Nitrones olefins

Asymmetric bias generated by protected vicinal diol controller and its application to asymmetric nitrone-olefin cycloaddition reactions 98YGK86. 

The Michael addidon of allyl Mcohols t nitronate olefin cycloaddidon fSecdon 8.2, 4.15. ... 

Several elegant synthetic strategies have been devised for biotin (1) this chapter describes one of the total syntheses developed at Hoffmann-La Roche. This insightful synthesis employs a derivative of L-cysteine, a readily available member of the chiral pool,2 as the starting material, and showcases the powerful intramolecular nitrone-olefin [3+2] cycloaddition reaction. 

The class of 1,3-dipolar cycloadditions embraces a variety of reactions that can accomplish the synthesis of a diverse array of polyfunctional and stereochemically complex five-membered rings.3 The first report of a 1,3-dipolar cycloaddition of a nitrone (a 1,3-dipole) to phenyl isocyanate (a dipolarophile) came from Beckmann s laboratory in 1890,4 and a full 70 years elapsed before several investigators simultaneously reported examples of nitrone-olefin [3+2] cycloadditions.5 The pioneering and brilliant investigations of Huisgen and his coworkers6 have deepened our under-... 

The elegant, enantiospecific synthesis of biotin (1) by Hoffmann-La Roche1 is based on a strategy that takes advantage of the powerful intramolecular nitrone-olefin cycloaddition reaction. Our analysis begins with model studies in which the straightforward conversion of L-cysteine (2) into aldehyde 3 (see Scheme 1) constitutes... 

The total synthesis of biotin (1) described in this chapter provides an impressive example of the intramolecular nitrone-olefin [3+2] cycloaddition reaction. Aiming for a practical process, the Hoff-mann-La Roche group utilized relatively simple and inexpensive starting materials, and ingeniously controlled the crucial [3+2] cycloaddition reaction to give only one stereoisomer by confining the cycloaddition precursor to a ten-membered ring. 

Intramolecular Silyl Nitronate-Olefin Cycloaddition (ISOC)... 

The Michael addition of allyl alcohols to nitroalkenes followed by intramolecular silyl nitronate olefin cycloaddition (Section 8.2) leads to functionalized tetrahydrofurans (Eq. 4.15).20... 

Intramolecular cyclizations of silyl nitronates were also used in the preparation of aminosugars. In 2003 Kudoh et al. reported the stereoselective conversion of 2-nitroalkanols by silyl nitronate generation followed by an intramolecular nitronate-olefin [3 + 2] cycloaddition reaction (Scheme 51).88... 

Like the nitrone-olefin [3 + 2]-cycloaddition, the nitrone-allene [3 + 21-cycloaddition also takes place regioselectively to furnish methylene-substituted isoxazoli-dine derivatives. The substituents of the 3- and 4-positions of the cycloadducts 72a and 72b are disposed cis in contrast, the reaction of 70 with allenyl sulfone 71c gives rise to the trans-cycloadduct 72c exclusively (Table 12.4). On treatment with base or heating, methyleneisoxazolidines 72 readily rearrange to isoxazo-lines via a 1,3-hydrogen shift. 

Nitro compounds, synthesis of, 12, 3 Nitrone-olefin cycloadditions, 36, 1 Nitrosation, 2, 6 7, 6 Nucleosides, synthesis of, 55, 1... 

In this case, the exojendo selection coincides with the ratio of cis and trans annulation of ring A (formed by the cycloaddition) and ring B (formed from the tether which connects the reactive sites in the starting material). By means of the intramolecular cycloaddition, the exojendo selection may be significantly improved in a predictable manner (Section 2.3.6.). The exojendo problem is characteristic of all types of cycloadditions and enc reactions for instance, nitrone-olefin cycloaddition. 

The [3 +2] Nitrone-Olefin Cycloaddition Reaction Pal N. Confalone, Edward M. Hula... 

A variant of the above method is the nitrone-olefin cycloaddition, exploited for the synthesis of shikimic acid [349] and for the synthesis of the carbocyclic nucleoside neplanocin [350]. RCM has been recently exploited in Ziegler s synthesis of cyclophellitol [351]. This synthesis encompasses a number of steps described throughout this chapter, including chain extensions at Cl, at C6 and branching by Michael addition followed by carbocyclization. 

Using this methodology, the first total synthesis of the natural product (-)-(19R)-ibogamin-19-ol was reported [106]. The crucial ISQ core containing the entire configurational stereochemistry of the final target compound was prepared in 15 steps. The two key steps involved chirality transfer in an Ireland-Claisen rearrangement and an intramolecular nitrone-olefin 1,3-dipolar cycloaddition described earlier. 

The complex Cp2Ti(OTf)2 induces [3 + 2]-nitrone-olefin cycloaddition reactions by a concerted process. The reaction proceeds via the intermediate [Cp2Ti(nitrone)2]2+. The crystal structure of one of these adducts is reported. Asymmetric induction has been observed when the chiral [Pg.592]

The chiral catalyzes asymmetric [3 + 2]-nitrone-olefin cycloaddition reactions.1559... 

Chiral Cyclopentane Synthesis From Sugars Transformation of monosaccharides into enantiomerically pure penta- substituted cyclopentanes via fragmentation and nitrone-olefin dipolar cycloaddition. 

There is no comprehensive review on the chemistry of isoxazolidines except for the specific aspect of the nitrone-olefin cycloaddition (1964)14 and on the cycloaddition of nitronic esters with olefins (1969).13 This review will cover the chemistry of isoxazolidines in the literature up to May 1974. 

Generally, the inversion at nitrogen is rapid and invertomer pairs are not isolated. In a few rare cases, however, a high nitrogen inversion barrier renders the invertomers separable.31 This point will be discussed later in more detail. The kinetic aspects of the nitrone-olefin cycloaddition have been extensively investigated by Huisgen32 and by Boyle.33... 

The [3 + 2J Nitrone-Olefin Cydoaddition Reaction Pat N. Confalonc and Edward M. Huic... 

---