2.3- Dihydro-l,4-dioxins

Most partially saturated ring systems, 2,3-dihydro-l,4-dioxin 10 (sometimes named as 1,4-dioxene), 2,3-dihydro-1,4-dithiin 11, 2,3-dihydro-l,4-oxathiin 12, 2,3-dihydro-l,4-benzodioxin or 1,4-benzodioxane 13, 2,3-dihydro-l,4-benzodithiin 14, and 2,3-dihydro-l,4-benzoxathiin 15 are well investigated. Ring numbering for compounds 10-12 is followed as shown, independently of the presence of substituents. 

Another approach to substituted 2,3-dihydro-l,4-dioxins 239 involves the reaction between 1,2-diols 238 and rhodium carbenoids generated from a-diazo-/ -ketoester 237 (Scheme 23) <1999H(51)1073>. This method complements the intramolecular reactions described earlier <1997JOC3902>. 

Functionalised 2,3-dihydro-l,4-dioxins can be synthesised in a three step-sequence from P-keto esters. The key step is the insertion of a Rh-carbenoid derived from an a-diazo-p-keto ester into an 0-H bond of a 13-diol <99H(51)1073>. The reaction of 2-(l,4-dioxenyl)alkanols with silyl enol ethers yields 23-disubstituted 1,4-dioxanes. When 13-bis(trimethylsilyloxy)-cyclobut-l-ene is used, the expected cyclobutanone products are accompanied by a spirocyclopropane derivative <99TL863>. 1,4-Dioxane-monochloroborane 57 is a highly reactive hydroborating reagent <990L315>. 

Benzil undergoes a cycloaddition reaction with stilbene and 1,1-diphenyl-ethylene to form adducts containing the 2,3-dihydro-[l,4]-dioxin ring system and with visnagin to form an oxetane. These reactions and the cycloaddition reactions of the analogous o-quinones are reviewed by Schonberg.87... 

The identical vicinal coupling constants for 1,4-benzodioxin and 2,3-dihydro-l,4-dioxin, 3.73 Hz, have been cited as evidence to confirm the absence of aromatic character in the former (67AJC1773), and by the same reasoning (22) should also be non-aromatic (70ZC296). The corollary of this argument, however, is that the dissimilar 3/ values for 1,4-dithiin, 6.97 Hz, and its dihydro analogue, 10.02 Hz, indicate substantial aromatic character in the former. However, it has been pointed out that this simple type of comparison fails to allow... 

The double bonds in 2,3-dihydro-l,4-dioxin, 2,3-dihydro-l,4-oxathiin and 2,3-dihydro-1,4-dithiin undergo standard electrophilic addition reactions. Under acid catalysis, methanol adds to 2,3-dihydro-l,4-oxathiin to give 2-methoxy-1,4-oxathiane (66HC(21-2)842). Various examples are available of reactions of the double bonds with carbenoids to give bicyclo[4.1.0]diheteroheptanes (77LA910,78ZC15), and with alkenes in [2 + 2] cycloadditions (78CB3624). 

A suspension ofZn dust (39.4 g, 0.60 mol) in dry DMF (500 mL) was stirred under N2 while about one-third of 2,3,5,6-tetrachloro-2,3-difluoro-2,3-dihydro-l,4-dioxin (78.0 g. 0.30 mol) was added dropwise. The entire system, including a still head, was evacuated at 120 Torr with a N2 bleed-in to maintain the inert atmosphere, and heated slowly to reflux (98 C). As the reaction proceeded, a product was taken off slowly (bp 69-72 C/ 120 Torr), whilst dropwise addition of the starting material was completed. Distillation was continued at bp 91 C/120 Torr. The distillate was washed with H20 (100mL) in a N2 atmosphere, dried (CaS04), filtered, and fractionated under N2 to afford the product yield 20.9 g (37%) bp 42-43 C/55 Torr. 

Perfluoro(2,3 4,5-diepoxy-3,4-dimethylhexane) (14) undergoes a novel isomerization at 200CC to give perfluoro(2,3,5,6-tetramethyl-2,3-dihydro-l,4-dioxin) (15).49... 

Further examples of dehydrofluorinations of heterocyclic compounds can be found in refs 113 (formation of fluorinated dihydrofuran derivatives), 114 [formation of 3-fluoro-2,2-bis(tri-fluoromethyl)-2//-oxete], and 115 (formation of fluorinated 2,3-dihydro-l, 4-dioxin derivatives). 

Diphenyl-2,3-dihydro-l,4-dioxin reacts with chromium(VI) or molybdenum(VI) oxide diperoxides in chloroform in the absence of oxygen, affording ethane-1,2-diol dibenzoate and benzil as reaction products <88TL3145>. 

Vinyl-2,3-dihydro-l,4-dioxin (150 R = = H) was also prepared by Wittig alkenylation... 

The dechlorination of 2,3,5,6-tetrachloro-2,3-difluoro-2,3-dihydro-l,4-dioxin with zinc in dimethylformamide afforded 2,3-dichloro-5,6-difluoro-1,4-dioxin as a distillate contamination with dimethylformamide <91J0C3915>. In the oxathiin series, sulfur dichloride added to vinylic ethers gave 2,6-dichloro-l,4-oxathianes (197). Dehydrohalogenation of these products by potassium t-butoxide gives the fully unsaturated compounds 1,4-oxathiin and 3,5-dimethyl-l,4-oxathiin (198) in moderate to reasonable yields <80RTC12>. 

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