Endocyclic reactions compounds

Thus, if there are several nonequivalent endocyclic reaction centers in the aminoazole molecule (Scheme 3.50), then usually more nucleophilic ones participate in the interaction with the carbonyl compound (for example, N(2) in 3-amino-1,2,4-triazole or 5-aminopyrazole [158, 160]). 

The selective consecutive cycloisomerization/isomerization of 1,6-dienes to form exocyclic and then endocyclic olefinic compounds is not a common transformation. At 80 °C, with RuCl2(p-cymene)(N-mesitylmethylbenzi-midazole) activated by reaction with diphenylpropynol in the presence of AgOTf as the catalyst, as an attempt to generate allenylidene intermediate, diallyl tosylamide is selectively transformed into 3-mclhyl-4-mclhylene-N-tosylpyrrolidine. When this catalytic cycloisomerization is completed, the isomerization into 3,4-dimethyl-AMosylpyrrolidine starts without external modification of the catalytic system (Scheme 27) [68]. 

The reaction of ( n -arene)(ti - 2.21paracyclophane)-complexes of nithenium(II) with hydride afforded (q6-l,3-diene)(Tj -(2.2]paracyclophane)-complexes of ruthenium(0) whidi on protonation with HBF4 gave a series of highly fluxional endocyclic agostic compounds. 

As a further application of the reaction, the conversion of an endocyclic double bond to an c.xo-methylene is possible[382]. The epoxidation of an cWo-alkene followed by diethylaluminum amide-mediated isomerization affords the allylic alcohol 583 with an exo double bond[383]. The hydroxy group is eliminated selectively by Pd-catalyzed hydrogenolysis after converting it into allylic formate, yielding the c.ro-methylene compound 584. The conversion of carvone (585) into l,3-disiloxy-4-methylenecyclohexane (586) is an example[382]. 

Imino-4-thiazolines are far more basic than their isomeric 2-aminothiazoles (see Table VI-1). They react with most electrophDic centers through the exocyclic nitrogen and are easily acylated (37, 477, 706) and sulfonated (652). The reaction of 2-imino-3-methyi-4-thiazoline (378) with a-chloracetic anhydride yields 379 (Scheme 217) (707). This exclusive reactivity of the exocyclic nitrogen precludes the direct synthesis of endocyclic quaternary salts of 2-imino-4-thiazolines. although this class of compounds was prepared recently according to Scheme 218 (493). 

Allylic bromination of pregnenolone acetate with dibromodi-methylhydantoin affords the 7-bromo compound (155) of undefined stereochemistry. Dehydrobromination by means of collidine followed by saponification affords the 5,7 endocyclic cis,cis-diene, 156. This compound contains the same chromophore as ergosterol, a steroid used as a vitamin D precursor. The latter displays a complex series of photochemical reactions among the known products is lumisterol, in which the stereochemistry at both C9 and Cio is inverted. Indeed, irradiation of 156 proceeds to give just such a product (158). This reaction can be rationalized by... 

The driving force for this reaction is the formation of the thermodynamically more stable aromatic compound. The mechanism (E) involves doublebond isomerization, as was discussed previously, imtil an endocyclic diolefin is obtained. 

Addition of hydroxylamines to C=N double bonds produces stable compounds only for endocyclic C=N bonds. Addition of hydroxylamine to azirines of type 59 results in stable adducts 60 as seen in equation 40. Similar reactions have been observed in benzoxazoles and l,2,3-triazolo[4.5-rf]pyrimidones ° ... 

Harmata et al. have developed a tandem Sonogashira/nitrogen-addition reaction of acetylenes 261 to sulfonamide 262 to prepare S(vi)-oxidized compounds 263 and 264 (Scheme 36) <20050L143>. When R = alkyl (e.g., Pr"), the 1,2-thiazine 263 is the major product (70%) formed via a endocyclization process, along with a minor amount (20%) of exocyclized product 264, while the five-membered ring product 264 (81%) is preferred when R= Ph. 

Limonene is a liquid with lemon-like odor. It is a reactive compound oxidation often yields more than one product. Dehydrogenation leads to / -cymene. Limonene can be converted into cyclic terpene alcohols by hydrohalogenation, followed by hydrolysis. Nitrosyl chloride adds selectively to the endocyclic double bond this reaction is utilized in the manufacture of (—)-carvone from (+)-limonene (see p. 61). 

The Cu(I)-catalyzed decomposition of (alkynyloxysilyl)diazoacetates 119 furnishes the silaheterocycles 120 and/or 121 (equation 30) in modest yield63. In these cases, the photochemical extrusion of nitrogen from 119 does not lead to defined products and the thermal reaction is dominated by the 1,3-dipolar cycloaddition ability of these diazo compounds. In mechanistic terms, carbene 122 or more likely a derived copper carbene complex, is transformed into cyclopropene 123 by an intramolecular [1 + 2] cycloaddition to the triple bond. The strained cyclopropene rearranges to a vinylcarbene either with an exo-cyclic (124) or an endocyclic (125) carbene center, and typical carbene reactions then lead to the observed products. Analogous carbene-to-carbene rearrangements are involved in carbenoid transformations of other alkynylcarbenes64. 

In related work, carbohydrate 1,2-diol dibutylstannylene acetals have been regiospecifically oxidized by IV-bromosuccinimide to give a-hydroxy-ketones. The reaction is illustrated for the preparation of 3-deoxy-l,2-O-isopropylidene-a-D-eryt/zro-hexofuranos-5-ulose (223) in Scheme 24. The endocyclic diol is also regioselectively oxidized to one product (224) in 44% yield, although 55% of the starting compound was recovered. (Scheme 24).439a... 

Noteworthy also is the reaction between L42 and Cul in the presence of KI. The crystal structure determination reveals the formation of hitherto unknown exocyclic Cu g2- clusters, which are linked by L42 to generate a polymeric ID array of composition pC2(Cu4I6)L42]n 42". Within the crown ether cavities of L42, two potassium ions are trapped in an endocyclic manner. Also, upon inclusion of the K+ ions the crown ring shrinks leading to opening of opposite aromatic rings in the host calixarene unit, a rare behavior for this class of compound.162... 

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