Alkaloids nitrone 1,3-dipolar cycloadditions

Enantioenriched (-)-rosmarinecine, which belongs to the group of pyrrolizidine alkaloids [413], has been synthesized by Goti, Brandi and coworkers applying an intramolecular 1,3-dipolar cycloaddition as the key step [414]. The required nitrone was obtained in situ from L-malic acid. Moreover, 1,3-dienes as precursors for a cy-... 

Since Huisgen s definition of the general concepts of 1,3-dipolar cycloaddition, this class of reaction has been used extensively in organic synthesis. Nitro compounds can participate in 1,3-dipolar cycloaddition as sources of 1,3-dipoles such as nitronates or nitroxides. Because the reaction of nitrones can be compared with that of nitronates, recent development of nitrones in organic synthesis is briefly summarized. 1,3-Dipolar cycloadditions to a double bond or a triple bond lead to five-membered heterocyclic compounds (Scheme 8.12). There are many excellent reviews on 1,3-dipolar cycloaddition, in particular, the monograph by Torssell covers this topic comprehensively. This chapter describes only recent progress in this field. Many papers have appeared after the comprehensive monograph by Torssell. Here, the natural product synthesis and asymmetric 1,3-dipolar cycloaddition are emphasized.630 Synthesis of pyrrolidine and -izidine alkaloids based on cycloaddition reactions are also discussed in this chapter. 

Dipolarophiles D3. 1,3-Dipolar cycloadditions of suitably functionalized cyclic nitrones with terminal alkenes, which have potential leaving groups X at the end of the alkane chain -(CHo),- (D3), were successfully used for the synthesis of pyrrolozidine, indolizidine and quinolizidine alkaloids, such as (+ )-and (—)-lentiginosine, a potent amyloglucosidase inhibitor (Scheme 2.243) (742). Reductive cleavage of the N-0 bond in the cycloadduct is important for the subsequent cyclization to pyrrolozidines, indolizidines, and quinolizidines. 

Highly selective 1,3-dipolar cycloaddition reactions of nitrone (154) with acrylates have been used in the total syntheses of pyrrolizidine alkaloids, 7-deoxy-casuarine (572) and hyacinthacine A2 (573) (Scheme 2.263) (772). 

Dipolarophiles D14. The 1,3-dipolar cycloaddition of nitrones to dimethyl maleate and dimethyl fumarate is widely used in the synthesis of polyhydroxy alkaloid derivatives of dihydroindolizidinone (81), pyrrolizidine (119), (—)-codonopsinine, and (+ )-hyacinthacines Ai and A2 (312). In cases of unstable nitrones, syntheses of cycloadducts are performed in situ (81). 

Recent research deals with stereoselective 1,3-dipolar cycloadditions of nitrones for the syntheses of alkaloids and aza heterocycles asymmetric synthesis of biologically active compounds such as glycosidase inhibitors, sugar mimetics, /3-lactams, and amino acids synthesis of peptido-mimetics and peptides chemistry of spirocyclopropane heterocycles synthesis of organic materials for molecular recognition and photochemical applications. 

The key step of the synthesis, which involved a classical Mannich condensation in the synthesis by Evans and Scott (as well as in the biogenesis of alkaloids in general [10]), is substituted by a 1,3-dipolar cycloaddition of a nitrone to a carbon-carbon double bond [11] which provides an alternative route for the formation of a new C(l)-C(2) bond with the concomitant creation of a 1,3-consonant relationship between an oxygen atom and a dialkylamino group. In order to arrive at a typical Mannich base two more steps are, however, necessary. The similarity between the two processes is shown is Scheme 13.2.7 ... 

An impressive enantiopure synthesis of Amaryllidaceae alkaloids has been achieved through the formation of sugar-derived homochiral alkenyl nitrone 265 (Fig. 1.7).[280] While this reagent required lengthy preparation, it underwent an intramolecular dipolar cycloaddition to establish the required stereochemistry of the polycyclic pyrrolidine skeleton of (—)-haemanthidine (266), which was converted to (+)-pretazettine and (+)-tazettine by established procedures (281). 

The dipolar cycloaddition of nitronates has been applied to the synthesis of several natural products in the context of the tandem [4+2] / [3 + 2] nitroalkene cycloaddition process. All of these syntheses have focused on the construction of pyrrolidine, pyrrolizidine, and indolizidine alkaloids. For example, the synthesis of ( )-hastanecine (316), a necine alkaloid, involves the elaboration of a p-benzoy-loxynitroalkene 311 via [4 + 2] cycloaddition with a chiral vinyl ether (312) in the presence of a titanium based Lewis acid, to provide the nitronate 313 with high diastereo- and facial selectivity (Scheme 2.30) (69). The dipolar cycloaddition of... 

Alkenyl nitrones, having the alkene connected to the nitrone nitrogen atom, have been used in another approach to intramolecular reactions (231-235). Holmes and co-workers have this method for the synthesis of the alkaloid (—)-indolizidine 209B 137 (210,231). The alkenyl nitrone 134, was obtained from the chiral hydroxylamine 133 and an aldehyde. In the intramolecular 1,3-dipolar cycloaddition, 135 was formed as the only isomer (Scheme 12.45). The diastereofacial selectivity was controlled by the favored conformation of the cyclohexane-like transition state in which the pentyl group was in a pseudoequatorial position, as indicated by 134. Further transformation of 135 led to the desired product 137. 

