Stereoselective nitrile

Two potentially useful routes to alkenyl nitrites have been studied in 1982. The first of these, and the most practical, uses Ni species to catalyse the formation of the unsaturated nitrile stereoselectively from vinyl halides and potassium cyanide. This new procedure complements existing methodology. The second route investigates the stereochemistry, mechanism, and preparative applications of Ni -catalysed additions of hydrogen cyanide to alkynes. ... 

A very efficient and universal method has been developed for the production of optically pue L- and D-amino adds. The prindple is based on the enantioselective hydrolysis of D,L-amino add amides. The stable D,L-amino add amides are effidently prepared under mild reaction conditions starting from simple raw materials (Figure A8.2). Thus reaction of an aldehyde with hydrogen cyanide in ammonia (Strecker reaction) gives rise to the formation of the amino nitrile. The aminonitrile is converted in a high yield to the D,L-amino add amide under alkaline conditions in the presence of a catalytic amount of acetone. The resolution step is accomplished with permeabilised whole cells of Pseudomonas putida ATCC 12633. A nearly 100% stereoselectivity in hydrolysing only the L-amino add amide is combined with a very broad substrate spedfidty. 

L-Amino adds could be produced from D,L-aminonitriles with 50% conversion using Pseudomonas putida and Brembacterium sp respectively, the remainder being the corresponding D-amino add amide. However, this does not prove the presence of a stereoselective nitrilase. It is more likely that the nitrile hydratase converts the D,L-nitrile into the D,L-amino add amide, where upon a L-spedfic amidase converts the amide further into 50% L-amino add and 50% D-amino add amide. In this respect the method has no real advantage over the process of using a stereospecific L-aminopeptidase (vide supra). 

The method is very useful for the synthesis of physiologically interesting a-mcthylamino acids, e.g., methyl dopa from the 3,4-dimethoxybenzyl derivative. The excellent stereoselection achieved in the process, however, is caused by the preferential crystallization of one pure diastereomerfrom the equilibrium mixture formed in the reversible Strecker reaction. Thus, the pure diastcrcomers with benzyl substituents, dissolved in chloroform or acetonitrile, give equilibrium mixtures of both diastereomers in a ratio of about 7 347. This effect has also been found for other s-methylamino nitriles of quite different structure49. If the amino nitrile (R1 = Bn) is synthesized in acetonitrile solution, the diastereomers do not crystallize while immediate hydrolysis indicates a ratio of the diastereomeric amino nitriles (S)I(R) of 86 1447. 

Wagner and co-workers explored the different selectivity of 1,3-dipolar cyclo additions of nitrones 140 and cinnamonitrile 139 leading to oxadia-zolines 141 derived from an exclusive CN attack instead of a C = C attack (Scheme 50). This behavior was observed when cinnamonitrile was coordinated to a transition metal like Ft or Pd [89]. A similar approach to platimun-promoted nitrile-nitrone cyclo additions was reported using cychc nitrones. In this case, the authors reported a higher stereoselectivity of cychc nitrones with respect to the acyclic nitrones, due to a rigid E conformation adopted by cyclic nitrones [90]. 

The stereoselectivity of conjugate addition and cyclopropanation of the chiral nitrovinyldioxolanes 17 can be effectively controlled <96TL6307>, and good selectivity is observed in the ultrasound-promoted cycloaddition of nitrile oxides to alkenyldioxolanes 18 <95MI877,95JOC7701 >. Asymmetric Simmons-Smith cyclopropanation of 19 proceeds with... 

Keywords Intramolecular 1,3-dipolar cycloadditions. Stereoselectivity, Nitrile oxides, SUyl nitronates. Oximes, H-Nitrones, Azides, NitrUimines... 

The versatility of the INOC reaction is evident from the synthesis of tetrahy-drofurans fused to an isoxazoline 22a-f (Eq. 3) [181. a-Allyloxyaldoximes 21, formed by the reduction of jS-nitrostyrenes 19 with SnCl2 2H2O in the presence of an unsaturated alcohol 20, are transformed to isoxazolines 22 in high yield on treatment with NaOCl via stereoselective ring closure of a nitrile oxide intermediate (Table 2). 

