihydrofuran and cyclobutanone (Agosta and Foster, 1972). The thermolysis of XVII in decalin at 180° for 15... 

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