Annulations allylic ylides

In 2003, Lu and coworkers reported the first phosphine-catalyzed yhde reactions in an allylic ylide [3-t2] annulation reaction. Allylic bromides 26 reacted with dually activated olefins 27 or 28 smoothly at 90 °C in the presence of a catalytic amount of PPha, affording the [3-1-2] cydoaddition products 29 and 30 respectively, in moderate to good yields with high diastereoselectivity (Scheme 20.24) [4d, 44]. 

Dipolar cycloaddition of alkenes with carbonyl ylides generated in situ is a versatile method for tetrahydrofuran synthesis. The synthetic potential of such transformations has been reviewed <2005JOM(690)5533, 2003BMI6-253>. In addition, the stereoselective [3 + 2] annulation of allyl silanes has become a reliable protocol for the synthesis of tetrahydrofurans as demonstrated in several total syntheses . Such a [3 + 2] annulation, for example, affords the tetrahydrofuran product 11 as a single stereoisomer (Scheme 15) <2002OL2945>. Lanthanide salts serve as efficient Lewis acid catalysts in similar [3 + 2] cycloaddition reactions . 

A phosphane-catalyzed [4 - -1] annulation between nitroalkenes and Morita-Baylis-Hillman carbonates was performed by He and co-workers. " The authors claimed the in situ formation of an allylic phosphorus ylide as an active intermediate. Allenoates and enones were able to form cyclopentenes via two cycloaddition reactions they underwent a [3 -F 2] or a [2 -F 4] process in the presence of catalytic phosphines or amines, respectively (Scheme 15). To explain such a different reactivity, Huang, Lankau and Yu, carried out M06-2X/6-31- -G calculations to study the role of ylide intermediates. ... 

Scheme 20.24 Phosphine-catalyzed ylide annulation reaction of allylic bromide.

The introduction of the activated allylic bromides and Morita-Baylis-HiUman acetates and carbonates pioneered the development of a number of phosphine-catalyzed reactions in subsequent years [45]. Interestingly, the asymmetric variant of this type of transformation only appeared in the literature seven years later. In 2010, Tang, Zhou, and coworkers disclosed a highly enantioselective intramolecular ylide [3-1-2] annulation using spirobiindane-based phosphine catalyst 31 (Scheme 20.27). BINAP was found inactive in this reaction even at an elevated temperature (70°C). Notably, both optically active benzobicyclo[4.3.0] compounds 32 and 32 with three continuous stereogenic centers could be obtained as major products in high yields and stereoselectivities just by a choice of an additive [Ti(OPr )4], which can block the isomerization of the double bond [46]. 

Heteroatom Wittig chemistry also includes reactions of N-sulfonyl imines. It was demostrated that these compounds underwent olefination reactions with nonstabilized phosphonium ylides under mild conditions to afford an array of both Z- and E-isomers of 1,2-disubstituted alkenes, allylic alcohols, and allylic amines.Additionally, studies of the reactions of 5-bromo-4,6-dimethyl-2-thioxo-l,2-dihydropyridine-3-carboni-trile and thiazolidinone with phosphorus ylides have proved the formation of new phosphonium ylides. Annulations via P-ylides are a common occurrence in the literature. For example, on photochemical irradiation, phosphonium-iodonium ylides were shown to undergo 1,3-dipolar cycloaddition reactions with triple bonds, via a carbene intermediate, to yield furans. " Even more common are the reactions of Morita-Baylis-Hillman (MBH) acetates and carbonates. Zhou et al. demostrated that these substrates were able to generate very reactive 1,3-dipoles in the presence of tertiary phosphines the dipoles then underwent cycloaddition reactions to yield annulation products (Scheme 16). ... 

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