Isoxazolines tricyclic

The photolysis of 3-( p-cyanophenyl)-2-isoxazoline in benzene produced a tricyclic product along with six other materials (Scheme 46) (B-79MI41616). Irradiation of the bicyclic 2-isoxazoline (155) produced benzonitrile, /3-cyanonaphthalene and polymer via a proposed biradical intermediate (156) (Scheme 47) (B-79MI41615). 

Chiral tricyclic fused pyrrolidines 29a-c and piperidines 29d-g have been synthesized starting from L-serine, L-threonine, and L-cysteine taking advantage of the INOC strategy (Scheme 4) [19]. L-Serine (23 a) and L-threonine (23 b) were protected as stable oxazolidin-2-ones 24a and 24b, respectively. Analogously, L-cysteine 23 c was converted to thiazolidin-2-one 24 c. Subsequent N-allylation or homoallylation, DIBALH reduction, and oximation afforded the ene-oximes, 27a-g. Conversion of ene-oximes 27a-g to the desired key intermediates, nitrile oxides 28 a-g, provided the isoxazolines 29 a-g. While fused pyrrolidines 29a-c were formed in poor yield (due to dimerization of nitrile oxides) and with moderate stereoselectivity (as a mixture of cis (major) and trans (minor) isomers), corresponding piperidines 29d-g were formed in good yield and excellent stereoselectivity (as exclusively trans isomers, see Table 3). 

In the seven-step stereoselective total synthesis of ptilocaulin 44 [21 ], a potent antileukemic and antimicrobial agent isolated [22] from marine sponges, the oxime 36 was treated with NaOCl providing the tricyclic isoxazoline 38 in 89% yield without isolation of the nitrile oxide intermediate 37 (Scheme 5) [23]. Isoxazoline 38 was obtained as a mixture of four diastereomers and their ratio was... 

The reaction of the a-bromo aldoxime 52e (R = R = Me) with unsaturated alcohols has been extended to the heterocyclic systems furfuryl alcohols and 2-thiophene methanol [29b]. The furanyl and thiophenyl oximes 63a-c were treated with NaOCl and the resulting heterocyclic nitrile oxides were found to undergo spontaneous intramolecular dipolar cycloaddition to produce the unsaturated tricyclic isoxazolines 64a-c in high yield (Eq. 5). In these cases, the heterocyclic ring acts as the dipolarophile with one of the double bonds adding to the nitrile oxide [30]. 

Intramolecular 1,3-dipolar cycloaddition of cyclo-1,3-diene- and -1,3,5-triene-tethered nitrile oxides give tricyclic isoxazolines, for example, 353, as a single stereoisomer. 

In later work, Mioskowski and co-workers (320) used cyclohexenone 160 to prepare oxime 161 as part of a twofold nitrile oxide strategy to synthesize the basic taxol ring system. Cycloaddition of 161 was effected by means of sodium hypochlorite and gave tricyclic isoxazoline 162, which feamres rings A and C of taxol (320) (Scheme 6.79). Nagaoka and co-worker tried to apply a related intramolecular cycloaddition toward the synthesis of the taxane A/B ring but this approach failed, producing only the oxime derivative (248) (see Scheme 6.44, Section 6.3.1). 

Intramolecular cycloadditions of alkenyl-substituted nitrile oxides produce bicyclic isoxazolines. When monocyclic olehns are used, tricyclic structures are obtained. This approach was pioneered by both Kozikowski s and Curran s groups. A typical case involves the cycloaddition of nitro compound 191 [mixture of diastereomers derived from pentenose pyranoside 190], which produced a diaster-eomeric mixture of isoxazolines that contain cis-fused rings (i.e., 192) in near quantitative yield (326) (Scheme 6.85). Further elaboration of this mixture led to epoxycyclopentano-isoxazoline 193, which was then converted to the aldol product in the usual manner. The hydrogenation proceeded well only when rhodium on alumina was used as the catalyst, giving the required p-hydroxyketone 194. This... 

A related sequence was used by Kozikowski and Park (74) to prepare the ring skeleton of streptazolin (200), a compound that exhibits antibacterial and antifungal effects. In this approach, the tricyclic isoxazoline intermediate 198 was formed in the key cycloaddition step (Scheme 6.86). Thus, the reaction of oxime 197 (obtained from 4-piperidone) with sodium hypochlorite-triethylamine afforded tricyclic isoxazoline 198 in very good yield. This cycloadduct was converted to p-hydroxyketone 199 by reduction/hydrolysis using Raney Ni in the presence of acetic acid. Racemic streptazolin (200) was obtained from 199 in several additional steps (74). 

The intramolecular nitrile oxide-alkene cycloaddition has further been used for the construction of a tricyclic isoxazoline intermediate containing a decaline ring. 

