Cycloaddition Cyclohexene oxide

Reactions of 1 with epoxides involve some cycloaddition products, and thus will be treated here. Such reactions are quite complicated and have been studied in some depth.84,92 With cyclohexene oxide, 1 yields the disilaoxirane 48, cyclohexene, and the silyl enol ether 56 (Eq. 29). With ( )- and (Z)-stilbene oxides (Eq. 30) the products include 48, ( > and (Z)-stilbenes, the E- and Z-isomers of silyl enol ether 57, and only one (trans) stereoisomer of the five-membered ring compound 58. The products have been rationalized in terms of the mechanism detailed in Scheme 14, involving a ring-opened zwitterionic intermediate, allowing for carbon-carbon bond rotation and the observed stereochemistry. 

The ring-opening of the cyclopropane nitrosourea 233 with silver triflate followed by stereospecific [4 -i- 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-cyclohexen-l-ones [131], allylic oxidation of oc, -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

A Ag -induced dipolar cycloaddition of 2-chloromethylpyridine A-oxides (101) with cyclohexenes in the presence of silver tetrafluoroborate afforded a mixture of cycloadducts 102 and substitution products 103 (81AGE481). In some cases, the cycloadduct could be detected only by NMR spectroscopy. 

Regioselective [4-1-2] cycloadditions to Cjq are also possible with 2,3-dimethyl-buta-1,3-diene (4) and with the monoterpene 7-methyl-3-methylideneocta-l,6-diene (5, myrcene) [22]. These monoadduct formations proceed under mild and controlled conditions. Most of these addition products of 1,3-butadiene derivatives (e.g. 4, 5, 8-12) are unstable against air and light [25]. The dihydrofuUerene moiety in the Diels-Alder adducts act as a 02-sensitizer and promotes the oxidation of the cyclohexene moiety to the hydroperoxide. Reduction of the hydroperoxide with PPhj yields the corresponding allylic alcohols [25]. 

A special case involves the thermal decomposition of 3,4-dinitrofuroxan (104). The cycloreversion is already observed at room temperature and the nitroformo-nitrile oxide could be trapped with electron-deficient nitriles. The cycloadditions with styrene, phenylacetylene, frani-stilbene, and cyclohexene, however, led to complex mixtures of products that could not be separated (104). In the related case of a furoxan with an a-hydrogen adjacent to the sulfonyl group, the reaction was proposed to proceed according to course (b) (Scheme 6.7). 

Side reactions that occur with intramolecular cycloaddition, such as linear oligomerization or dimerization of the nitrile oxide, are not very common when shorter chain lengths n < 1) are used due to the entropically favored intramolecular process. A rather unusual result in this regard involves the formation of a fused cyclooctane instead of the less-strained six-membered ring (also fused) in the cycloaddition of the nitrile oxide derived from p-naphthoquinone (Scheme 6.43). This result is consistent with the effect of electron-withdrawal in the enedione part, leading to increased reactivity (247), and also reflects the known sluggishness of cyclohexenes towards nitrile oxides (cf. Section 6.2.1.2). 

This reduction has also been used to effect ring annelation by an intramolecular [3 + 2] cycloaddition of a nitrile oxide to a cyclohexene.4 Example ... 

The high-pressure cycloaddition of 2-vinylbenzothiophene 848 with 3-nitro-2-cyclohexen-l-one gives adduct 849, which is treated with l,5-diazabicyclo[4.3.0]non-5-ene (DBN) to afford benzothiophene 850. The reaction with indenone affords the thiophene 852 via oxidation of 851 with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQ) (Scheme 130) <2001T4959>. 

The cycloaddition of diethyl diazenedicarboxylate and l-(l-cyclohexenyl)cyclohexene has been achieved in good yield. The adduct 1 resisted hydrogenation (hydrogen, palladium on carbon or Raney nickel, 414 kbars) and reconverted to the starting diene on hydrolysis and oxidation... 

In addition to the trihalo compounds, many other 1,2,4-triazines have been found to react with cyclopentene, cyclohexene, cycloheptene and cyclooctene to give condensed pyridines. In most cases, 1,4-benzoquinone is added to oxidize the initially formed dihydropyridines and to prevent a second [4 I 2] cycloaddition.386 Other 1,2,4-triazines also react with bicyclo[2.2.1]hept-2-ene to give either the condensed pyridines and/or the bisadducts. The ratio of the two products depends on the reaction conditions and the relative proportions of the reactants.386 For a detailed survey, see Houben-Weyl, Vol. E7b, p471ff. 

Regioselective Diels-Alder reactionsThe intramolecular [4+2]cycloaddition of 2, prepared by reaction of a vinylsilane with the diunsaturatcd alcohol 1, provides an adduct (3), which on oxidative dcsilylation provides the cyclohexenes 4 and S with marked preference for the d.v-l,3-diol 4. 

Dipolar cycloaddition of nitrile oxides to cyclohexenes and the action of hydroxylamine on 2-acylcyclohexanones constitute the main synthetic routes to 4,5,6,7-tetrahydroindoxazenes. A convenient one-step synthesis utilizes the addition of a nitrile oxide to cyclohexenones of type 69 (X = OH, OMe, OAc, Cl, NH2, or NR2) (Eq. 4).88... 

Contrary to earlier reports,1 3a,4,5,6,7,7a-hexahydroindoxazenes can be prepared by cycloaddition of nitrile oxides to cyclohexene.102,103 Similar cycloadditions with 1,2-dihydronaphthalene produce a mixture of the regioisomers 91a and 91b, and with cyclohex-2-en-l-one a mixture of 3-aryl-4-oxo- and 3-aryl-7-oxo-3a,4,5,6,7,7a-hexahydroindoxazenes, the former predominating.104... 

Oxidation of Alkenes to Oxirans by Peroxy-acids. The mechanism of the reaction of m-chloroperbenzoic acid with double bonds has been investigated through a study of the epoxidation of a series of cycloalkenes (of ring sizes 5,6,7,8, and 12) and substituted cyclohexenes.The second-order rate constants were determined in CHCI3 at 0—30°C, and the data support a 1,3-dipolar cycloaddition reaction. 

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