1.3- Dioxane, ring contraction

Dioxane ring contraction via Wolff rearrangement is the basis of the synthesis of dioxolane 565 by photolysis of diazo diketone 566 in methanol (85KGS321). 

Cyclobutenes prepared by the Ramberg-BScklund reaction are almost always unsubstituted at the vinylic positions. In 1974, Paquette introduced a related and complementary ring contraction which is particularly suitable for the synthesis of 1,2-dialkylcyclobutenes. The reaction takes place when carbanions of five-membered cyclic sulfones are treated with LAH in refluxing dioxane (Scheme 19). The mechanism of the reaction is not clear. Good yields are obtained only when the sulfone is 2,5-dial-kylated in the absence of these groups, simple reduction of sulfone to sulfide becomes a serious side reaction. Dibenzyl sulfone is converted into a mixture of stereoisomeric stilbenes (56%) and dibenzyl sulfide (23%) under the same conditions. ... 

There are very few examples of ring contraction from cyclobutyl to cyclopropyl. Irradiation of a benzene solution of 2-diazo-3,4-bis(diphenylmethylene)cyclobutanone (60) in the presence of water, methanol, 2-propanol or aniline afforded the carboxylic acid derivatives (61) in 13, 87, 75 and 45% yield respectively. Thermolysis of an aqueous dioxane solution of (60) surprisingly gave (61 XR = OH) in higher yield (52% equation 20). ... 

There have been relatively few applications to the contraction of larger rings. Irradiation of solutions of 4-diazo-/ra j-bicyclo[6.1.0]nonan-5-one afforded the strained carboxylate derivatives (107 XR = OH, OMe) in ca. 22-45% yield.Small to medium ring diazo ketones afford the ring-contracted carboxylic acids (108 n = 4-10) in 25-95% yield on photolysis in dioxane-H20 or THF-H20. Thermolysis of the diazo ketones in aniline at 150-160 C gave the anilides of (108) in 78-91% yield. Transannular reactions of the ketocarbene intermediates can be a complication under some conditions. 4-Carboxy[8]paracyclophane (109) has been prepared (25%) by the photochemical method. ... 

Intramolecular C-acylation. Treatment of 4-acyl-l,3-dioxan-2-ones such as that appearing as an intermediate for taxol synthesis with LTMP results in ring contraction of the heterocycle. 

Solvolysis of methyl 6-deoxy-2,3-0-isopropylidene-4-0-(methyl-sulfonyl)-a-D-mannopyranoside (175, R=Ms) at 170° in 9 1 p-dioxane-water containing an excess of sodium hydrogen carbonate or sodium hydroxide gave,437 as the major product, 6-deoxy-2,3-0-isopropyli-dene-5-O-methyl-D-talofuranose (189). The authors postulated initial ring-contraction, to give an intermediate 5-0-(methylsulfonyl)-)3-L-... 

Ring contraction accompanying nucleophilic substitution has been observed when cw-2-phenyl-5-hydroxy-l, 3-dioxan (254) is treated with triphenyl-phosphine and carbon tetrabromide in refluxing benzene giving (255). ... 

Ring contraction. A soln. of 3-diazonopinone in dioxane-water irradiated 190 hrs. in a Pyrex vessel with 15-w. Sylvania Blacklite tubes 5,5-dimethyl-bicyclo[2.1.1]hexane-2j -carboxylic acid. Y 68%. J. Meinwald and P. G. Gass-man, Am. Soc. 82, 2857 (1960). 

The only reported X-ray structure of a it-bonded diiodine exists in the 12/coronene associate [75], which shows the I2 to be located symmetrically between the aromatic planes and to form infinite donor/acceptor chains. -Coordination of diiodine over the outer ring in this associate is similar to that observed in the bromine/arene complexes (vide supra), and the I - C separation of 3.20 A is also significantly contracted relative to the stun of their van der Waals radii [75]. For the highly reactive dichlorine, only X-ray structures of its associates are observed with the n-type coordination to oxygen of 1,4-dioxane [76], and to the chlorinated fullerene [77]. 

The transannular formation of sulfonium ion intermediates is not limited to six-membered rings. A similar contraction of a benzo[6]thiepin derivative occurred when (288) was refluxed in dry dioxane, giving a quantitative yield of (289) (74CC626). 

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