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 intramolecular cycloaddition of the norbornadiene-tethered nitrile oxides 110 (Eq. 11 and Table 11) was reported to be highly regio- and stereoselective, providing the exo cycloadduct 111 as the exclusive product out of the four possible regio/stereoisomers [36]. The cycloadduct 111 provides a stereoselective entry into tricyclic (e.g., 112) and spirocyclic (e.g., 113) frameworks. 

A regio- and stereospecific INOC reaction of unsymmetrical silaketals 114, synthesized in one pot from unsaturated alcohols, nitro ethanol, and dichloro-silanes, via the nitrile oxide 115 to isoxazolines 116 has been described (Scheme 14) [37a]. The intermolecular version of the cycloaddition, under similar conditions, proceeds with poor regio and stereoselectivity. 

Although nitrile oxide cycloadditions have been extensively investigated, cycloadditions of silyl nitronates, synthetic equivalent of nitrile oxides in their reactions with olefins, have not received similar attention. Since we found that the initial cycloadducts, hl-silyloxyisoxazolidines, are formed with high degree of stereoselectivity and can be easily transformed into isoxazolines upon treatment with acid or TBAF, intramolecular silylnitronate-olefin cycloadditions (ISOC) have emerged as a superior alternative to their corresponding INOC reactions [43]. Furthermore, adaptability of ISOC reactions to one-pot tandem sequences involving 1,4-addition and ISOC as the key steps has recently been demonstrated [44]. 

Intramolecular nitrone cycloadditions often require higher temperatures as nitrones react more sluggishly with alkenes than do nitrile oxides and the products contain a substituent on nitrogen which may not be desirable. Conspicuously absent among various nitrones employed earlier have been NH nitrones, which are tautomers of the more stable oximes. However, Grigg et al. [58 a] and Padwa and Norman [58b] have demonstrated that under certain conditions oximes can undergo addition to electron deficient olefins as Michael acceptors, followed by cycloadditions to multiple bonds. We found that intramolecular oxime-olefin cycloaddition (lOOC) can occur thermally via an H-nitrone and lead to stereospecific introduction of two or more stereocenters. This is an excellent procedure for the stereoselective introduction of amino alcohol functionality via N-0 bond cleavage. 

Scheme 8 Stereoselective reactions of non-racemic 2-sulfonyloxy nitriles with...

Cycloadditions of nitrones, nitrile oxides or diazo compounds to thiete dioxides do not show the high stereoselectivity observed with acyclic vinyl sulfones, and mixtures of the two possible adducts are formed . The charge-transfer stabilization energy calculated according to the Klopman-Salem perturbational approach is able to account for the experimental trends of the isomer ratio in terms of the major stereochemical structural differences between the acyclic vinyl sulfones and the four-membered ring sulfones (see Section IV.B.3). 

The synthesis in Scheme 13.64 was carried out by E. Carreira and co-workers at ETH in Zurich, Switzerland. A key step in the synthesis in Scheme 13.64 is a stereoselective cycloaddition using a phosphonyl-substituted nitrile oxide, which was used to form the C(16)-C(17) bond and install the C(15) oxygen. 

The C(6)-C(15) segment was synthesized by Steps C-l and C-2. The stereoselectivity of the cycloaddition reaction between the nitrile oxide and allylic alcohol is the result of a chelated TS involving the Mg alkoxide.39... 

The construction of novel tetracyclic ring systems 1-142, which can be considered as hybrids of the I e Ira h y dropy r ro I o 12,3 - fo] i ndo le and tetrahydroimidazol[l,2-o]in-dole ring system, has been described by Herranz and coworkers [40]. The exposure of tryptophan-derived a-amino nitrile 1-140 to acidic conditions triggers a stereoselective tautomerization to give 1-142 in quantitative yield (Scheme 1.35). 

Hassner and coworkers have developed a one-pot tandem consecutive 1,4-addition intramolecular cycloaddition strategy for the construction of five- and six-membered heterocycles and carbocycles. Because nitroalkenes are good Michael acceptors for carbon, sulfur, oxygen, and nitrogen nucleophiles (see Section 4.1 on the Michael reaction), subsequent intramolecular silyl nitronate cycloaddition (ISOC) or intramolecular nitrile oxide cycloaddition (INOC) provides one-pot synthesis of fused isoxazolines (Scheme 8.26). The ISOC route is generally better than INOC route regarding stereoselectivity and generality. 

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