Oppolzer et al. (321) applied his own sultam as the auxiliary for a cychc nitrone in the synthesis of (—)-allosedamine (Scheme 12.60). The enantiomerically pure nitrone 209 was synthesized from 208 by base treatment, attack of the enolate on 1-chloro-l-nitrosocyclohexane at the nitrogen atom, and subsequent elimination of chloride. Subsequent addition of aqueous HCl gave the cyclic nitrone 209. The nitrone participated in a 1,3-dipolar cycloaddition with styrene, proceeding with complete exo-specificity. The product, 210, was obtained with a de of 93%. Two further reaction steps yield the piperidine alkaloid ( )-aUosedamine 211 in an overall yield of 21%. 

The 1,3-dipolar cycloaddition of nitrones to alkenes has been shown to be very useful in the field of synthesis of alkaloids. The reaction is normally efficient and the inherent features of carbon-carbon bond formation, oxygen transfer and nitrogen incorporation have been joined by high regioselectivity and even stereoselectivity (79ACR396). The... 

Several isoxazolidines or their quaternary salts derived from the 1,3-dipolar cycloaddition of rr/i-hcxcnolidcs (6,7-dihydrooxepin-2(5//)-ones) to cyclic nitrones, were converted into the corresponding piperidyl- and pyrrolidyloxepi-nones by reduction of the nitrogen-oxygen bond (Equation 4). These amino alcohols provide a new access to the l-azabicyclo[5.3.0]decane core of the Stemona alkaloids <2004EJ04215>. 

Tufariello has reviewed his strategy for the synthesis of alkaloids (including necine bases) using the 1,3-dipolar cycloaddition of nitrones to alkenes.5 This work began with the synthesis of ( )-supinidine (7) from 1-pyrroline 1-oxide (see these Reports, Vol. 2, Ch. 4). A related approach has been used by Iwashita et al. in their synthesis of ( )-isoretronecanol (5).6 The stereochemistry of the exo-product (8), formed by regiospecific 1,3-dipolar cycloaddition of 1-pyrroline 1-oxide to dihydrofuran (Scheme 2), was confirmed by its conversion into... 

Recent strategy for the synthesis of phenylquinolizidine alkaloids involves inter- or intramolecular [3 + 2] dipolar cycloadditions of nitrones. The intermo-... 

If nitroalkenes are employed as heterodienes in hetero Diels-Alder reactions instead of nitrosoalkenes, cyclic nitrones are formed. These cycloadducts undergo numerous subsequent reactions, and especially the combination of this hetero Diels-Alder reaction with a 1,3-dipolar cycloaddition is an extremely powerful tool for the synthesis of polycyclic alkaloids. This domino [4+ 2]/[3+ 2] cycloaddition chemistry has been comprehensively reviewed by Denmark and Thorarensen very recently, and this review also covers many hetero Diels-Alder reactions of nitroalkenes being not part of this sequential transformation [5]. Therefore the present article will focus on some selected examples which might highlight the advanced state of the art concerning stereocontrol of these reactions. On the other hand, an insight shall be given into the multitude of polycyclic structures accessible by means of nitroalkene cycloaddition chemistry. 

Dipolar cycloadditions. Nitrones such as 1 combine with alkenes to form isoxazolidines. Tufariello has employed these 1,3-dipolar cycloadditionS as key steps in the synthesis of several alkaloids. Monosubstituted olefins react with nitrones regioselectively to afford 5-substituted iSoxazolidines thus addition of 2 to 1-pyrroline-l-oxide produces the cycioadduct 3, which serves as an intermediate in a total synthesis of elaeocarpine (4). ... 

The asymmetric 1,3-dipolar cycloaddition of nitrones to 3-(( )-2-butenoyl)-2-oxazoli-dinone [214] is catalyzed by nickel(II) [215] and palladium(II) [216] complexes (Sch. 66). The resulting isoxazolidines are of interest because they are readily converted into y-amino alcohols, precursors to alkaloids and /3-lactam antibiotics. 

Werner, K. M., De los Santos, J. M., Weinreb, S. M., Shang, M. A Convergent Stereoselective Synthesis of the Putative Structure of the Marine Alkaloid Lepadiformine via an Intramolecular Nitrone/1,3-Diene Dipolar Cycloaddition. J. Org. Chem. 1999, 64, 686-687. 

Dipolar cycloaddition reactions of nitrones, prepared by the oxidation of suitable hydroxylamines, have been used in a number of alkaloid syntheses. The preparation of tetrahydropyridine /V-oxides by means of the oxidative cleavage of suitable bicyclic isoxazolidines has been studied in connection with projected syntheses of... 

In an alternative approach to the nitrone method for the synthesis of Elaeocarpus alkaloids (cf. vol.ll, p.61), a 1,3 dipolar cycloaddition of 1-pyrroline 1-oxlde to a substituted enone gives the isoxazolidine (2), which was converted in a number of steps into the compound (3), the C-7 epimer of elaeokanine C. Oxidation of the epimer (3) afforded the diketone (4), an intermediate in a previously published synthesis of ( )-elaeokanlne C. 

Dipolar cycloadditions. This dipolarophile reacts with nitrones to give 4-toluenesulfinyl-5-methylisoxazolines, which on hydrogenation are transformed into /3-amino ketones. Some simple piperidine alkaloids can be synthesized by this method. 

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