Olefins nitroso, cycloadditions

SCHEME 10.42 In situ generated nitroso olefins undergo facile cycloadditions with enol ethers. 

Isoxazolidines 92 were converted into 3-nitro-4-hydroxymethyl tetrahydrofurans 94 by treatment with TBAF. The process is believed to occur through the formation of nitroso intermediates 93, that undergo a spontaneous aerobic oxidation. The two-step sequence of intramolecular silylnitronate olefin cycloaddition (ISOC) followed by oxidative ring cleavage was diastereoselective and allowed complete control of the relative configuration of the newly created stereocenters <040L2027>. 

Main Aspects of Chemistry and Stereochemistry of Cyclic Nitroso Acetals Chemistry of cyclic nitroso acetals or nitrosals (the term was introduced by Prof. Seebach) has attracted interest only after the discovery of the 1,3-dipolar cycloaddition reaction of nitronates with olefins in 1962 by the research group of Prof. Tartakovsky. (Principal data on nitroso acetals up to 1990 were summarized in the review by Rudchenko (395).)... 

Besides, cyclic nitroso acetals have attracted considerable interest because [3+ 2]-cycloaddition of nitronates to olefins provides a simple and efficient procedure for the synthesis of five-membered cyclic nitrosals (see Scheme 3.153). Hence, knowledge of the configurations of these products allows conclusions about the mechanism of this very important process. However, it is impossible to distinguish kinetic and thermodynamic products of [3+ 2]-cycloaddition of nitronates to olefins without studying such a fundamental problem as the stereodynamics of five-membered cyclic nitroso acetals, that is, the ring and nitrogen inversions. 

The hetero Diels-Alder reaction of nitrosoalkenes with electron-rich olefins has been known for a long time [363]. A detailed mechanistic study carried out by Reissig et al. has given evidence that this inverse electron demand cycloaddition is a concerted process [364]. Recent ab initio calculations dealing with the reaction between ethylene and nitroso ethylene strongly corrobate this view [365]. In this work, Jursic and Zdravkovski have also investigated the influence of BH3 as Lewis acid catalyst. However, cycloadditions of nitrosoalkenes already pro-... 

In efforts towards a synthesis of eremolactone, the dienolate (158) has been found to react with the cyano-ester (159) to give (160) (probable stereochemistry shown) most likely via a double Michael addition rather than a Diels-Alder reaction. Highly substituted cyclohexanes have been prepared by cycloadditions between cyclohexa-1,3-dienes and nitroso-compounds followed by N—O and olefinic bond cleavage (Scheme 12). ... 

Asymmetric induction can be also accomplished through the use of a chirally modified nitro olefin. Sugar-based nitroalkenes participate in thermal [4 + 2] cycloaddition to form enantiomerically pure nitronates [55,97]. Alternatively, diastereoselective cycloadditions are possible with chiral nitroalkenes as illustrated on Scheme 16.15 [47]. The tandem double intramolecular cycloaddition of enantiopure nitro-alkene 62 containing a single stereogenic center provides nitroso acetal 63 with high diastereoselectivity (relative to the existing center) in moderate yield. The product is isolated as a mixture of isomers that is formed due to epimerization of the intermediate nitronate (not shown) and used toward total synthesis of daphnilactone B. 

The most extensively stodied double intermolecular cycloadditions involve vinyl ether dienophiles (F = -OR, Scheme 16.53) and unsaturated ester dipolarophiles (A = — CO2R)- In this case, the final product of hydrogenolysis is pyrrolizidinone (such as 250, Scheme 16.54), which may contain five of the six original stereogenic centers in the nitroso acetal. The pyrrolizidine is a common motif in many natural products and the double intermolecular cycloaddition of nitro olefins has been extensively employed for its construction in syntheses efforts. 

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