Kozikowski and Li (268) also made use of this protocol for the construction of the hexahydronaphthalene portion of the hypocholesteremic agent compactin (256) (see Section 6.4.2.2). The oxime derived by from alcohol 253 (via y-lactone 252) was heated with aqueous sodium hypochlorite in the presence of triethylamine to give the tricyclic isoxazoline adduct 254 (Scheme 6.96). Reductive hydrolysis and dehydration afforded enone 255, which in several further steps led to compactin (256) (268). 

An intramolecular cycloaddition of the tetradecatrienyl nitroethyl ether 263 was used in the synthesis of the 14-membered bicyclic precursor 265 of crassin acetate 266, a cembrane lactone possessing antibiotic and antineoplastic activity (332). Nitro compound 263 was obtained from farnesyl acetate (262) in several steps and was then treated with phenyl isocyanate and triethylamine to give the tricyclic isoxazoline 264 (Scheme 6.98). Conversion to ketone 265 was accomplished by hydrogenation of the cycloadduct with Raney Ni and boric acid followed by acetylation (332). In this case, the isoxazoline derived from a 3-butenyl nitroethyl ether moiety served to produce a 3-methylenetetrahydropyran moiety (332). 

Few examples of total syntheses have been reported that involve an intramolecular nitrile oxide cycloaddition and ensuing reduction to an aminoalcohol. The very first example was reported by Confalone et al. (334) and involved a synthesis of the naturally occurring vitamin biotin (287). The nitro precursor 284 was easily prepared from cycloheptene. When treated with phenyl isocyanate-triethylamine, cycloaddition led to the all-cis-fused tricyclic isoxazoline 285 with high stereoselectivity (Scheme 6.102). Reduction with LiAlFLj afforded aminoalcohol 286 as a... 

For alkenyl nitrile oxides having the alkene in a cyclic structure, such as compound 141, high diastereoselectivities can be obtained (Scheme 12.47). Compound 141 is formed in situ, and undergoes a spontaneous cyclization to furnish 142 as the sole diastereomer. Toyota et al. (239) used the tricyclic isoxazoline 143 in the synthesis of (+)-pumiliotoxin C. 

Ring fused products can be elaborated from isoxazolines (80S757). Several nitrocyclo-alkenes (516) were prepared and reacted with phenyl isocyanate to generate the intermediate nitrile oxides which underwent internal cycloaddition to afford the tricyclic isoxazolines (517). Cleavage of the N—O bond by hydrogenation in the presence of a catalytic amount of Raney nickel and subsequent hydrolysis afforded the /3-ketol (518 Scheme 113). 

The cu-dioxane (97) underwent cyclization to afford a single tricyclic isoxazoline in which the new 5,5-fused bicyclic system formed rapidly at 0 C (Scheme 27).45 The isomeric tranr-dioxane also gave a single tricyclic isoxazoline (96) as product but only at 20 C. The results were rationalized as follows cyclization of the nitrile oxide derived from (96) occurred with the vinyl group equatorial, while the cis isomer required an axial vinyl group in the transition state. The isoxazoline (96) was converted in several steps to a known PGF2a precursor. 

A one-step synthesis of tricyclic diazadihydroacenaphthylenes with an isoxazoline ring has been developed from 1 -benzylamino-1 -methylthio-2-nitroethene derivatives induced by a large excess of triflic acid715 [Eq. (5.265)]. Dications 169, similar to those detected by Coustard,197 were observed by NMR spectroscopy. Quenching with water gives a reactive intermediate nitrile oxide, which undergoes an intramolecular cyclization to furnish the final products in fair yields. 

As shown in Equation (10), the 1,3-dipolar cycloaddition of in situ generated carboethoxy-formonitrile oxide to a dihydropyran has been used to construct the terminal isoxazoline ring of the tricyclic heterocycle (30) <86T1407>. The [k4s + 7t2s] pericyclic product is obtained as a 70/30 inseparable mixture of diastereomers in 50% yield. 

Stable chromium and tungsten Fischer dienyl carbenes bearing a 2,3-dihydroisoxazole ring have been treated with isocyanides to give highly functionalized 2,3-dihydro-l,2-benzisoxazoles and indazoles. Treatment of chromium isoxazoline 110 with benzyl isocyanide afforded the tricyclic compound 111 selectively in 82% yield (Equation 10) <2001CEJ5318>. 

A 1,3-dipolar cycloaddition using a nitrile oxide dipole was described by Roth and Singh as an approach to the 3-hydroxy-3-alkyl oxindole scaffold (2011OL2118). Spirocychc isoxazoline 307 was obtained by cycloaddition of nitrile oxide 305 with 3-methylene oxindole 306 and was further elaborated to 3-hydroxy-3-cyanomethyl oxindole 308. This novel protocol was then used to synthesize the tricyclic pyrrolidinoindohne natural product ( )-alline 310 via the tricycHc intermediate 307 (Scheme 55). Thus... 

---