Tandem transesterification

Tandem transesterification and diastereoselective intramolecular 1,3-dipolar cycloaddition of a-methoxycarbonylnitrones with chiral allyl alcohols give polycyclic compounds in one step with high stereoselectivity (Scheme 8.14).76 Transition state Ain Scheme 8.14 is more favorable than B because B has severe steric interaction (allylic 1,3-strain).77... 

NHCs have also been shown to promote the reaction of benzoins and methyl acrylate to produce y-butyrolactones (Scheme 44) [143], In the absence of dimeth-ylimidazolium iodide, the reaction does not proceed. The mechanism is still under investigation, although the authors propose that the transformation may proceed via a tandem transesterification/intramolecular Michael addition LXVin or Michael... 

Tamura et al. (170-172) discovered that, when reactions of ester-substituted nitrones with allylic alcohols are performed in the presence of an equimolar amount of titanium tetraisopropoxide under heating or at room temperature, transesterification takes place to form new nitrones bearing an inner alkene dipolarophile. The resulting nitrone substrates undergo regio- and stereoselective intramolecular cycloaddition reactions to give the ring-fused isoxazolidines (Scheme 11.52). This tandem transesterification/[3 + 2]-cycloaddition method leads to the selective... 

The tandem transesterification/[3 + 2]-cycloaddition methodology is be a powerful synthetic tool, since it guarantees high diastereoselectivity even under thermal conditions. It has been successfully apphed to synthetic work of the N-terminal amino acid component of Nikkomycin Bz (Scheme 11.53) (173). Thus, the sugar-based oxime is condensed with a glyoxylate hemiacetal to produce a chiral nitrone ester, which is then reacted with ( )-p-niethoxycinnamyl alcohol in the presence of a catalytic amount of TiCU at 100 °C. After the intramolecular cycloaddition, the... 

Achiral ester-substituted nitrones as well as chiral nitrones can be employed in diastereoselective asymmetric versions of tandem transesterification/[3 + 21-cycloaddition reactions, as shown in Scheme 11.54 (174). High diastereoselectivity and excellent chemical yields have been observed in the reaction with a (Z)-allylic alcohol having a chiral center at the a-position in the presence of a catalytic amount of TiCl4- On the other hand, the reaction with an ( )-allylic alcohol having a chiral center at the a-position, under similar conditions, affords very low selectivities. Tamura et al. has solved this problem with a double chiral induction method. Thus, high diastereoselectivity has been attained by use of a chiral nitrone. 

Furthermore, the Dieckmann condensation strategy has also been employed in the synthesis of pulvinic acids [61]. Interestingly, 30 years after the first relevant report by Weinstock et al. [61a], the group of Le Gall [61b] presented a versatile route for that purpose. Le Gall improved the synthesis of tetronic acid 61, previously prepared by Campbell et al. [58], by devising a tandem transesterification/Dieckmann... 

Tandem transesterification and intramolecular cycloaddition of achiral C-aUcoxycarbonylnitrones with aUyl alcohols... 

TANDEM TRANSESTERIFICATION AND INTRAMOLECULAR CYCLOADDITION OF C-ALKOXYCARBONYLNITRONES WITH ALLYL ALCOHOLS... 

Tandem Transesterification and Intramolecular Cycloaddition of Achiral C-Alkoxycarbonylnitrones with Allyl Alcohols [10]... 

SCHEME 6.5 Representative examples of tandem transesterification and intramolecular cycloaddition. 

TABLE 6.4 Tandem Transesterification and Intramolecular Cycloaddition of C-Methoxycarbonylnitrones 1 with Allylalcohol (Z)-12... 

The synthesis of tricyclic compound 120 was first reported by Tamura and co-workers using a tandem transesterification-cycloaddition reaction of nitrone methyl ester 118 and cyclohexen-3-ol (Scheme 16.17). This reaction was catalyzed by titanium tetrachloride to make nitrone cyclohexenyl ester 119. A subsequent 1,3-dipolar cycloaddition reaction proceeded presumably through a transition state involving the i-cis-like conformation of the ester C—O bond and the E configuration of the nitrone C=N bond (Scheme 16.17). 

SCHEME 16.17. A tandem transesterification-cycloaddition reaction. 1995 Elsevier Ltd. AU rights reserved. 

Tamura O, Okabe T, Yamaguchi T, Gotanda K, Noe K, Sakamoto M. Studies on tandem transesterification and intramolecular cycloaddition of nitrones. 1. Sequential bicyclization of a -methoxycarbonylnitrones with allyl alcohols. Tetrahedron 1995 51 107-118. 

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